Gravity - a fundamental force in nature

AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy Gravity, black holes and physics

Chapter 1
1.1. Development of knowledge about nature, space, gravity
1.2. Newton's law of gravitation
1.3. Mechanical LeSage hypothesis of the nature of gravity
1.4. Analogy between gravity and electrostatics
1.5. Electromagnetic field. Maxwell's equations.
1.6. Four-dimensional spacetime and special theory of relativity

1.1. Development of knowledge about nature, space, gravity

Gravitation is a force with which every person is in direct and constant contact from birth to life, in the form of Earth gravity - weight (Greek gravis = heavy), and humanity thus encounters gravitational phenomena from the very beginning. Nevertheless (or perhaps because of it), as with most basic natural phenomena, there have been very misconception opinions about gravity for a very long time, and we can say that the origin and essence of gravity is not fully explained even now. In this introductory part, we will very briefly summarize how views on gravity, space, time, matter, universe, and nature in general, have gradually evolved and refined from antiquity to the present day.

Science in antiquity
The origins of science in antiquity arose from entirely pragmatic motives: to systematically and correctly solve the problems that life brought. Such specific problems were, for example, the construction of cult buildings and the construction of irrigation systems, the rational management and cultivation of land, the distribution of food or other objects and their exchange, and so on. To solve such tasks, it was necessary to learn to determine distances, height differences and areas, to study and predict the weather, to count and distribute goods in terms of quality and quantity.

The needs of exchange and distribution led to the introduction of the basic arithmetic operations of addition, subtraction, multiplication and division. These operations, which are an expression of the properties of ordinary material objects, display (model) the actual processes with real bodies. Scales - units *) have been introduced to determine the distances and sizes of objects , ie certain standard objects, which can be used to express the sizes of other objects by comparison. In other words, the amount and sizes of real bodies were assigned numbers - their number and dimensions - which were operated according to the rules of arithmetic, and the resulting numbers were converted back to the corresponding number and size of real objects. Thus mathematics appeared in human activity as the science of models and a general scheme began to be used (initially subconsciously) :

reality ® Model ® reality
n n

*) Units of length and time
In the past, people chose basic mechanical units for measing distances, time, and weight (the amount of matter) according to the environment in which they lived and using the objects they encountered. For length, these were first human measures such as "foot", "elbow" or "inch", later, when the shape and approximate size of the Earth were already known, the unit of length "kilometer" was determined as 1/40000 of the length of the Earth's equator, from which then "meter".
  Time was derived from the duration of one Earth orbit around the Sun - "year", the time of the Moon's orbit around the Earth - "moon", the time of one rotation of the Earth around the axis - "day"; it was divided into 24 "hours", an hour into 60 "minutes", a minute into 60 "seconds". The name "minute" comes from lat. pars minuta - reduced part (from hours), "second" then from pars minuta secunda - second reduced part.
   Although today these units are defined and metrogically measured much more accurately than by the properties of the Earth, for historical reasons they have basically been preserved and their factual random selection overshadows some fundamental relationships in natural laws, where there are complex constants whose numerical size is given by the choice of units.

Astronomical observations. Astrology, alchemy; quackery
Already in prehistoric times, people have observed that not only day and night but also seasons recur periodically, and there is a close connection between these daily and annual seasons and the movement of the Sun, Moon and planets across the sky. The need to determine and predict the time of day and season, ie natural conditions for agricultural and other work, therefore naturally led to astronomical observations and the introduction of corresponding time units: day, month, year (measuring time both short-term - hours and long-term - calendar) . The Babylonians and Egyptians excelled in observing the sky in ancient times.
   The study of objects outside the Earth - in space - is dealt with by astronomy
(Greek astron = star,nomos = rule, order, law; that is, the " laws of the stars ") *), popularly stargazing. Although current astronomy includes the study of planetary systems, interstellar matter, and the overall evolution of the universe (cosmology), stars and their systems (galaxies) are still the main objects for astronomers.
*) Originally, it was a "regularity of star motion", because the apparent shifts of stars and constellations in the sky were considered star motion. Nothing was known about the very nature of the stars in antiquity and the Middle Ages; they were considered a sort of unchanging distant points of light - perennials, against the background of which the moon, planets, or the Sun, moving. The true nature of the stars was gradually revealed only during the 19th and 20th centuries - that they are very distant huge hot gas spheres, in the interior of which thermonuclear reactions take place (for most of their evolution) ; The sun is also a star, they are a kind of "distant sun". They are not immutable and eternal, but during their evolution they shrink and expand, they can even explode violently, "live" for a finite time - details are in §4.1 "Gravity and evolution of stars".
   Astronomy is sometimes considered the "queen of science", from two points of view :
a) It is the oldest natural Science. Astronomical observations, records and calculations of the positions of celestial bodies in the sky have been made since ancient times. At that time, however, it was not considered a natural science, because it was not known that earthly and "heavenly" nature are one and the same.
b) Everything is part of the universe, and our Earth is a cosmic body; everything around us was formed by astrophysical processes in the universe (§4.1 "The role of gravity in the formation and evolution of stars" and §5.4 "Standard cosmological model. The Big Bang. Shaping the structure of the universe."). In the concepts of unitary field theory, contemporary fundamental physics tries to solve the problems of the microworld of elementary particles and the megasworld, the origin and evolution of the universe, on the same basis (Chapter B "Unitary field theory and quantum gravity", §B.6 "Unification of fundamental interactions. Supergravity. Superstrings.") .
   Ancient civilizations generally had very poor and completely distorted knowledge of the universe. No wonder - after all, we wouldn't be better off if we just looked around the sky without optical instruments and without prior knowledge. There are fundamental obstacles in the immediate visual cognition of the universe :
1. Cosmic objects are very distant. The optical system of our eye creates such small images of these objects on the retina that it is impossible to distinguish the details of their structure. Very little light comes from distant objects in space , which due to the small diameter of the eye lens (approx. 5 mm) is usually not enough to see them even with a relatively high sensitivity of the retina.
2. We live on the planet Earth, which rotates ("cosmic carousel" ) and orbits the Sun. These movements of us observers (which we are not aware of) give the false impression of the movements of observed objects in space.
3. The Earth's atmosphere absorbs a lot of light and our eye is sensitive only to a narrow region of the electromagnetic radiation spectrum. Other important "windows" into space are hidden from our sight.
4.Earth's gravity binds us tightly to the earth and prevents me from "going to take a closer look" at cosmic objects. After all, our close connection with terrestrial living conditions and our "snail" slowness in overcoming vast distances in space also prevent us from doing so.

The observed connections between periodic natural events and the movement of celestial bodies, the causes of which ancient observers did not know, led to the idea that the positions and movements of celestial bodies are related even to other phenomena on Earth *) - various catastrophes, wars and even human destinies. From this false notion developed astrology, which until the end of the Middle Ages was the main motive for astronomical observations. However, already Copernicus' and Kepler's findings about planetary movements made astrological claims highly unlikely: individual planets, when viewed from another planet - Earth - are randomly projected into different constellations as they move.
(also randomly projected) in the starry sky; there is no reason to attribute to these random projections any real influence on the course of events. Indeed, since then, educated people have mostly not believed in astrology; let us remember only the words of J.A.Komenský: "Astrologers - they are not astronomers, but liars from the stars!". Other scientific findings have confirmed this view even more certainly. It is hard to imagine that the apparent projections of sunlit planets on random constellation patterns in the sky could somehow affect the complex structure of DNA macromolecules inside homo sapiens germ cells on one of the other planets orbiting the Sun! No distant cosmic bodies can affect on the personality characteristics of people or on their fate. So astrology is no longer a science, but it can be a nice game using astronomical props ...
) It is interesting that the ideas of cosmic action are again encountered in modern physics in connection with some interpretations of Mach's principle, according to which local physical laws are determined by the distribution and motion of all matter in the universe - see "Appendix A". Of course, this has nothing to do with astrological ideas!
   It is similar with numerology. Numbers fascinate many people, and some attribute magical powers to them - that a particular number affects a certain characteristic of a person. Astrology, numerology, chiromancy, irodology, magic balls, card unloading ... etc., belong to the same category of superstitious "oracle" or "prophetic" techniques that do not work, but many people believe them ...

Closely related to astrology was another false way *) to explore nature - alchemy, which, based on some metaphysical principles and philosophical ideas, sought to achieve the transmutation of the elements **) - especially to make gold - and to find a universal "sage stone". But alchemists in their attempts
(in terms of the then required goals inevitably unsuccessful!) gathered a large amount of empirical knowledge, which later, after abandonding alchemical misconceptions, have become an important basis for understanding the true nature of the chemical merging of substances - the basis for building of chemistry.
*) This critical evaluation applies only to science page of alchemy and astrology! Some spiritual and philosophical aspects, especially the effort for a unified conception of being, for spiritual improvement, the transformation for improvement, the unification of art and science, were at a high level for their time and can still appeal to us today. However, among current proponents of alchemy and astrology, we often encounter misunderstandings related to confusing and merging the misguided scientific ideas of the past with valuable spiritual and philosophical ideas of enduring validity.
**) However, alchemists had no idea not only about atoms and their nuclei, but also did not recognize elements and compounds. They judged the substances according to their external manifestations and a few simple chemical reactions that they were able to carry out. Today, with the methods of nuclear physics, we can perform transmutations of elements - this is "
Nuclear Alchemy".
  From the turn of the 17th and 18th centuries, when feudalism and the church lost their absolute power, the former alchemists - charlatan and often fraudulent "gold-producers" in the service of the rich and powerful - were gradually replaced by serious naturalists, who no longer wanted a recipe for making gold or different elixirs, but they tried to penetrate the essence of the construction of matter. This eventually succeeded in atomic and nuclear physics and chemistry (see Construction of atoms).

Quackery versus science
For erroneous, untrustworthy or fraudulent schools of thought and behavior of people is being called quackery or charlatanism. According to literary sources, the name "charlatan" comes from the Italian word "ciarlatano", meaning the inhabitants of the town of Cerreto, from where in the 15th century several prominent magicians and healers came. In French, the word "charlatan" began to be used for an untrustworthy healer and impostor using unproven and medically unrecognized practices. More generally for other talkers, shaders, swindlers...
   From the mental roots of astrology, alchemy, and religion, are growing up some newer quacks directions - a parapsychology denoting itself by misleading name psychotronics, various ideas about the aura, cosmic energy, quantum consciousness, geopathogenic zones, megalytic legends, homeopathy, alternative medicine. They are a frequent part of the line of thought called the esotericist
(cf. the passage "The Meaning of Phenomena and Events" in the essay "Science and Faith") . On this occasion, we will make a brief mention of these worrying mistakes, and even frauds , which, paradoxically, have become more and more common in people's consciousness in recent decades.
   Proponents of these ideas often claim that our ancestors already knew in ancient times
("megalytic culture") and used the mysterious "cosmic energy" and possessed miraculous abilities. Today's science is accused of ignorance for not acknowledging it... Modern charlatans equip old superstitious ideas with new "scientific" props, they boast the latest quantum, holographic, relativistic physical theories - so that the lay public can trust them. They talk without knowing what it is, on quanta of energy, gravitons, unified interactions, information fields, relativistic effects, tachyons and superluminal velocities, and a number of other concepts that, without further study and understanding, they borrowed from the arsenal of valid theories of contemporary science. They use computers and spread their phantasmagories with the help of information technology ...
   Most of the charlatan schools are characterized by a misunderstanding of the way they treat the concept of energy. There is talk of mental, life, psychic, magnetic, cosmic, divine, etheric, negative or positive energy, of energy zones. The physical meaning of energy is erroneously confused with the common folk notion of biological and mental "energy", which is, in fact, a combinatorial and biochemical property of the arrangement of complex molecules and their systems in an organism; it has nothing to do with the physical concept of energy. Not surprisingly, such confusion often creates very bizarre nonsense . They talk about different energy transmitters, receivers and interference suppressors, zones, auras, "energetically active" water and other substances, energy miraculous properties of pyramids and other structures ...
   No disguise into a modern, seemingly scientific garb, can not change anything to the fact that all these claims are only completely unsubstantiated assumptions, dating from the pre-scientific period and from misconceptions from the past. Now these starting points have long been refuted. Nevertheless, many trusting people, with underdeveloped critical thinking, continue to believe in the wrong conclusions from them. They fail to recognize that the apparent successes of alternative medicine are due to the placebo effect, and the proclaimed success of psychotronics is in fact only a selection effect - from a probability of 50% to 50%, failed cases are excluded and forgotten, while (randomly) successful cases are glorified and widely published. Objective comparison and confrontation (independent "blind" experiments with subsequent statistical evaluation) is strongly opposed by charlatans; they argue that "scientific supervision" leads to the inhibition of their mental faculties, or disrupts their association with the transcendent, cosmic energy, gods or demons, and so on. If some such comparisons were still made
(eg wells for finding water with dividing rot), the statistical significance of paranormal phenomena was not proven.
   Quackery and fraud are most common in the field of health and disease, in medicine -miraculous means and methods that can cure all diseases, from cancer to AIDS. Whether they are pharmacological means (universal "miracle drugs") or physical means - various biolamps, magnets, generators of electromagnetic frequencies or some mysterious fields, etc. The use of these methods, as well as psychotronics and homeopathy in medicine, can sometimes subjectively improve the health by the placebo effect; or it may be a mere coincidence
(even without treatment, the body would "help itself")... However, in serious diseases, their use carries the risk that there will be a delay in waiting for the alleged effects effective treatment, which may cause irreversible damage to the patient's health.
Quantum consciousness, quantum medicine 
Concepts and knowledge of quantum physics are a popular tool of fictional directions. This field of physics is sufficiently mysterious (moreover, in connection with the theory of relativity) and human consciousness is also shrouded in mystery, so that in the opinion of some people they should have something in common..?.. The common name "quantum consciousness" or "quantum mind" means a group of hypotheses proposing putative quantum-mechanical phenomena in the function of the brain to explain thinking and consciousness. Immediately after the creation of quantum mechanics in the works of Planck, Bohr, Schrödinger, Heisenberg, Pauli and others (see eg "Quantum Physics"), quantum theory became an attractive option for solving the philosophical-epistemic conflict between the strict determinism of classical physics and our conscious free will : quantum randomness can open up new possibilities for free will. In most discussions of the relationship between quantum physics and consciousness, the basic ideas of quantum theory are accepted in a purely metaphorical way, only as analogies, without a relevant reference to their physical meaning. This can lead to interesting sci-fi ideas, which are, however, unconvincing and scientifically erroneous. Quantum-mechanical terms are often used to seek greater persuasiveness - to add "scientific weight" to subjective hypotheses.
   The latest hypothesis of this kind was created by R.Penrose *) and S.Hameroff, who assumed cells or structures with "quantum sensitivity" in the brain, through which quantum mechanics is involved in brain activity. Hameroff hypothesized that suitable structures for quantum behavior are microtubules (for microtubules see eg "Cells - basic units of living organisms", passage "Cytoskeleton - skeleton and carrier of cell functions"), which form the cytoskeleton of cells, including neurons, where they could mediate synapses. Quantum vibrations could occur in their regular lattices of tubulin protein, and coherent superpositions of tubulin states could interfere with many neurons. However, those skilled in the art of biochemistry, molecular cell biology, and neurobiology are skeptical of this microtubule function...
   Simultaneous quantum collapse of these microtubulin states is interpreted as an "individual elementary act of consciousness". Penrose attributes this act to "quantum gravity", which has not yet been created. From the point of view of unitary field theories, however, quantum gravity acts on much smaller spatial scales than the dimensions of microtubule molecules, so that it cannot affect any of their quantum states at all.
More similar irrelevant constructions (such as twistors), are being introduced here, probably to give more "scientific weight" to these speculations...
   The idea arose that quantum information from microtubules is not lost after the physical death of man, but they remain embedded in the "information" structure of the universe as a kind of "counter of consciousness"..?.. This is already completely in the field of sci-fi..!.. The quantum consciousness hypothesis is often (but unjustifiably) linked to the hallucinatory experiences of people who have undergone clinical death but have returned to life. In fact, these frequently described typical experiences are probably due to altered brain activity in a state of hypoxia during circulatory arrest. The relationships between matter and consciousness are briefly discussed in the passage "Soul and Matter" of the work "The Anthropic Principle or Cosmic God".
   Despite extensive treatises and great efforts by Penrose *) and Hameroff, their hypothesis is not a convincing model of brain function or explanation of the essence of consciousness and mind. It is based on unverified and debatable assumptions physical and neurobiological. Microtubules in cells probably do not play a significant role in the function of neural networks in the brain. It is apparently not necessary to use the props of quantum mechanics to truly explain the human mind and consciousness. Quantum physics works at the molecular and atomic levels everywhere in nature, in our everyday life, but in the behavior of macroscopic objects - which are also brain neurons and their networks - quantum effects do not manifest any way. Consciousness probably arises algorithmically - combinatorially - from the enormous complexity of signal connections between billions of neurons in the brain network...
*) Note: It is difficult to understand, that this "flight-mistake" happened to the excellent physicist Roger Penrose, who had previously been very successful in research in the field of black holes and cosmology. Together with S. Hawking, they developed important theorems about spacetime horizons and singularities (§3.8 "Hawking's and Penrose's theorems about singularities"). After all, this collaborator and his friend Stephen Hawking flatly rejected the hypothesis of "quantum consciousness"! The ways of the human mind are unpredictable...
   Other fictitious claims of "quantum medicine" are based on hypotheses about quantum behavior, according to which the body is quantum composed of energy and information influenced by quantum consciousness. Quantum healing can therefore cure any disease based on a person's state of mind..?.. Some charlatans even claim that they can heal in this way even from a distance..!..
   Rational approach to reports of miracles and supernatural phenomena have already expressed the Scottish philosopher David Hume (1711-1776) words: "No testimony is not able to prove a miracle - it would have to be a witness of this kind that the error of his would be more miraculous than the event. which he is trying to witness". In other words, a mistake or a lie is more likely than a miracle. This attitude is sometimes referred to as "Hume's Razor", cutting off credible information from improbable and perhaps erroneous information. Within science itself, criteria called "Occam's razor" and "Popper's razor" are used - see " "New" and "Old" physics"and "Knowledge: experience + science. Awareness - education - wisdom." in the monograph "Nuclear Physics and the Physics of Ionizing Radiation".
   All this would not be worth mentioning in a physically oriented treatise, if these irrational ideas did not spread more and more in recent years (and a lot of people believe them)..!.. For a comparison, see also the essay "Science and Religion".

Space, time, matter, universe
In the empirical observation of nature and finding their laws, important abstract concepts such as space, time, motion, matter were formed , which are certain images of the general (universal) properties of being. Efforts were made to put individual isolated knowledge into context, to generalize it and to extrapolate it. Questions like "When and how did the world (universe) come into being?", "How big is the universe?", "What is matter composed of?" etc., they are probably as old as people's conscious thinking about nature.
   One of the basic questions that philosophy asks is where to find the essence of all being - the basic "primordial matter" (the simplest and indecomposable primordial substance) from which all things, plants, animals and people are formed and composed. Ancient philosophy, which could not penetrate into the true essence of things and phenomena, naively considered to be the essence of some superficial and purely phenomenal aspects of reality, based on immediate sensory sensations. Thus, water (Thales *) , air (Anaximenes) or fire (Herakleitos) were considered the basic precursors. Later, four basic (independent and non-intertwining) primates were postulated, or elements (natural elements) from which the whole world is built: water (as the essence of liquids), air (as the representative of the gaseous state), the earth (as the carrier of solid properties ) and fire (as a cause of mobility and variability).
*) However, in the light of today's knowledge, it can be said that Thales was not so far from the truth: according to current astrophysics, all elements were actually formed in stars by nuclear reactions from hydrogen formed from elementary particles generated during the Big Bang (see §5.4 " Standard cosmological model. The Big Bang. Shaping the structure of the universe. ") .
   This teaching on the four basic elements of the world was developed especially by Aristotles (384-322 BC), who supplemented it with the concept of four basic properties (essences), which are heat, cold, humidity and dryness. The interconnection of such "essences" was to create individual basic elements of the world (eg water from moisture and cold, fire from heat and drought). Similar ideas persisted until the Middle Ages, where they were the basis of alchemy.
   Philosophers have also addressed the question of how matter is composed of the basic elements. As for the structure of matter, there are basically two possibilities: either matter is continuous (infinitely divisible), or it consists of certain small "grains" (atoms) that are no longer divisible. In ancient times, it was not possible to make a reliable decision between these two possibilities, so both concepts were coined in different philosophical schools. The science of atoms was developed mainly by Democritos (ca. 460-370 BC), who justified the necessity of the existence of indivisible atoms by the fact that, with unlimited divisibility, there would be nothing left that could carry the properties of the substance. This speculative reasoning is based on the assumption that the properties of substances never change by division and that the substance itself is the bearer of all its properties
(current atomic and nuclear physics already looks at it differently...) . The philosophical thesis of the structure of matter is almost universally accepted in the methodology of the natural sciences.

Gravity and the Universe in Antiquity
In the earliest times, before conscious exploration of nature began, humans did not think about gravity at all ; it was so common and mundane that people got used to it and ignored it. They took the gravity of the earth as a matter of course and a natural effort of objects to fall to the ground. Cognition of what we call gravity today was previously associated with astronomy. Astronomy and philosophy in general - all teaching about nature was part of philosophy at the time - reached a particularly high level in the period of ancient Greek culture. Some ancient philosophical schools
(represented by Thales Miletsky, Pythagoras, Aristarchus, in India by Arjabhata) at the time had a surprisingly realistic picture of the shape, location and motion of nearby planets (including the Earth) around the Sun *).
*) Opinions on the true level of ancient science sometimes differ. There are sensational claims about the use of electricity and atomic energy and about knowledge of outer space in ancient civilizations. However, these claims are completely unfounded. The legacy of ancient thinkers is so rich and extensive that among hundreds and thousands of ideas and statements (often contradictory) one can find those that, more or less by accident , coincide with the conclusions of modern science. In these statements, we sometimes insert a sense that their authors might be very surprised by ...
   The development of natural sciences, especially astronomy and the overall worldview, for a long time significantly (and unfortunately mostly negatively)influenced the teachings of the most important representative of Greek philosophy - Aristotle. This teaching was the culmination of ancient natural philosophy. Aristotle came from the basic sensory experience of earthly life, that heavy bodies try to fall down to the ground, while "light" objects like smoke or fire rise. Based on this, Aristotle proclaimed the concept of " natural places " and " natural movements": the natural place of heavy substances (soil and water) is" to be below ", the natural place of light substances (fire and air) is" above ". The natural movement of earth and water is to descend, the natural movement of air and fire is to ascend *). All other movements are forced by an external force.
*) The philosophical thesis that "the like goes to the like" was already expressed by Platon, who thus explained the fact that material bodies fall to the ground.
   This concept, together with the assumption that the Universe has only one center of gravity, led Aristotle to the following image of the world: in the center of the universe is a motionless Earth, in which the heaviest matter of the universe gathered - earth and water; the earth is composed of soil and water located in its natural place, so it is at rest. The universe (ie the Earth and its surroundings) consists of individual spherical layers: earth, water, air, fire. All celestial bodies are composed of the lightest and "most perfect" substance - ether - and perform a "perfect" uniform circular motion over some spheres through which they are carried. Thus, in Aristotle's conception, the universe is composed of two diametrically opposed parts: earthly and celestial.
   As for movementas such, Aristotles taught that bodies move only when they are propelled by some force - "the cart drawn by the donkey stops, when the donkey stops towing". Aristotele did not know inertia because he was not experimenting and could not reduce or relieve friction enough. Regarding the fall of bodies to the ground, Aristotle claimed that the rate of fall of a body is proportional to its weight; this erroneous conclusion again arose from the common experience that light sparse bodies fall much slower than dense heavy bodies.
   Aristotele's geocentric cosmology further elaborated by Ptolemaios (ca. 100-160n.l.), who solved the discrepancies between the assumed perfectly uniform motion and the observed irregularities of the planets' motion together with changes in their brightness (indicating changes in distances between Earth and planets) by hypothesizing that real planetary motions or more uniform circular movements, the so-called deferent, epicycle and equant. Ptolemaios thus reached a relatively good agreement with astronomical observations, but at the cost of considerable complexity and sophistication. Aristotle-Ptolemy's geocentric teaching was then canonized by the Church and maintained as a dogma throughout the Middle Ages
(the era of "intellectual darkness"); the development of astronomy and the natural sciences has thus been hampered for more than a thousand years.

Development of scientific astronomy and physics
Heliocentric system
The first significant breakthrough into the long-prevailing misconception of the structure of the universe was made by M. Copernicus (1473-1543), who noticed that is the Sun, around which the planets and the Earth orbit. He thus assembled a heliocentric system and showed that the Earth is only one of the other orbiting planets; in addition, the Earth revolves around its axis with the diurnal period, giving the impression that all cosmic bodies, stars and planets, orbit it. This laid the foundation for eliminating the senseless contradiction between "terrestrial" and "celestial" and for bringing astronomy closer to other sciences, especially physics. The knowledge that the Earth
(and as it turned out later, neither the solar system nor our Galaxy) has no any privileged place in the universe, it is called the "Copernican principle" and plays an important role in contemporary cosmology (Chapter 5, §5.1 "Basic starting points and principles of cosmology ") .
   Copernicus also realized that it was probably not correct to assume only one center of gravity in the universe, but that each body should have its own weight - gravity. At Copernicus, we thus already encounter a hint of a realistic conception of gravity as the effort of bodies and their parts to join together into the whole, ie with a hint of the concept of general gravity. Copernicus' conception was followed by J.Bruno
(infinity of the world in space and time, the same nature of the perrenials-stars and the Sun) and especially J.Kepler (1571-1630), who, on the basis of astronomical observations, formulated his three important laws of planetary motion around the Sun (§1.2). Kepler sensed that the cause of these planetary motions was a force emanating from the Sun, but in the absence of mechanics he could not come to a correct explanation; it was later filed by Newton.

Experiment - the birth of scientific physics and natural science
Galileo Galilei (1564-1642), who can be considered the founder of physics as a scientific discipline, made a decisive contribution to the development of astronomy and physics. He introduced experiment into physics as a decisive tool of knowledge. On the basis of simple experiments with the motions of bodies, Galileo formulated the law of inertia (which denied Aristotle's teaching on motion), the composition of motions, and also arrived at the principle of relativity of motion (see §1.2 and §1.6). He thus became a pioneer in the mechanics of motion of bodies, especially kinematics. In astronomy, Galileo was a staunch supporter of Copernicus' heliocentric system, which he decisively supported with his discoveries using telescopes.
Galileo was also the first scholar in history to have made a direct and significant contribution to the knowledge of gravitational phenomena. Through his experiments with free-falling bodies (allegedly from the Leaning Tower of Pisa), he arrived at the famous law of free fall, according to which in free fall all bodies fall to the ground with a constant acceleration, which is independent of weight (mass) and composition of the body. Hi thus refuted Aristotle's concept of natural movements upwards or downwards: these are always the movements of bodies under the influence of gravity, but in an environment with greater or lesser density. The law of free fall, generalized to the principle of universality of gravitational action and the principle of equivalence, has become one of the main starting points of modern gravity physics - Einstein's general theory of relativity (see Chapter 2, especially §2.2 "Universality - a basic property and the key to understanding the nature of gravity").
Isaac Newton (1642-1727) was a decisive milestone in the development of physics, astronomy, and the natural sciences in general. Above all, Newton followed up on the Galileo's findings and built mechanics, in which he formulated precisely and mathematically expressed the three basic laws of motion (§1.6, passage " Newton's classical mechanics "). He also discovered the basic laws of hydrodynamics, acoustics and optics. Newton then completed his epoch-making work by combining his and Galileo's mechanics of the motion of terrestrial bodies with Kepler's kinematics of planetary motion, thus arriving at his great law of general gravity and creating the dynamics of the solar system; more on this in §1.2 "Newton's law of gravitation".
   Through the work of I. Newton, the pre-scientific period of erroneous conjectures and dogmas ended in human cognition, and a period of scientific research, precise experiments and logical thinking begins. Newtonian physics also provided a new, more realistic view of the universe.   

Observing the universe with telescopes - telescopic astronomy
We know from everyday life that we do not see what is too far away, we do not recognize the details and we can only guess at the true nature of distant objects. This is even more true of distant objects in
universe. G.Galilei first looked into space with a simple self-constructed telescope in 1610 and was surprised to see craters on the Moon ("moon mountains"), Jupiter's moons, the phases of Venus illumination, Saturn's rings. Larger telescopes *) then revealed many stars invisible to the naked eye, nebulae, new planets (Uranus r.1781, Neptune r.1846), spiral "nebulae" - galaxies during the 19th century. New tools placed in the foci of telescopes - photography and spectroscopy - led to the fascinating discoveries of previously unsuspected structures in space, a huge number of stars and paved the way for astrophysics to study the physical properties of stars and galaxies (see below "Electromagnetic radiation - a fundamental source of information about the universe ").
*) Small binoculars are often lens refractors - the primary lens consists of a larger converging lens with a large focal length, the eyepiece (used as a magnifying glass to observe the image created by the lens) is a smaller converging or diverging lens with a short focal length. The optical disadvantage of the refractor is a color defect (chromatic aberration) - due to the dispersion of light, the focal length is somewhat different for different wavelengths - colors - light. In addition, it is technically difficult to produce quality large diameter lenses. In 1668 I.Newton assembled the first mirror telescope - a reflector, the "lens" of which was a concave mirror creating the image in the focal plane by means of reflection of light. The reflector has no color defect and, in addition, large-diameter spherical or parabolic mirrors can be precisely ground (their deformation is prevented by mechanical reinforcement "from behind"). That's why all large binoculars are mirrored. Large diameter telescope (aperture) collects more light and therefore we can observe fainter and more distant objects. In addition, larger telescopes have better spatial resolution, so we can recognize finer details of the structure of multiple stars, star clusters, spiral arms of galaxies, gas clouds.

Electrodynamics, atomic physics, theory of relativity, quantum physics
In the middle of the 18th century, the development of mechanics was seemingly completed. Fundamental physics focused on the study of other physical phenomena - thermal and especially
electrical and magnetic.
Electricity and magnetism
At this point, it may be useful to briefly recapitulate the development of knowledge about the extremely important natural phenomena
of electric and magnetic. The first observation of electrical (electrostatic) phenomena comes from ancient Greece. For articles made of amber, which is fossilized natural resin from which jewelry and ornaments were made, at friction were observed the attraction of small light bodies - hair, feathers, yarn (Thales Miletus in the 6th century BC described that the amber tool used at spinning flax began to attract various small bodies, while the flax fibers began to repel each other). Amber is called the electron (electron) in Greek, which later gave a collective name to all these phenomena (the name elektricitas derived from amber was used by W. Gilbert in the study of static electricity, although he observed attractive forces at friction even in some other materials, especially glass). For many centuries, these phenomena served only as an curiosity interest and for juggling demonstrations, nothing was known about their cause and nature.
   Only the existence of two kinds of electric charges (called conventionally positive "+" and negative "-") has been discovered, with charges of the same kind repelling each other and of the opposite kind attracting each other. Later, the law of conservation of electric charge was discovered (B.Franklin). In 1784, Ch.A.Coulomb, with the help of sensitive torsional weights of his own construction, measured the interaction of electric charges (Priestley and Robinson dealt with these experiments independently) and discovered the basic law of electrostatics - Coulomb's law (1.20b), similar to Newton's law of gravitation (the comparison of the laws of electrostatics and gravity is discussed in detail in §1.4 "Analogy between gravity and electrostatics").
   In 1789, in his well-known experiments with frog legs, Galvani observed muscle contraction when touching an iron railing - indirectly observing the biological effects of discharging electric charges, ie electric current (at that time there was still a distinction between "galvanic" and frictional electricity). In 1799 A. Volta first designed a source of "galvanic current" - an electrochemical Volt cell; this current has been shown to be of the same nature as the "discharge current" generated for a short time by the conductive connection of electrostatically oppositely charged bodies. Electrochemical sources - Volt cells assembled into batteries - made it possible to study the continuous passage of an electric current through a conductors, to assemble the first electrical circuits.
   Completely separately and independently of electrical phenomena, other phenomena of "mysterious" force action were observed - magnetic phenomena. It has been observed already in ancient times, that some minerals attract or repel each other and that they attract iron objects. Iron ore mined near the city of Magnesia in Asia Minor was most well-known in this respect; this ore (it is iron oxide Fe3O4) was called magnetite, which gave the collective name to magnetic phenomena. When placed on a cork float on water, or when hung on a thread, this magnetic ore was always rotated in the same direction - one end to the north and the other to the south. Thus, two magnetic poles were marked out - north and south; magnetic "arrows" found important use in compasses (the Chinese used such a magnet 4,000 years ago to determine the correct geographical direction when traveling). In Europe, experiments with magnets were studied in detail around 1600 by the English physician W.Gilbert. As with electrical phenomena, no one had any idea about the nature of magnetic phenomena until the end of the 18th century (the fluid idea vaguely spoke of northern and southern "magnetic amounts", which, however, unlike electric charges, cannot be separated from each other).
   The first important breakthrough into the essence of magnetic phenomena and their connection with electrical phenomena began with the accidental discovery of H.Ch.Oersted in 1820, who noticed during experiments with electrical circuits, that the magnetic needle deflects near the conductor through which the current passes - that is, that the electric current causes a magnetic field in the same way as if a permanent magnet had been applied instead of the conductor with the electric current. It turned out gradually that the mysterious magnetic action, which until then was the domain of only natural substances, permanent magnets, probably has an electrical origin - it is created by the movement of electric charges. And the magnetic field, in turn, exerts a force on the moving charges, on the electric currents.
   This was soon shown even more certainly by the experiments of A.Ampere (1775-1889), who discovered the law of mutual force (magnetic) action of electric currents. In 1820, Biot and Savart measured the intensity of the magnetic field around the conductor flowing through the el. current, these results were further generalized by Laplace - the Biot-Savart-Laplace law (1.33a) was created, indicating the dependence of the intensity of the magnetic field excited by the current in the conductor element on the magnitude of the current and on the distance. These laws led to the construction of "artificial magnets" powered by electric current - electromagnets. The magnetism of permanent magnets was later explained by atomistics.
   Another key finding was the law of electromagnetic induction discovered in 1831 by M.Faraday, according to which the time change of the magnetic field evokes (induces) an electric field, whereas the induced voltage is proportional to the rate of time change of the magnetic flux through the surface of the considered conductor loop - relation (1.37a). These findings became the basis not only of electrodynamics (the merger of the science of electricity and magnetism), but also the practical application of electromagnetic phenomena - electrical engineering was created.
   Faraday further expressed the idea, based on his experiments, that the electric and magnetic forces do not take place directly from one charge to another, but spread through the environment lying between them. He thus laid the foundations of the teaching about the electromagnetic field, which he further elaborated, generalized and mathematically formulated by J.C.Maxwell (1831-1879) in the 1860s. The theory of the electromagnetic field led Maxwell to the knowledge of the finite speed of propagation of electromagnetic action equal to the speed of light *), to the prediction of electromagnetic waves and to the hypothesis of the electromagnetic nature of light. The experiments of H.Hertz and his followers, which proved the existence of electromagnetic waves and found out some of their properties, fully confirmed the correctness of Maxwell's theory. From the physical-mathematical point of view, the theory of the electromagnetic field is discussed in §1.5 "Electromagnetic field. Maxwell's equations".
The speed of light  
- or the speed of propagation of electromagnetic waves, or the speed of photons - is extraordinarily high compared to all other terrestrial speeds (a million times the speed of sound in air), so in earlier times it was not easy to measure it more accurately (it was often considered infinite). The first approximate determination was made astronomically in 1675 while observing the eclipse of Jupiter's moons, but the real measurement of the speed of light using terrestrial sources and optical-mechanical means was not made by Fizeau until 1849. In this classic experiment, a beam of light was reflected back and forth through the teeth of a rotating gear when reflected from mirrors. As the speed of the gear increased, it was observed that at a certain speed the reflected beam no longer passed through the gear - the beam that passes through the gap between the teeth of the gear returns to the gear space after overcoming the mirror distance, reflection and overcoming the distance back, when the wheel rotates by such an angle, that instead of a gap there is already a tooth in the path of the beam. If there is a distance between the gear and the reflecting mirror d and a wheel rotating at a frequency f has N teeth around the circumference, between the speed of light c and the first frequency f , when the reflected beam stops passing, a simple relation c = 4.d.f.N (coefficient 4 arises from the fact that the distance d is overcome twice and the rotation time of the wheel from the gap to the tooth is
1/2.fN ).
   In further experiments, the measurement of the speed of light was gradually refined, the current value being c = 299,792.458 km/second for vacuum. In material optical environments, the speed of light is slightly lower
(it is analyzed in §1.1, part "Electromagnetic and optical properties of substances" monograph "Nuclear physics and ionizing radiation physics") and depends somewhat on the wavelength of light (so-called dispersion ). Eg in water the speed of light for red light is (rounded) 226 000 km/s, for violet 223 000 km/s; it is even slower in crystals and glass. Of all natural materials, the highest refractive index is diamond (n = 2.42), in which the speed of light is only 123,881 km/s - this leads to significant optical effects of refraction and reflection of light in crystals of diamond, which results in its aesthetic popularity as a piece of jewelry.
   The  speed of light in a vacuum does not depend on the speed of movement of the source. Measurements of Michelson and Morley in 1881 to 1904 (measuring the speed of light in the direction and against the direction of the Earth's motion) even showed that the speed of light in a vacuum does not depend on the state of motion of the source or observer - it is the same in all inertial systems, no matter how fast they move . This fact, expressed in the principle of constant speed of light, became the basis of the special theory of relativity (§1.6 "Four-dimensional spacetime and special theory of relativity") and thus of the whole relativistic physics.
Note: The specific numerical value of the speed of light c is by no means exceptional, it is basically given by units selected for length [m] and time [s]. In §1.6 and many other places in our explanation of the theory of relativity and gravity, we will often use a system of units in which the speed of light is c = 1.
   The theoretical result of electrodynamics and the special theory of relativity, that the velocity of photons in a vacuum is exactly equal to c , holds for a plane unlimited wavefront of electromagnetic radiation. However, if the radiation beam is narrowed in the transverse direction (the wavefront is spatially limited), quantum uncertainty relations begin to apply, leading, among other things, to fluctuations in the velocity of photons in vacuum
(these subtle differences are difficult to measure) .
How fast is gravity ? 
- or how long does it take for a change in the gravity of one body to take effect on another (distant) body? We can put it simply by the example of our Earth, which is bound in its orbit by the gravitational force of the Sun. If, hypothetically, the Sun suddenly ceased to act its attraction ("the devil would steal it at once"), the motion of the Earth would change from an ellipse to a straight line heading into distant space. But how long after the removal of the Sun would this happen? Immediately? - as assumed by the classical Newtonian idea of instantaneous action at a distance, according to which gravity propagates infinitely fast. Or in about 8 minutes, which is about the time it takes for the sun's rays to fly to us? Or for some other end time? 
From the point of view of the speculative example outlined, this is certainly an insignificant question. However, the value of the speed of gravity has a fundamental influence on phenomena in outer space - in astrophysics and cosmology *). If the gravitational force acted at any distance immediately, the evolution of the universe would proceed completely differently than if gravity affected all material bodies with a delay given by its finite velocity. We will outline this question first at the end of §1.2, part "
Modification of Newton's law of gravitation", but mainly in §2.5 "Einstein's equations of the gravitational field" and in §2.7 "Gravitational waves", where in the passage "How fast is gravity?" will be briefly discussed as well as general questions propagation velocity changes in the gravitational field and the possibility of its experimental determination. We will see that the "speed of gravity" is the same as the "rate of electromagnetism", ie changes in the gravitational field spreads speed of light, like a stir in the electromagnetic field .
*) In addition, perhaps in the multidimensional "gateway" theory of superstrings (§B.6 "
Unification of fundamental interactions. Supergravity. Superstrings.", passage "Another dimension, M-theory, 11-dimensional theory of superstrings"), where the assumed gravitational action between the gates cannot be instant ..?..

Classical Faraday, Ampere and Maxwell electrodynamics is a macroscopic and phenomenological theory - it perfectly describes the properties of electric and magnetic fields in vacuum and in material environments, their temporal changes and mutual transformations. However, it does not take into account the details of the structure of matter, the nature of its own and basic "carriers" of electric and magnetic forces. The first "microscopic" theory of electromagnetism was developed in 1895 by H.A.Lorentz, but a full understanding of the relationship between electromagnetism and the structure of matter was made possible by the development of atomic and nuclear physics - see below.
   Great stimulus for the development of physics during the 19th century was the technical problems arising during the Industrial Revolution. This created fundamental discoveries that gave physics the character of a comprehensive science . Some methodological issues of the construction of physics and its incorporation into other natural sciences, as well as in the context of scientific knowledge in general, are discussed in §1.0 "Physics - fundamental natural science" monograph "Nuclear physics and physics of ionizing radiation".

Microstructure of matter - atomic and nuclear physics
At the turn of the 19th and 20th centuries, research into electrical phenomena opened the door to an understanding of one of the most basic unresolved issues - the structure and composition of matter. And in turn, the discovery of the basic building blocks of matter has made it possible to better understand the nature and origin of electric forces.
   When chemists (especially J.Dalton) revived the idea of atoms at the end of the 19th century, practically nothing was known about the nature and structure of the atoms themselves. Faraday's experiments with electrolysis in 1836 suggested that chemical fusion had much in common with electrical phenomena. In 1895, J.J.Thomson, during experiments with gas discharges, discovered an elementary particle carrying a negative charge - an electron and proposed the first idea of the atom ("pudding model"). E.Rutheford along with Geiger and Marsden in 1911 made an important experiment with scattering particles alpha which led to the discovery of the atomic nucleus and created the planetary model of the atom. In 1913, N.Bohr supplemented the planetary model with three quantum postulates; the resulting Bohr model of the atom is still used with certain modifications.
   Atomic and nuclear physics has shown that the origin of electric and magnetic forces lies in the basic elementary particles that make up matter - electrons and protons, which are carriers of negative and positive electric charges. It also explains all electrical and magnetic properties of substances, including the cause of the magnetic properties of permanent magnets. Atomic physics further explains the mechanical and optical properties of substances and especially chemical fusion - the essence of chemical fusion is the electrical attractive forces between atoms, which, when sufficiently close to each other, share part of the envelope electrons in the valence shell.
The structure of atoms and atomic nuclei is discussed in more detail in §1.1 "Atoms and atomic nuclei" of the monograph "Nuclear physics and physics of ionizing radiation". Research in the field of radioactivity has played a crucial role in elucidating the properties of atoms and atomic nuclei (discovered in 1896 by H.Becguerel) and ionizing radiation - see §1.2 "Radioactivity" and §1.6 "Ionizing radiation" in the same treatise.
   By applying laboratory knowledge of atomic and nuclear physics to phenomena occurring in space, nuclear astrophysics was created, which clarifies the origin of radiation in space, evolution of stars, release of energy by thermonuclear reactions inside stars, formation of elements by cosmological and stellar nucleosynthesis
(§4.1 "The role of gravity in origin and evolution stars"), dramatic events of supernova explosions (§4.2 "Final stages of stellar evolution. Gravitational collapse").

Living Nature - Biology
In parallel with physics, astronomy, chemistry and other sciences of inanimate nature, it has been occurring since the 18th century to significant discoveries in biology as well - the science of living organisms. Earlier descriptive examination of external manifestations and often random similarities was replaced by a systematic examination of the structure, development, metabolism, species classification and interrelationships of living organisms. The basis of biology has become the science of the structure and activity of the cell as the basic building block of organisms (in more detail "
Cells - the basic units of living organisms").
   An important circumstance for a correct understanding of life was the abandonment of so-called vitalism - the assumption that complex "organic" substances are created by the action of some specific "vital forces" which are inherent only in living organisms (they are different from the forces controlling inanimate nature). Careful physicochemical research has shown with absolute certainty that not a single atom in any living organism is any different from that in an inanimate "inorganic" nature. Also all complex "organic" molecules in organisms can be (at least in principle) prepared by synthesis of inorganic atoms hydrogen, carbon, oxygen, nitrogen, sulfur, phosphorus and possibly next. What makes an organism alive is not some mysterious "life-vital force", but an amazing combination and interplay of the myriad chemical and biophysical processes that take place in a living organism. This is mainly due to the ability of carbon to form an extraordinary variety of compounds. Carbon atoms can combine with each other and with other atoms not only into simple molecules (linear or cyclic), but also into chains that can have hundreds, thousands to millions of links - to form macromolecules. During the cessation of vital functions - the death of an organism - none of its atoms are "lost", only the coordination of the mentioned complex processes is lost; and many complex "organic" molecules eventually decompose into simpler ones.
   Biological processes in cells and in the whole organism are therefore based on chemical reactions, especially of complex organic compounds of carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus and other elements. The structure of the cell nucleus and the role of deoxiribonucleic acid (DNA) as a carrier of information in cells were recognized, and the basic features of evolutionary theory are developed. The processes in living organisms are so complex that much remains unknown. Nevertheless, the application of physical and chemical knowledge at the molecular and atomic levels makes it possible to gradually understand increasingly complex details and contexts in biology and to integrate this science fully into the context of other natural sciences.

Theory of relativity, quantum physics
The physics of the 17th and 18th centuries tried to explain all phenomena using mechanical models - movements of either atoms and molecules (kinetic theory of heat, hydrodynamics, thermodynamics) or elastic "ether" *) - the carrier of electromagnetic phenomena. At the end of the 19th century, it seemed that almost everything in physics was basically solved; all that remained was the problem of the ether, the ambiguity surrounding the radiation spectrum of the "absolutely black" heated body (leading to the so-called "ultraviolet paradox") and some properties of the photoelectric effect. Efforts to solve the problem of radiation of an absolutely black body, together with research in atomistics, led to the creation of quantum physics - see, for example, the passage "Corpuscular - wave dualism" in §1.1"Atoms and nuclei" monograph "Nuclear Physics, ionizing radiation". Problems ether became a springboard to create the theory of relativity.
*) Problems ether is briefly discussed at the end of §1.5 "Electromagnetic field. Maxwell's equations. ".
   As tmore detailed experimental finding accumulated, mechanistic models and concepts of classical physics in general, encountered increasing difficulties. It is manifested particularly when examiningthe microworld  which began in the late 19th century. When observing the motion of fast electrons in electric and magnetic fields (which Lorentz dealt with in particular), it was found that classical Newtonian mechanics is no longer consistent with the experiment. Various partial hypotheses were stated, until finally A.Einstein built a new and more general mechanics, well describing even very fast particle motions , within his special theory of relativity. The special theory of relativity (§1.6 "Four-Dimensional Spacetime and Special Theory of Relativity ") is, together with quantum mechanics, the most important and most versatile interdisciplinary theory of today's physics; it is also the "springboard" of the general theory of relativity as the physics of gravity and spacetime.
Note: Discrepancies in experiments and progress in scientific knowledge
Experiments that deny the current well-established knowledge are almost always erroneous - they are often the result of measuring errors or hidden disturbing and distorting influences. From time to time, however, they prove to be right - and then they open the way to significant progress in our understanding of nature! To generalize and clarify previous laws, sometimes to set new horizons and concepts.
A new concept of force in physics
The central concept in classical physics is force, which appears in the laws of motion as the cause of dynamic changes; allows you to predict the movements of bodies. Force works well as a quantity in the mathematical description of natural processes, but the "physical" definition of the nature of force is not easy (in classical physics it is tautological - so we didn't really know what force is ...). Modern physics has brought two views on the nature of force :
1. The general theory of relativity explains the gravitational force as a geometric manifestation of curved spacetime: Each body curves spacetime around itself, and in this "deformed" spacetime, bodies move in geodetic paths. This gives the impression of force. E.g. the planets orbit the Sun not because it is acted upon by a "physical" gravitational force, but because the Sun has curved spacetime in its surroundings, and the orbit (circular or elliptical) is the "straightest" orbit in this deformed spacetime.
2. Quantum field theory explains the interaction of particles by the fact that these particles are constantly exchanging virtual intermediate "field" particles. In the electromagnetic interaction they are photons, for a strong interaction the field particles are gluons,
for weak interactions they are intermediate bosons W+,W-, Z0.
  It is hoped that this somewhat "schizophrenic" view of the nature of force will unite in the future in the unitary theory of all 4 interactions - "theory of everything".
  The study of the laws of the microworld has greatly enriched and deepened knowledge of the structure of matter. However, the physics of the microworld has yielded very little to elucidate the properties of space and time, as well as to understand the nature of gravity. However, it is hoped that this will change in the future. Quantum theory of gravitation and especially unitary field theory, after their successful completion, will probably combine quantum physics, electrodynamics, nuclear and particle physics with gravity, with the theory of space and time (Chapter B "Unitary field theory and quantum gravity").

A wide range of sizes of objects in our world, studied by various fields of physics and natural science using various tools
(it is discussed in more detail in §1.0 "Physics - fundamental natural science", passage "Methods and tools of nature research", monograph "Nuclear physics and ionizing radiation physics")

Electromagnetic radiation - the basic source of information about the universe
Practically all information about bodies and processes in the universe is still obtained through
electromagnetic radiation. Originally it was visible light, now electromagnetic waves of other lengths also approach it - radio waves, infrared, ultraviolet, X and gamma radiation *).
*) It is good to realize, that the light emitted from stars and other objects in the universe is several times transformed by radiation originating originally from nuclear and subnuclear processes with much higher energies, corresponding primarily to radiation g. Recently, the "observation" of the universe using other types of radiation - neutrinos - also seems promising (see "Neutrinos"), protons and other particles in cosmic rays (see "Cosmic rays"), gravitational waves (§2.7 "Gravitational waves").
  Although these new "non-optical " observation "windows" into space will not captivate us with beautiful color images of nebulae and galaxies
(known from large classical astronomical telescopes), but they provide us with very important information about structures and significant processes in outer universe that are invisible in the optical field - see below "More 'windows' into the depths of space".

Astronomical telescopes - cosmic "time machines"
Telescopes for observing distant cosmic objects must be much "larger" than the telescopes used in everyday life here on Earth
("theatrical peepholes", binoculars, hunting binoculars). In order to achieve sufficient magnification, they must have a long focal length and, in order to make very distant or dimly glowing objects visible, they must also have a high luminosity - a large lens diameter. Today's astronomical telescopes usually have a lens diameter of approx. 2 - 6 meters, their lens is usually made of a precise parabolic mirror with a focal length of approx....-... m. These large terrestrial telescopes have provided since the middle of the 19th century, during many decades , the vast array of astronomical images and measurements on which our current understanding of the structure of the universe is essentially based.
 Terrestrial and space telescopes
However, even the most technically advanced telescopes, located here on the earth's surface, have
two limitations when observing cosmic objects, reducing their performance :
1. Turbulence of the atmosphere ; distracting light
A well-known visual experience when observing the night sky is the
"twinkle" of stars - a subtle, rapid twinkling of star brightness. In reality, however, the stars shine absolutely calmly and stably, with a short-term constant unchanging brightness (we do not mean variable stars here, which occurs over long time intervals). The observed subtle changes in their brightness are caused by atmospheric air turbulence, which constantly gently oscillates the rays of starlight. Therefore, even the most perfect telescope, placed on the earth's surface, can never produce a perfectly sharp image.
  Furthermore, the scattering of sunlight or moonlight, light from the stars, as well as light "smog" from artificial lighting, in the atmosphere leads to the fact that it never gets completely dark. This adversely reduces the contrast of telescope images - limiting the ability to observe extremely faint objects.
2. Absorption of radiation in the atmosphere
Atmospheric air is not completely transparent to light and other electromagnetic radiation, some of the light is absorbed and scattered. The wavelengths of light visible to our eyes represent only a small fraction of the range of electromagnetic waves in nature and space. Electromagnetic radiation of shorter or longer wavelengths than the visible spectrum can carry important astronomical information, but much of it fails to penetrate the Earth's atmosphere.
  Both of these disadvantages of terrestrial telescopes are removed by their placement in outer space, outside the Earth's atmosphere. These space telescopes are not bothered by clouds or mists, no turbulence in the atmosphere, no disturbing "light smog", sunlight (alternation of day and night). The light background of the sky here is completely black, so with a sufficiently long exposure, even the faintest or most distant objects can be recognized in the image. And they are able to measure and display all wavelengths, from radio waves, through infrared and visible radiation, UV, X-rays and gamma rays. Even those wavelengths that relatively penetrate the atmosphere (visible, radio) are sometimes advantageous to be detected by space probes for accurate and sensitive measurements. Among other things, this eliminates the disturbing effects of radiation of natural and artificial origin. An example is the measurement of the relict microwave cosmic background CMB, which is so weak that terrestrial microwave telescopes hardly detect it against the background of disturbing natural and artificial signals, but space probes (WMAP, Planck) mapped it in detail - §5.4. passage "Microwave relic radiation - a unique messenger of news about the early universe".
  Currently, there are mainly 3 space telescopes in operation :
- Above all, it is the legendary Hubble space telescope, launched in 1990, which examines the universe primarily in the optical and ultraviolet spectrum. In addition to a large amount of astrophysical data, he also provided many beautiful astrophotographs, which helped the general public to get a better idea of the structure of universe.
- The Spitzer Space Telescope, launched in 2003, explores space primarily in the infrared range.
- The James Web Space Telescope, launched in 2022, is also sensitive primarily to the infrared part of the spectrum.
  All astronomical telescopes show us light that was emitted in the past - either nearby (seconds, minutes, hours - for objects in our solar system), or more ancient, depending on the distance of the observed object. The more distant the object, the earlier its light started its journey to us. They are thus a probe into the past; with a bit of exaggeration, we can consider them as a kind of "time machines" that virtually return us to the ancient past of the universe - when observing very distant objects, it can be more than 13 billion years !
  Electromagnetic radiation brings from space information of two kinds (the third type of information, so far potential, will be mentioned below in the section "Polarization Measurement "), according to the methods of its detection and analysis :
¨ Optical display
In visible light we see
hot objects in space such as stars, hot gases and objects reflecting light of primary sources or glowing deexcitation of electron levels of atoms. By visual observation and optical display, we can obtain information about the position of individual objects (with a given resolution), their relative intensity (brightness, clarity) and possibly some details of their structure. Based on the change in position over time, we can monitor the movements of bodies and determine them speed. However, this is only possible with the nearest cosmic bodies - planets in the solar system. Stars, nebulae, galaxies, and other features are so distant that direct visual observation of changes in their positions over time and their velocities is not possible (but see Doppler spectrometry below).
Visual, photographic and optoelectronic imaging
In earlier times, until the middle of the 19th century, astronomers spent nights with their telescopes, in the eyepiece of which they observed with their eyes. Later, by the middle of the 20th century,
photographic plates and films were placed in the focal points of astronomical telescopes, on which even very weak objects, completely unobservable to the naked eye, were displayed after a long exposure (sometimes even several hours) and development. Optoelectronic image sensors (most recently based on CCD elements) are installed in the focal points of today's large astronomical telescopes . The image is electronically recorded in computer memory and displayed on the screen, computer image processing is often used .
Infrared astronomy
Observing the universe through
infrared radiation (electromagnetic waves with wavelengths greater than 700nm but shorter than about 1mm) is important for two reasons :
Most objects in space are relatively cold and according to Planck's law it shines dominantly (or even exclusively) in the infrared field of the electromagnetic spectrum. These are, for example, protostars, red and brown dwarfs, nebulae. The transitions between rotational and vibrational energy levels of molecules in cold gases are often in the infrared range of the spectrum, and infrared spectrometry makes it possible to know the chemical composition of gas clouds around stars, planets, nebulae, and interstellar gas.
2. Due to cosmological expansion of the universe all radiation from distant objects for observers on Earth shifts to the long-wavelength part of the spectrum. In the most distant objects, the original physically emitted visible light and UV radiation of the primordial stars, and even the X-rays of the quasars, shifst so far that they reach the infrared region.
Infrared astronomy belongs to
optical astronomy because it uses the same optical imaging components - mirrors, lenses. Semiconductor diodes (most often based on HgCdTe) or superconducting bolometers (Microcalorimetric detectors ) are used as detectors of displayed IR radiation. For areas from 1 to 4 micrometers, the atmosphere is relatively permeable, so observations can be made from the earth's surface. For longer wavelengths and submillimeter waves, the permeability of the atmosphere is very low and observations must be made from space probes.
Radio telescopic imaging
Visible light forms only a very narrow window in the spectrum of electromagnetic radiation, and in addition, clouds of interstellar dust are opaque to visible light (light is scattered and absorbed here). In this respect,
radio waves of wavelengths of the order of millimeters to tens of centimeters are more preferred. This radio radiation is abundantly generated in the environment of flowing gases (with turbulence and shock waves) by the mechanisms of braking and synchrotron radiation, when moving charged particles in a magnetic field at high speed along curved paths. Due to the expansion of the universe, the radiation of the most distant galaxies and relic radiation from the period of separation of radiation from matter is also shifted into the radio submillimeter region of the spectrum (§5.4, passage "Microwave relic radiation - messenger of early space messages" are discussed ). In the millimeter spectrum, we observe cold universe such as clouds of molecules and dust clouds.
Radio radiation is very well measurable by focused ("parabolic") antennas -
radio telescopes. The larger the diameter of the receiving antenna, the better the angular resolution. To observe radiation of a wavelength of the order of millimeters (which is much longer than that of visible light), the parabolas for sensing this radio radiation must be much larger than that of optical telescopes, min. 100 times larger than optical telescope with comparable resolution. However, increasing the size of antennas has its technical limits. However, the method of radio astronomical interferometry is very promising: the signal is received simultaneously by two or more "coupled" antennas, electronically connected in coincidence, distributed in different places at a greater distance from each other. The resulting image is obtained by electronic reconstruction of amplitudes and phases of signals from different antennas. The resolution of such an interferometric system is given by the spatial distance of the receiving antennas *), not by the actual size of these antennas (only the sensitivity of radiowave reception depends on the size of the antennas). When placing antennas over long distances (interferometry with a very long base - the planned placement of antennas on different continents or even in space) a very good angular resolution can be achieved.
*) Angular resolution is given by the ratio of the wavelength of the received radiation to the length of the base - the larger the base, the finer details of distant objects can be displayed.
¨ Spectral analysis
Electromagnetic waves carry not only optical information about the position and "strength" of radiation sources. No less important information is "encoded" in the wavelength or frequency of electromagnetic waves - in its
Spectral analysis
of visible light is performed by its distribution in an optical prism (light dispersion - different refractive index for different wavelengths, ie colors) or optical grating (bending and interference of light waves according to different wavelengths). In both cases, a spectrum is obtained - graphic image, where the wavelength is on the horizontal axis, the light intensity of individual wavelengths is on the vertical axis or brightness scale. Spectral analysis of radiation of wavelengths other than light is performed using appropriate methods of detection and electronic processing of the measured signal (for high-energy radiation, see "
Detection and spectrometry of ionizing radiation").
  The term spectrum was introduced in optics by I.Newton in the 17th century, when he discovered that white sunlight is a mixture of many colors of the rainbow. It is based on Lat. spectrum = image, revelation, delusion - an image of something that is not physically present.

Chemical spectrometry

Each atom of a given element and molecule of a particular compound has very specific, fixed and characteristic
energy levels of electrons, in the excitation of which electromagnetic radiation of a certain wavelength l (or a photon corresponding to the energy h.c /l) is absorbed and in the deexcitation of which the radiation of this certain wavelength is again enitted (see "Radiation of atoms"). By analyzing the spectral lines "light" (emission spectrum) and "dark" (absorption spectrum) it is possible to obtain reliable information about the atoms of elements or molecules of compounds that emit this radiation or that absorb it as it passes - chemical analysis of substances in outer space can be performed .
Doppler spectrometry

Since the energies (wavelengths) of the spectral lines are fixed and precisely known, in addition to detecting elements and compounds, spectrometry also allows the
analysis of motion - measuring the velocities of stars, galaxies and their parts. If there are lines, resp. a series of lines, systematically shifted in the spectrum to the red or purple end of the spectrum, this means that there act Doppler effect *) - a change in wavelengths causes by the the movement of the source relative to the observer. If the source moves away from the observer, the wavelength lengthens (redshift), as the source moves towards the observer, the wavelength shortens (shifts towards the purple end of the spectrum).
  The situation is more complicated in the general theory of relativity in the presence of gravity and curved spacetime. This can also be a gravitational redshift (see §2.4). In cosmology, we then encounter Hubble's redshift of very distant objects (§5.1), which is the result of the expansion of space itself (see the discussion in §5.4, the passage "What expands and does not expand during the expansion of the universe? "). From an observational point of view, however, both interpretations of the spectral shift are essentially equivalent.
*) The Doppler effect is a kinematic effect arising from the relative movement of a wave source and an observer (wave detector). It generally applies to all types of waves. If the wave source of a certain constant frequency fo moves towards the observer (receiver), this observer registers a higher frequency f than the source actually emits. Conversely, when the source moves away from the observer, the registered frequency is lower than the actual one. The relative difference between the actual fo and the observed f frequency (Doppler frequency shift) increases in proportion to the velocity V of the source relative to the observer: f = [1 + (V/v)] fo , where v is the propagation velocity of the given wave; Df/fo = (f-fo)/f = V/v . The same applies to the wavelength l = v/f. By measuring the frequency difference or wavelengths of the primary transmitted wave and received wave, we can determine the mutual speed of movement of the source and the observer. For electromagnetic waves, of course, v = c. And we know the actual (primary) frequency or wavelength of the emitted spectral lines exactly from laboratory measurements.
Note: This rule also applies when the source of the received wave is the reflection of the wave from a certain moving object (including a flowing gas or liquid). It is used in radar technology and ultrasonic sonography.
  Accurate spectrometric analysis can measure not only the speeds of translational motions, but also the rotation, pulsation or turbulence of gases in stars and galaxies; these events are manifested by a corresponding broadening or doubling of the spectral lines as a result of the Doppler effect.
Spectral analysis of radiation coming to us
even from the remotest observed objects in the universe shows that there apparently to apply the same laws of classical and quantum mechanics, electrodynamics, atomistic, thermodynamic and gravity as here on Earth. Although this cannot be strictly proven, this entitles us to believe that the same laws of physics apply even where we have not yet "looked" - and perhaps even in places we will never be able to see ..!..
  Polarization measurement 
An electromagnetic wave is a transverse wave in which the electric field intensity vectors E and the magnetic induction B oscillate perpendicular to each other and perpendicular to the direction of propagation . Both of these vectors lie in a plane perpendicular to the direction of propagation and together with the Poynting vector P
(§1.5 "Electromagnetic field. Maxwell's equations.") form a clockwise system. In addition to the intensity (amplitude of oscillations E and B ), frequency (wavelength) and phase, the electromagnetic wave is characterized by the direction in which the vectors E and B oscillate in a plane perpendicular to the direction of propagation (since they are always perpendicular to each other, just consider one of them, usually the intensity of the electric field E is chosen). If the vectors E of individual waves (rays, photons) always have a different, essentially random direction in the plane perpendicular to the direction of propagation, it is non-polarized radiation. In the opposite case, when the vectors E have the same direction in the imaginary plane, the vertically intersecting beam of radiation, we speak of polarized radiation. If the vector E still oscillates in one constant line, it is called linear polarization. If the ends of the vector E describe a circle, we speak of circular polarization, if more generally they describe an ellipse, it is elliptical polarization.
  Only coherent radiation - waves of the same frequency, the same direction of oscillation and the same phase (or phase difference) shows complete (perfectly 100%) polarization . Coherent radiation arises mainly during stimulated emission in the laser. In most common sources of radiation (sun, flame, light bulb), electromagnetic processes in a large number of atoms and electrons take place in a disordered manner, the electrical and magnetic components of the resulting wave randomly change direction, the resulting radiation is non-polarized. Partial, sometimes complete, polarization of light occurs during the reflection and refraction of light in optical environments.

Faraday twisting of polarization of electromagnetic radiation

When passing through areas where there is a
magnetic field and free electrons - it is an ionized plasma - there are interactions of the electromagnetic wave with electrons, which subsequently move in circular (spiral) paths due to the magnetic Lorentz force. This process - interaction + re-emission - of an electromagnetic wave leads to partial twisting of the plane of its polarization by the angle Dj :
Dj = l2 ò ne B cosf ds ,
where l is the wavelength of the electromagnet. wave, ne is the local density of free electrons in the ionized medium, B is the intensity (induction) of the magnetic field, f is the angle between the beam and the direction of the magnetic field, s is the path of radiation in a given environment. Integrates n over the entire path of s electromagnetic waves in ionized environments.
Note: The name "Faraday" in this phenomenon comes from the fact, that M.Faraday was generally concerned with rotation in a magnetic field, although electromagnetic waves were not yet known at that time.
This phenomenon is used mainly in
radio astronomy, where waves from distant radio sources pass through large space regions containing a sparse ionized substance - plasma - in the interstellar or intergalactic  magnetic field. For different wavelengths, the twisting of the plane of polarization is different, so from these differences it is possible to estimate the magnetic field and the electron density in interstellar space. In certain cases, this effect can also be used to determine the distance of radio sources (if known values determined in another way are used for electron density and magnetic field value) as an additional method in the "ladder" of cosmic distances (§4.1, passage "Determining distances of space objects basic condition of astrophysics") .
Polarization of relic microwave radiation
From an astronomical point of view, perspective may be the
measurements polarization of relic microwave radiation (the origin and properties of the relic microwave cosmic background are discussed in §5.4, passage "Microwave relic radiation - a messenger of early space news"). Both electromagnetic and gravitational waves have the property of polarization. In the early universe, at the end of the radiation era, a special kind of polarization of relic radiation (so-called mode B) could occur due to the action of primordial gravitational waves generated during the inflationary expansion of the very early universe (§5.5 "Microphysics and cosmology. Inflationary universe."). This future possibility of measurement, discussed in §2.7, passage "Measurement of polarization of relic microwave radiation" could reveal important information from the earliest period of the universe.
Astronomical Telescopes - " time machine "

The universe is well transparent to astronomically used radiation (at least at the present stage), so we gradually gain a lot of informations about the distant universe (paradoxically,
we know more about it than about the interior of our Earth). Thanks to the known constant speed of light, astronomical telescopes can also be considered as a "time machine" (in the sense of cognition), with which we can observe events in space even many billions of years ago (about the possibilities of "traveling in time "see the link "Journeys through time: fantasy or physical reality? ").
Astronomical observations and measurements in various fields  electromagnetic radiation (visible light, infrared, ultraviolet, X-ray, gamma) also allowed to determine the distances of various objects in space using the interconnected methods of the "ladder" of cosmic distances - see §4.1, passage "Determining the distances of space objects - the basic condition of astrophysics".
Other "windows" into the depths of universe
In addition to light and other bands of electromagnetic radiation, other types of particles and radiation come to us from space, which can
(at least in principle, or in the future) also be used to study structures and processes in universe. They can serve as new observation "windows" into space, which are so far slowly "opening up", but they will certainly be very promising in the future! There are three new "windows" and we will only briefly list them here :
Cosmic radiation (primary) 
high-energy radiation of cosmic origin, which is formed mostly by protons (88%), followed by helium nuclei (10%) and other elements (1%); the content of the various nuclei in cosmic rays roughly corresponds to the representation of elements in the universe, as established by primordial and stellar nucleosynthesis. From light particles then fast electrons and neutrinos. High-energy photons of gamma radiation are also part of cosmic radiation . The properties and detection of cosmic radiation, the mechanisms of its origin, as well as its effect on life, are discussed in detail in §1.6 "Ionizing radiation", part "Cosmic radiation" in monograph "Nuclear physics and physics of ionizing radiation".
  From the point of view of astronomical use, cosmic radiation has one major disadvantage : electrically charged
particles, which we capture, on their long journey to Earth by magnetic fields within the galaxy and in intergalactic space, have undergone very complex curved orbits, which unfortunately loses directional information about the source in which they formed. Only at very high energies is the curvature of the trajectory small and the particles at least approximately keep their direction. Cosmic radiation detection and spectrometry could provide us with useful information about processes in outer space, often the most tumultuous processes in the extinction of stars by gravitational collapse (supernova explosions) or the accretion of matter to a black hole (in quasars). Methods of detection and spectrometry of cosmic radiation, including the possibilities of their astronomical use, are discussed in the section "Detection and Spectrometry cosmic rays" in §1.6 "Cosmic radiation".
l Neutrinos 
are very small particles
(the rest mass close to zero), which have no electrical charge and did not show a strong nuclear interactions, show only weak interactions. This is so weak and short-range, that neutrinos hardly interact with matter and fly freely through it. Neutrinos in huge quantities arise in a number of processes in space - from the "big bang" lepton era, to thermonuclear reactions in stars, to supernova explosions. Neutrine detection and spectrometry may be important for the study of various processes here in space. Neutrinos, due to their extreme penetration, are the only particles that are able to "bring out" information about nuclear and particle processes from the interior of massive, large or compact objects, from which no other radiation absolutely penetrates. Detection of the flow of solar neutrinos makes it possible to test the instantaneous intensity of thermonuclear reactions inside the Sun (especially the proton-proton cycle). Even the high density and thickness of plasma in the solar interior do not prevent neutrinos from leaving the region of their birth almost immediately and thus "bring out" relevant information (unlike photons, which for hundreds of thousands of years "penetrate" plasma, with gradual energy degradation, from interior to surface before they radiate; they can only carry information about the surface layers of the Sun). Neutrinos also provide important information about turbulent processes in outer space. These are mainly supernova explosions in which a colossal amount of neutrinos (electron ne) is emitted. Relic neutrinos, originating from the lepton era, can provide important information about the dynamics of the earliest stages of the evolution of the universe and the formation of its structure.
  The main disadvantage of neutrinos is the very difficult detection. The wider use of the possibilities provided by neutrinos is thus linked to the improvement of neutrino detection techniques. The properties of neutrinos and the methods of their detection are discussed in detail in §1.2 "Radioactivity", part "Neutrinos - "ghosts" among the particles" of the book "Nuclear physics and physics of ionizing radiation ".
Gravitational waves 
they are a time-varying (oscillating) gravitational field that detaches from its source and propagates into space at the speed of light. According to the general theory of relativity, this is a ripple in the curvature of spacetime. They arise generally in any physical system with a time-varying mass distribution. The properties of gravitational waves, their origin and detection possibilities are discussed in detail in §2.7 "Gravitational waves".
The most important permanent (periodic or quasi-periodic) sources of gravitational waves in the universe are massive bodies that orbit each other (orbit around a common center of gravity). Powerfull sources of gravitational waves can be systems of compact gravitationally collapsed objects such as neutron stars or black holes, orbiting close to each other. Another intense source of gravitational waves can be the gravitational collapse of a star, if it is asymmetrical. The "collision" of two compact objects, the extinction and fusion of their tight binary system, as well as the strongly asymmetric gravitational collapse, is accompanied by a massive flash of gravitational waves, which carry away a significant part of the total rest mass.
  Detection of gravitational waves
, measurement of their frequency and intensity, together with the display of the direction from which they come, will make it possible to detect important dynamic processes with compact objects, often invisible in other ways, including the most tumultuous processes of gravitational collapse and collisions of neutron stars and black holes. Perhaps also about the dynamics of the earliest stages of the evolution of the universe, when the universe was impermeable to all other forms of radiation, but the "primordial" gravitational waves originating from that period could be detected in principle (interesting possibility of indirect detection of primordial gravitational waves is mentioned in §2.7, passage "Measurement of relic microwave radiation polarization").
  Gravitational waves are the most difficult to detect radiation.
The possibilities of future gravitational-wave astronomy are discussed in §2.7, in the section "Astrophysical significance of gravitational waves".

Natural laws, models and physical theories
Let us briefly consider some general gnoseological aspects of revealing natural laws, creating their models and formulating physical theories. Some other aspects of this kind are discussed in §1.0 "Physics - Fundamental Natural Science" of the book "Nuclear Physics and the Physics of Ionizing Radiation".
Order and laws in nature

In ancient times, when people did not know the causes and interrelationships of phenomena and events in nature
(or in human life), they looked at everything anthropomorphically and explained all events with the help of the gods or demons who control all aspects of nature and life - the gods of the Sun, fire, sea, war, storms, etc... These gods were, in their opinion, unpredictable, people were given to them at their grace and disfavor (they could possibly be reconciled by sacrifices and rituals). However, during the long process of knowledge development (which began in ancient Greece, around 500 BC and was mainly attended by Thales of Miletus, Anaximandros, Democritus, Pythagoras, Aristarchus, Archimedes), was increasingly asserted the cognition, that nature is governed by certain unchanging rules - natural laws that can be revealed. People began to understand that nature (the universe) has its inner order, which can be understood through observation and thinking about these observations. That the world is recognizable and can be understood without reference to myths, religions, and unsubstantiated assumptions, but on the basis of reliably established and carefully categorized knowledge.
   Natural law is a statement that describes a certain observed phenomenon or group of phenomena and generalizes it to other analogous phenomena on the basis of logical conclusions. At the same time, complex phenomena in which we do not observe direct regularity can be understood using simpler laws and principles - the so-called reductionism (cf. below). The individual laws of nature form a larger, interconnected system of laws; this system should be logically consistent. In contemporary science, the laws of nature are mostly formulated mathematically. Under clearly defined conditions, it must apply without exception. Under changed conditions, they may not lose their validity completely, but may apply approximately *).
*) An example is Newton's laws of mechanics, which apply with high accuracy in normal conditions of everyday life, but lose accuracy when objects move at high speeds, close to the speed of light. Or Archimedes' law applies exactly to "ideal" fluids (in which the pressure spreads evenly in all directions), located in a homogeneous gravitational field; in viscous liquids it applies only approximately, in mud it does not apply at all.
And, of course, it is invalid (or pointless) in a weightless state.
Regularities and randomness in natural events
   Everywhere in the universe, matter behaves according to the same laws of physics and chemistry. However, the specific behavior, the course of events and their outcome, depend on current conditions - these have also evolved according to these exact laws, but often through a complex combination of circumstances, that already have the character of chance.
   The relationships between the exact physical laws and the role of random circumstances are analyzed in more detail in §3.3, section "Determinism - chance - chaos ?". The role of chance in the origin and evolution of life is discussed in the work "Anthropic principle and/or cosmic Good ", passage "Origin and evolution of life".
Models and physical theories 
Descriptions of phenomena, relevant physical laws and explanations (or attempts to explain them) of their nature and connections with other phenomena are summarized in a broader framework of the system of opinion, learning or system of thought -
physical theory. Each physical theory is based on a certain basic idea or image of the studied phenomena - a model of natural reality, which captures the basic features of the studied phenomena, but abstracts from some more secondary, accidental or disturbing influences. Physical theory is then actually a model + set of laws (mostly mathematically formulated) that connect this model with the results of observations or experiments.
The importance of idealization and models for scientific knowledge
We create models not only in science, but also subconsciously in everyday life, so that we can reflect and understand the complex world around us. Even when seen, our brain receives electro-chemical signals from the optic nerves and creates a mental image in the appropriate brain center - a model of reality. In fundamental science, however, it is of fundamental gnoseological importance :
l a) Initialy, conceptually (programmatically, but judiciously - distinguishing which phenomena are of fundamental importance and which are marginal), we neglect those aspects of nature (studied phenomenon) that are difficult to understand and describe accurately.
b) We will examine the remaining simpler aspects in more detail, we will successfully solve and understand - we will create a simplified but functional model.
l c) After we fully understand the simpler aspects, we return to the more complex ones and try to solve them at an improved level of knowledge, using a more complex model .
   We consider the model or theory to be correct, resp. adequate when it meets the following 4 criteria (the fifth criterion will be added below) :
1. It is in line with existing observations and explains their results.
2. Provides clear predictions of future measurements and observations. The results of these measurements can confirm or refute the model, depending on whether they agree or disagree with the model's prediction.
¨ 3. It contains as few arbitrary optional parameters as possible, the values of which do not follow from the theory and must be artificially set so that the model corresponds to the experimental results.
4. It is logically simple, "elegant", naturally and credibly explains the nature of phenomena.
   Not always and for all phenomena is doing well to create such an ideal model or theory. The above criteria of a "good" model or theory are largely met by classical mechanics and Newton's theory of gravitation (within its limits),  relativistic mechanics (in inertial frames of reference) and electrodynamics (it is even universal). It is interesting, that some difficulties with point 3 have such a successful theory as the standard model of elementary particles (discussed in §1.5 "Elementary Particles and Accelerators" of the book "Nuclear Physics and Physics of Ionizing Radiation"); it contains dozens of free parameters (masses, charges, spins and other characteristics of various particles), the values of which do not follow from the theory and must be set on the basis of experimental data so that the model corresponds to the results of measurements and observations.
   The basis of scientific thinking is unification : in the enormous diversity of phenomena and events to seek general laws and a common essence, to try to explain the diversity of phenomena on the basis of as few basic laws as possible. Thoughtful people have always longed for a theory that would describe and understand all the observed complexity and diversity of nature. The ultimate (monistic) ideal is to explain all the laws of nature using a single universal principle - to create a definitive final theory or a unified "theory of everything". And it is physics, which examines the most basic laws of nature, that has the main unifying role among all the natural sciences. A characteristic feature of the physical view of nature is therefore the already mentioned reductionist approach and the effort to uniformly understand the widest possible class of phenomena - unitarization. In this context, we can supplement the above-mentioned 4 criteria of adequate theory with the fifth criterion :
¨ 5. The theory should describe and explain the widest possible class of natural phenomena on a uniform basis.
   These aspects are discussed in more detail in §B.1 "The Process of Unification in Physics".
Duplicity and duality of models and theories
As models and theories gradually evolve and improve during our knowledge of natural laws, a number of models and theories of various phenomena and groups of phenomena have been formulated. It happens that two different theories describe the same phenomenon well. How to decide which one is right, or at least "better"? Here are two examples :
l The motion of celestial bodies - the planetary system.
Around 150, Ptolemy created a geocentric model (system) of the distribution and motion of celestial bodies. According to him, the Earth is spherical, motionless and lies in the center of the universe. Stars and the planets around it at great distances orbit evenly in circular orbits.
Discrepancy between predicted perfectly uniform motion and the observed irregularities in the motion of the planets with changes in their brightness (indicating changes in the distance between the Earth and planets) resolved by hypothesis that the actual movements of the planets are formed by folding two or more uniform circular motion (called deferent, epicycle and ekvant). Ptolemy thus reached a relatively good agreement with astronomical observations, but at the cost of considerable complexity and artificiality. The geocentric system with the motionless Earth looked quite natural, for in ordinary life we do not feel the Earth moving beneath our feet (no one realized then that the same thing was happening on a ship floating evenly on the calm surface of a lake).
In 1543, M.Koperník developed an alternative heliocentric model according to which the immobile center of the universe is the Sun and the planets orbit it in (approximately) circular orbits. The earth is one of the planets that revolves around its axis with the diurnal period, giving the impression that all cosmic bodies, stars and planets, orbit it.
The observed movements of the Sun and the planets are thus explained much more easily and naturally; this in itself is preferred by the heliocentric system. Most importantly, however, the orbit of the planets around the Sun has been explained precisely using Newton's law of general gravity + the laws of classical mechanics, from which Keppler's laws of planetary orbiting follow. Thus, it can be said that Ptolemy's geocentric system is erroneous and the heliocentric system *) corresponds to reality - but with the clarification that the Sun is the center only of our planetary solar system, not the universe.
*) At present, it is sometimes believed that the geocentric and heliocentric models are equivalent, that it is not possible to decide which one is more adequate. We can't agree with that! Although the introduction of other "epicycles to epicycles" can be achieved in such a precision that the geocentric system will satisfy even the last accurate observations. Or, from the point of view of the principle of relativity, we can equally use reference systems connected to the Sun or the Earth to describe the universe, ie from the point of view of the standing Sun or the standing Earth. Here, however, we come into conflict with the above-mentioned criteria 3 and 4.adequate model. The refinement of the geocentric model is associated with the growing complexity and need to introduce other optional parameters, lacking credibility and explanation of the nature of such behavior. From a purely kinematic point of view, the Earth-related frame of reference can be used (unfortunately we have to do so in astronomical observations from Earth, with the introduction of complex corrections), but when analyzing the motion of planets, their equations are much simpler in the frame of reference as a motionless beginning. Above all, however, the heliocentric system is supported and justified by the dynamics of gravitational and centrifugal forces in the orbit of the planets around the Sun - a mechanism that explains the causes of this motion very well. In short, "such is the truth" !
l Theory of light.
Using Newton's older corpuscular theory (light is made up of tiny moving particles), the rectilinear propagation of light in "rays" and, in essence, the refraction of light as it passes from one optical medium to another can be well explained. However, it cannot explain the bending of light and interference phenomena (characteristic light and dark stripes or Newton's rings). Therefore, a newer wave theory of light was created, which can naturally explain all the phenomena of propagation, refraction, diffraction and interference of light. In addition, it unites optics with electrodynamics: light is an electromagnetic wave of very short wavelength. The wave theory of light has been confirmed, and it would seem that the corpuscular theory is incorrect.
However, in the early 20th century. Einstein showed thatthe photoelectric effect can only be explained by the fact that a quantum of light - as a particle - hits the surface of a metal or an atom and emits an electron from it. Thus, light behaves both as a wave and as a particle. Each of the two theories can describe and explain some properties of light, and neither can be said to be "better" or "more realistic" than the other. This apparent gnoseological contradiction was only explained in the corpuscular-wave dualism of quantum physics, which also manifests itself in electrons and other particles (see the passage "
Particle-wave dualism" in §1.1 of the book "Nuclear Physics and Physics of Ionizing Radiation"). Here, the situation is completely different from that of the geocentric and heliocentric systems. Both theories proved equally correct and were unified in the spirit of criterion 5 .
Based on the above criteria
1. - 5. it is therefore possible, in principle, to decide which of the duplicate model is "better" and more adequate; and under what conditions.
Accurate and effective laws of nature; reductionism

The goal of science is to constantly seek a full understanding of reality and perfect predictions, especially through accurate mathematical models of the real world (in the spirit of classical Newtonian mechanics). However, this theoretical "ideal" encounters enormous diversity and complexity in practice
  natural phenomena. At the level of the microworld, the stochastic character of quantum regularities, corpuscular-wave dualism, quantum uncertainty relations contribuce to this. Therefore, in practice we usually have to resign ourselves to absolute accuracy and in our knowledge of reality and the ability to predict to admit more or less uncertainty...
   Each body is composed of atoms, so the exact theory describing its behavior and motion should include movements and interactions of all individual atoms. According to this concept, if we wanted to analyze, for example, the motion of a thrown stone, we would have to solve all the equations describing the gravitational action between each atom of the stone and each atom of the Earth, with a huge number of parameters. This is, of course, completely impossible. The mechanics proceed differently: the stone is modeled using an idealized mass point - center of gravity, of mass
M equal to the sum of the masses of all stone atoms and analyzes the interaction of this mass point with the total gravitational field of the Earth, described by gravitational acceleration g. This is incomparably simpler - one equation with two parameters (in this case actually only one parameter g, because due to the universality of the gravitational action the motion does not depend on the mass M), while we obtain the result (path of the thrown stone) with absolutely sufficient accuracy for the problem.
   A theory that adequately models certain phenomena without describing in detail all the processes that participate in and lead to these phenomena is called effective theory. Sometimes the name phenomenological theory is used.
   The whole of classical mechanics with the concept of a material point is based on "effective laws". The study of mechanical processes at the molecular level has led to statistical mechanics and thermodynamics. It is assumed here that the individual particles move according to the exact laws of Newtonian mechanics, but in practice their use for larger systems would be extremely complex and impossible. Therefore, at the turn of the 19th and 20th centuries, statistical methods were introduced, where the positions and velocities of individual particles were replaced by statistical averages (assuming that the probabilities of all microstates are the same). The analytical methods used in Newtonian mechanics have thus been replaced by probability-based methods of statistical physics; it has proved very successful especially in the field of gas behavior and thermodynamics (kinetic heat theory).
Effective theories are also the science of elasticity and strength, hydrodynamics, electrodynamics and optics of the material environment. In all these fields of physics, we deal with collective ones
movements of atoms and molecules and their collective electrical interactions, without analyzing all the details of the behavior of individual atoms. Also, much of astrophysics, which examines planetary motions, the evolution of stars and galaxies, including relativistic astrophysics. Here, however, there is an interesting circumstance that in the physics of black holes the approximate effective theory passes into exact theory (§4.5 "Theorem "black hole has no hair" ").
   An important effective theory is chemistry. The essence of chemical reactions is physical - the sharing of electrons of the atomic shell at close proximity of atoms and, as a result, the formation of an attractive electric force, binding atoms to each other, into molecules
(more details "Interaction of atoms - Chemical merging"). In the simplest cases, physical chemistry can analyze the bonds of atoms into simple molecules in detail and described them mathematically. But can not solve the equations describing the interaction of complex atoms and molecules. Chemistry but developed their efficient methods that adequately describe how atoms and molecules behave in mutual reactions and express them using chemical formulas and equations, without having to consider all the details of electromagnetic interactions between atoms.
   In modern biology - molecular biology- it turns out that all events in living organisms are based on very complicated physical and especially chemical (biochemical) reactions between complex molecules in cells. If we wanted to describe these life processes precisely analytically, we would have to know the physical state of many trillion-trillion molecules in the organism and solve a huge number of equations of their interactions. The behavior of living organisms is the result of such complex processes with such a huge number of parameters that it is completely impossible to describe them exactly physically. However, biology has its effective methods of examining life processes at the subcellular, cellular, and whole organism levels, without having to consider all the details of the reactions between individual atoms and molecules.
   The gnoseological process, which tries to explain more complex phenomena by means of simpler phenomena, is called reductionism. It is a basic thought platform for advanced research in areas of more complex phenomena, especially biological ones. Biological processes are explained by chemical reactions and chemical reactions by physical interactions of atoms. The basic laws of physics at the classical and macroscopic level are deterministic
(from the point of view of quantum physics it is more complicated). The behavior of living organisms is also internally governed by the laws of physics, but overall it is the result of such complex processes with such a huge number of parameters that it is practically impossible to predict (in a deterministic sense). We are basically "biological machines" and what we call "free will" is a mere illusion ?
Questions of determinism versus randomness are discussed in more detail at the end of §3.3, section "Determinism - chance - chaos?".
  From the point of view of the accuracy of the laws of nature we know, we can ask the question: What happens when we take the laws of nature that work perfectly in the laboratory conditions and the surrounding nature available to us and extrapolate them to the most extreme situations? Will there be any minor deviations, or even gross disagreement? These questions examine experiments with particles accelerated to high energies and astronomical observations of turbulent events in space.
The uniqueness and origin of the laws of physics
In the gnoseological analysis of our knowledge of natural processes, at least three questions about the laws of physics arise :

 Do the same physical laws apply throughout the Universe, now, in the past and in the future?
Is there only one set of possible physical laws?
3. What is the origin of the laws of physics?
Ad 1: Spectrometric analysis of radiation coming from the farthest reaches of the universe shows that natural processes taking place here on Earth and throughout the observation of the available universe are governed by the same universal laws of physics, gravity, electrodynamics, atomistics, nuclear physics, thermodynamics, plasma physics, etc. We usually assume that the laws of physics are the same over time, but we have no direct evidence for this. In the earliest stages of the evolution of the universe, the individual interactions may have been separated from the basic unified interaction, for which the hitherto unknown laws applied. Sometimes a slow change of physical constants with time is also considered , eg gravitational constants (see eg §A2 "Brans-Dicke's theory of gravitation"), which has also not been proven so far...
Ad 2:
More complex is the second question, which can be metaphorically (or theologically) formulated: "Did God have the freedom to make natural laws when creating the world? - or did he necessarily have to make them as they are?". From the point of view of classical physics, it seemed that there could be only one logically consistent set of laws of physics. However, according to quantum physics and unitary field theories applied to cosmology, the universe does not have only one possible history, but there may be many different universes with different physical laws and values of natural constants (discussed in more detail in §5.5 "Microphysics and Cosmology. Inflationary Universe." , §5.7 "Anthropic principle and existence of multiple universes" and §B6 "Unification of fundamental interactions. Supergravity. Superstrings. ').
Ad 3:
The origin of physical laws traditionally refers to the transcendent, to God who is the creator. This is perhaps the case with the most fundamental laws or starting points on which unitary field theories are based. The specific physical laws describing the phenomena around us, however, they are probably the product of turbulent events at the beginning of the universe, when the fundamental fields, particles and properties of interactions between them were formed. And in different universes it may be different (§5.5 "Microphysics and cosmology. Inflationary universe.")...
Existence of objective reality
Classical science is based on the so-called
objective realism: the assumption that there is a real "external" world whose properties are given and objective - independent of the observer who examines it. That all objects exist independently of us and have certain given physical properties, such as speed, mass, electrical action (charge). And these properties will have the same objects, whether someone observes them or not.
  This fully corresponds to our experience with all phenomena in the macroworld, described by the laws of classical physics. A more complex situation occurs in the field of the microworld - atoms and elementary particles. Here the process of "observation", respectively measurement, necessarily interferes with the behavior of microparticles and changes their parameters. The measurement is necessarily of a nature mutual interactions of the measured particle with the "test" particles. Thus, we cannot claim that the microparticles had a certain position and velocity until the parameters are measured. The measured quantity has acquired the appropriate value only at the moment of measurement - and as a result of the measurement process. However, one cannot agree with opinions that overestimate the role of the "subjective observer" and question the objective reality as such! Natural processes with innumerable interactions of particles and fields are constantly taking place in nature and their results are independent of us. Only our occasional probes into the events of the microworld are burdened by fundamental quantum uncertainties. However, this is not a consequence of our subjective intervention as an observer, but the influence of interaction with objectively existing particles used for observation or measurement. Quantum physics does not deny objective reality, it just points out some of its unusual and difficult to understand properties.
  In connection with this, it is sometimes discussed what actually means "to exist"? Whether we can say that there are things we do not see, such as atoms, electrons, protons, quarks. However, these particles, which we cannot "optically" see, are an adequate model, which explains many of the observed properties of matter - in fact, all known properties. In everyday life, for example, glowing dots forming an image on a television screen (vacuum tube - cathode ray tube): we do not see individual electrons directly, but they are "visible" by interaction with phosphor molecules on the screen screen. Other subatomic microparticles can also be indirectly observed, registered or detected by modern physics, often using very complex experimental techniques ("Detection and spectrometry of radiation", "Elementary particles and accelerators"). Therefore, we rightly "believe" that these microparticles exist - or that they are at least a very useful adequate model...
   The discussion of "existence" sometimes gets caught up in vain philosophizing. E.g. under normal circumstances, that there is a desk in the room: how do we know that the desk still exists when we leave the room and do not see it? We can go back and see him in the same place. But what if the table disappeared (or someone disassembled it, fell apart) when we left the room and reappeared (folded) in the same place when we returned to the room? That would certainly be an absurd (though in principle possible) idea that disagrees with experience (for example with the testimony of someone who remained permanently in the room)...
  This vain philosophizing sometimes goes even further and degenerates into agnosticism, nihilism and subjective idealism: There is nothing but our minds and our thoughts, the external world is not real and objective, but only to usit seems. This is a popular topic of "café philosophers", beer drinkers, or "alternatives" of various orientations. From the point of view of Eastern philosophy, it is reflected in the treatise "Anthropic Principle or Cosmic God", part "Is the world objective or subjective?". Although we cannot exactly prove the existence of objective reality, in our lives, practical actions and cognitions, including scientific research, objective reality is the only possible platform !

Space and time
Along with the development of knowledge of specific laws of nature, the most basic physical concepts
- space and time - developed. We all live in space and time, so we have a certain intuitive idea of what space and time are. However, we do not have a general and precise definition of space and time, because we lack some "higher" superior and more general concept by which to express it. Such a "triviality" as space and time, which we encounter constantly and within which we live, escapes us even in a deeper analysis...
   For people in antiquity and the Middle Ages, the whole space - the whole world, the universe - was a small district of our Earth, which could be overlooked from the nearest hill or bypassed in a few days. Later, thanks to sea voyages, this limited horizon expanded and the reality of the round Earth, on the surface of which we live, became apparent. The idea of a solar system and a vast universe inhabited by stars and galaxies was only brought about by modern astronomy. The details of the spatial arrangement of the interior of matter were then shown only by contemporary atomic and nuclear physics.
   And they used to perceive time cyclically as a periodically recurring alternation of day and night and a sequence of seasons - in a circle of ever-recurring life cycles. This idea had natural-agrarian roots, it was derived from four basic periods - after winter comes the time when plants sprout and new animals are born, in summer there is a harvest. People had the idea that this cycle, the cause of which they did not know (they knew nothing about the rotation of the globe and its orbit around the Sun, or the rotation of the Moon around the Earth), is the natural and fundamental nature of the world and the properties of time. And they considered various supernatural beings and deities, residing in trees, water, mountains, stones, the Sun, the Moon, and so on, to be the "driving force" of all natural events. These deities "enlivened" the world..
 How do we humans perceive space and time ?
We obtain basic information about the surrounding world through our senses. We perceive sound by hearing, light by sight, temperature of objects by touch, fumes of some substances by smell. But do we have a sense or a receptor for the mental perception of the passage of time (chronesthesia )? Neurophysiology shows that we do not have a direct "time sense", but the brain can create it additionally or virtually. The tool is memory - a more or less stable record in the structure of neurons in the brain. Using our senses, the brain registers various events as perceptions which it processes and sequentially writes them to the various layers of the neural network. By this sequential "addressing" of event records in neural networks, their chronological order is encoded in the brain. The sequence and continuity of these records of registered events we then understand and experience as time . The brain constantly compares events registered by the senses as well as neural records of past events and sorts them in terms of space and time. Psychological arrow of time
("psychic feeling for the passage of time"), according to which human memory in a way reflects the part of events that is called the past, is clearly different in our psyche from the future. The human mind currently perceives the present. The past, as unique and unrepeatable, is stored in memory and can form certain ideas and expectations about the future based on its past experience. Time is therefore a thought structure, which we rank consecutively one after the other the individual events and experiences.
   This mental experience of time is very subjective, time runs "differently fast" depending on a number of external and internal circumstances in our brain. The apparent "acceleration" or "slowing down" of time depends on the "density of experiences"
(It seems to us that during an interesting activity, time passes quickly, while during a boring waiting, time drags on endlessly...). Clocks seldom measure the same time that flows inside us... We can imagine our perception of time as the number of biological events taking place in our body with a given frequency - the "biological clock". Time is encoded in all living things. The perception of time by organisms is often influenced by biorhythms in cells that have evolved during evolution - the "biological arrow of time" (circadian clocks - the rhythm of alternating light and darkness - have evolved in the simplest organisms). The "arrow of time" relentlessly shows that all organisms age and eventually die, the opposite is never the case (cf. "Time Travel: Fantasy or Physical Reality? ").
   Classical physics objectively conceives time basically like a river that from somewhere to somewhere flows - at the time it is in principle from infinity to infinity
(yet here we do not consider the cosmological constraints). For us humans, however, is time some " mental dimension " reflecting that, how we perceive the world in our brain This is a kind of psychological arrow of time. Further analysis is in §5.6, section " Arrow of time".
   When we determine time using the hands of a clock, we do not actually measure time, but the distance they traveled on the dial during their regular movement... In later chapters of this book we will see that in space time does not have to run everywhere fast, it can be affected by gravity or speed, depends on the frame of reference and the geometry of spacetime. There is no single universal "now" (certain present), it depends on where the observer is and how he is moving. Time is a "flexible" quantity similar to space.
   We perceive space - spatial relations - primarily through the visual imaging of light, mostly light reflected from material bodies. The light image projected by the lens of our eye on the retina is transmitted through the optical nerves as a series of electrochemical signals to the brain center, where it is written into the neural network. Based on this information, we create an image of reality in the brain, including the spatial arrangement of objects, in our consciousness. Visual perception of space is based on the properties of electromagnetic wave propagation (optical wavelengths) - rectilinear propagation of waves in vacuum, air or other optically homogeneous medium, reflection of light from bodies and refraction of light during transition to an environment with different speed of light propagation.
   In addition to optical perception, we often supplement our spatial information with touch
(which is essentially the electromagnetic interaction of the electron shells of the atoms of the object under study and the atoms of the nerves of our touch) : "by touching" we get an idea of the size, shape and spatial arrangement of bodies. This mental idea and experience of space is established in early childhood (even higher animals have it) and allows for orientation and targeted movement in nature. In the actual observation, a comparison with earlier images is subconsciously used, we have the ability to guess, for example, the presumed missing part of the image or to insert our knowledge "haw it should be" into the observation. However, when observing phenomena with which we have no experience (such as some light phenomena in the atmosphere), optical illusions can occur ...
   The   material world, which we perceive with our senses, has three dimensions- we distinguish between width, length and depth. We can go up - down, left or right, forward - backward; and also combine these directions.

Space and time in nature
In physics and other natural sciences, space and time are the basis for the description of all phenomena. We try to explain these phenomena by some mechanisms that work in spacetime. However, we never see space and time itself, we rather deduce their existence from our everyday experience. Ancient Greek
s thinkers have already tried to solve questions about the nature of space, time, and matter through philosophical speculation. In his "Metaphysics", Aristotle expressed the view that matter still existed in infinite space (since the infinite past), with "God" (the first mover) merely putting motion and a planned order into its originally chaotic state -  natural laws. Later, prevailed faith in the creation of the world, and philosophy discussed the question of whether space and time already existed before the act of creation, or were created together with matter. Some philosophers have argued that without matter, space and time could not exist (eg Aurelius Augustinius, 354-430n.l.). Others have argued that "the existence of matter is not necessary at all for the existence of space and time, just as the existence of the Sun is not necessary for the existence of time, although by its motion we usually measure time" (J.Locke, 1690).
 Does space and time exist ?
It must be realized that space and time are mere
abstractions used to express the state and movements of matter. No space or time physically - "tangibly" - exists. They are just tools that make it easier for us to navigate the world, nature, in space. The impression of time and space is created by the distribution and movement of matter - the shape, distribution and movement of ordinary bodies, interactions of atoms and particles, different values of intensities and potentials of physical fields and their changes (including, where appropriate, a state of rest and immutability in a certain frame of reference). In the whole universe there is nothing "physical" but matter that moves or changes - no time, no space; these are just the quantities we use to describe it... Time does not exist independently, it is always part of some movement. And space is an expression of the positional relations of material bodies. Let's compare the "Operationalist concept of space and time" below.
   During the long development of natural science, space and time became the most important physical concepts and basic physical units were established for them. In ancient times, these units were based on an anthropocentric concept
(eg inches and feet for length). Later, when people began to recognize that they live on the spherical Earth, the units of length began to be derived from the dimensions of the Earth (the unit 1 "meter" was created). And time was generally derived from the alternation of day and night (unit 1 day, 1 hour), from the observed phases of the Moon (1 month) and the alternation of seasons (1 year). That is, from the rotation of the Earth and its orbit around the Sun.
   The close connection of movement with time led to the stabilization of the phrase "passing of time" for events when something changes, grows old, becomes the past. This does not mean that the physical quantity time "flows" physically, but that the given event can be described by a certain time interval in which it takes place, flows. It is somewhat similar to the usual verbal formulation that "a river flows through a certain valley or landscape": what is meant is that water flows through the given river bed
(under the influence of gravity gradient).
   Of course, we use these used formulations about the passage of time and the geometry of space in many places in our treatise, but we mean their real physical content
(primarily in classical physics and STR - including the relativity of reference systems). The situation is more complicated when considering the general theory of relativity, which equates gravity with curved spacetime. According to GTR, the gravitational field also has energy (§2.8 "Specific Properties of Gravitational Energy"), so curved spacetime takes on a material physical nature. In some geometric unitary field theories, spacetime is even considered to be the basic physical "tangible" entity from which all particles and all matter are composed ("condensed") (chapter B. "Unitary Field Theory", "Geometrodynamics") - matter is created from the emptiness... From an epistemological and philosophical point of view it can be difficult to cope with..!..
 Absolute space and time
It seemed self-evident to the ancient Greeks, that there is a state of absolute calm, acquired by every body that is not subject to the effects of external forces. This led to the concept of "absolute space", in which it is possible be determine whether at different points in time the events take place in the same location (point) of space, and the idea of "absolute time". Galilei and Newton, in principle of relativity and the law of inertia, the concept of absolute space partially rid of physical soil underfoot, because there is no way by which it is possible absolute calm or motion measure mechanically. Nevertheless, even Newton recognized the absolute space as seen from his work "Philosophiae naturalis principia mathematica" from 1678, in which he writes: "Absolute space, by its nature and without relation to anything external, always remains the same and immovable".
  Newton's three-dimensional absolute space is the environment of our everyday experience. We look or move from left to right, from east to west, from north to south, from top to bottom, each of us, whether we move through our environment in different ways and speeds, we perceive space in exactly the same way.
We all agree on the basic aspects of space both intuitively and when performing accurate measurements.
   However, the concept of absolute time has remained fully preserved, ie the possibility to absolutely determining the present of events even when these events take place in different places in space. Absolute time featured on Newton's laws of mechanics, flowed equally for arbitrary moving bodies, and its value could be determined absolutely from the "moment of creation of the world". "Absolute or mathematical time flows evenly on its own, without any relation to anything external.", Newton wrote in his "Principies". So, absolute time passes just as fast everywhere in the universe and independently of things and events - so the whole universe would have a single identical "cosmic" time. This universal absolute time, to which everything is subject, cannot be influenced, accelerated or slowed down. Creation, transformation, and extinction of all things (including living things), all phenomena and events, are part of this continuous stream of "cosmic time". Time is often compared to the one-way flow of the river, where backward movement is not possible. This idea persisted from antiquity, through the Middle Ages until the beginning of the 20th century.
  Newton's absolute time is an expression of our daily experience with the direction of phenomena and events. It flows relentlessly forward *) as we age and see many things disappear, our loved ones die, other things arise again or be born. It is a time, whose flow is perceived equally by all people, it governs the movements of the planets, the rotation of the Earth and the resulting alternation of day and night, as well as the apparent movements of the stars.
*) The direction of time is discussed from a physical and cosmological point of view in §5.6, section "Time arrow".

Space and time used in classical mechanics have the following four basic properties :

  Newton's idea of the structure of space and time was fully in line with experience and was not in doubt until the end of the 19th century. At that time, Faraday-Maxwell's electrodynamics showed that electromagnetic waves propagate at a speed of c = 300000 km/s; immediately, however, the question arose: speed relative to what? The introduction of the ether as an environment in which electromagnetic waves propagate has actually replaced absolute space here (see also the note on the ether at the end of §1.5). The famous interference experiments of Michelson and Morley between 1881 and 1887 showed that no ether existed and that light propagates at a constant speed regardless of the state of motion of the source or observer, contrary to the most basic mechanical concepts. Discrepancies between the ideas of classical mechanics and electrodynamics solved A.Einstein (1879- 1955) his special theory of relativity (STR), in which space and time are no longer absolute, but are part of a more general spacetime - §1.6 "Four-dimensional spacetime and the special theory of relativity".

However, to understand the relationship between space and time on the one hand and matter on the other, STR has brought very little. Spacetime continued to be a kind of "stage" on which the motion of matter takes place, but its properties are not affected in any way by the behavior of matter. Properties a), c) and d) have been preserved in the special theory of relativity. In his general theory of relativity completed in 1916, Einstein revises all the basic properties of space and time (see chapter 2 "General theory of relativity - physics of gravity"): spacetime not only does not have to be Euclidean or infinite (it is Riemannian and can be closed), but its geometric properties are directly determined by the distribution and motion of matter. There may be a different course of time in different places in space, the geometry of space can change over time. At the same time, the general theory of relativity marked a fundamental change in views on the nature of gravity - gravity becomes a manifestation of the curvature of space-time. In conceiving the general theory of relativity, Mach's views on the cosmic origin of inertial forces had a significant (mostly positive) influence on Einstein (see Appendix A).
   According to the theory of relativity, space and time are relative and depends on weight and speed. Different moving matter can "create" different space and time - space and time are the result of certain physical processes. And the very reason for introducing the concepts of space and time is to be able to assess the motion of matter - the motion of particles and changes in fields.
An deep revision of the concepts of space and time in Einstein's special and general theory of relativity stemmed from a careful analysis of the measurement process in physics. The definitions of the terms space and time and their properties must follow from the knowledge gained by physical measurements. If space and time are attributed to properties that do not inevitably result from physical measurements, we easily find ourselves on the path (astray) of metaphysics. Mathematical theory, which seeks to claim physical content, must be based on basic concepts that reflect natural reality. For a long time, the terms "space", "time", "matter" have been claimed by philosophy (sometimes we still meet it today). However, the history of scientific knowledge shows that philosophy can ask important fundamental questions, but it is not able to answer them reliably and definitively. With the help of various philosophical speculations, it is possible to reach completely contradictory conclusions and theses. Only fundamental natural sciences (especially physics) that dialectically combine speculative and experimental cognitive methods, theory and practice, can help find a realistic answer to basic philosophical questions such as the nature of space, time and matter, and perhaps to the problems of the relationship between being and consciousness or matter and "spirit". And the study of gravity has significantly contributed (and continues to contribute) to this knowledge.

Twoo concept of space and time
To summarize, from a general scientific and philosophical-gnoseological point of view, the categories of space and time can be conceived in two ways

Exact (ideal) measurement of space and time
For accurate measurement of physical quantities, it is necessary to use such methods, aids and devices that are sensitive enough to the measured quantity and are not affected by other influences and circumstances of measurement. If this is not the case, at least an accurate correction for these disturbances and distortions must be possible .
Idealized clocks and measuring rods are introduced for measuring space and time in fundamental physics, especially in the theory of relativity :
Ideal clocks
are those calibrated clocks whose speed (frequency of the periodic events used) is not affected by any non-universal influences such as temperature or forces acting. Thus, a pendulum or hourglass clock would be completely unusable here (whose running speed is directly determined by gravity, it stops in a weightless state); similarly, other mechanical clocks could be affected by mechanical deformations of their components. Electronic oscillators and atomic clocks are considered to be the most suitable in this respect.
Ideal measuring rods
are length-calibrated scales whose length is not affected by any non-universal influences such as temperature or forces acting. Ideal measuring rods should therefore be made of a non-thermally expandable material, sufficiently strong and rigid. 
  If the influence of non-universal factors cannot be avoided, a correction must be made for these non-universal influences. In practice, "clocks" and "rods" are usually not used directly to measure times and lengths, but more complex methods using electromagnetic radiation - optical and radar methods.

With the deepening and refinement of scientific knowledge and their extension into the microworld and mega-world, the concept of absolute space and time came into conflict with the results of observations and experiments. On the contrary, the operationalist conception of relative space and time made it possible to understand and mathematically describe even such phenomena that would be incomprehensible to our classical experience. In §4.3-4.9 and §5.2-5.7 we will see that especially in extreme situations around black holes or the beginnings of the universe, where all our usual ideas and methods of measuring space and time fail, the operationalist approach allows us to use new and unusual methods of measuring space and time, proportionate to the conditions; it is probably the only way to reflect on these exotic phenomena.
  In a dynamically evolving and ever-changing universe, only "flexible" space and time, whose properties derive from ongoing events, can adequately capture the structure and evolution of the universe on a global scale and in the scale of locally ongoing processes. Material bodies - in general every distribution of fields, matter, energy - in their surroundings co-create (complete) the properties of the space in which they move (§2.5 "Einstein's equations of the gravitational field").
The beginning and end of time?
Abstract, mathematical or absolute time has an eternal
infinite duration from - ¥ to + ¥. In the physical, operationalist conception, however, time has a finite duration. In §5.4 "Standard Cosmological Model. The Big Bang" we will see that the beginning of the universe in the Big Bang is also the beginning of time. And in §5.6 "The Future of the Universe. The Arrow of Time" it will be shown that the future evolution of the universe will lead to the actual end of time - either in great collapse or in the thermal death of the universe...
In addition to this "global" end of time, a situation where the "local" end of time occurs - only for certain observers or worldlines. In §3.
4 "Schwarzschild geometry" and in §4.9 it will be shown that an observer who passes through the event horizon of a black hole, in the final interval of his own time he arrives at a singularity, which represents a kind of "local end of time".
Quantum "atoms" of space and time?
In general, space and time are considered a
smooth continuum. However, in §B.4 "Quantum Geometrodynamics" it will be shown, that the quantum approach to gravity and spacetime leads to the idea of quantum fluctuations in spacetime . This effectively creates the smallest, elementary, spatial cells with a Planck length ~10-33 cm and the smallest meaningful time intervals with a Planck time ~10-43 sec. From a quantum point of view, the classic notion of a smooth flow of time is thus replaced by the notion of jumping discrete mini-intervals, so a bit reminiscent of the falling grains of sand in an hourglass. Only on larger space-time scales do chaotically bouncing time micro-intervals fold into a continuous one-way flow of time and the spatial "foamy" microstructure is smoothed into a continuous space with a certain metric.
Cf. also the discussion "
Is the world continuous or discrete at the deepest level?" in §1.1 "Atoms and atomic nuclei " of the monograph "Nuclear physics and physics of ionizing radiation".

We live in the curved spacetime of the evolving universe
By creating a general theory of relativity (
GTR) and experimentally confirming its basic predictions (especially the deflection of light rays in the gravitational field of the Sun measured in 1919 by an expedition to observe a solar eclipse), people were confronted with reality that they live in curved spacetime. Practical importance of general relativity, however, long after its creation has not been fully appreciated, because it was thought that the gravitational fields in nature can never be strong enough to significantly exhibited specific relativistic effects and deviations from Newtonian theory. Far greater interest was aroused by quantum physics, which celebrated immediate success in elucidating the laws of the microworld and the structure of matter.
   The first success of the general theory of relativity came in the 1920s in cosmology (Chapter 5 "Relativistic Cosmology"). A.Fridman found that Einstein's gravitational equations allow a solution describing a spatially homogeneous and closed universe that expands over time, which fully agreed with E.Hubble's discovery that the wavelength of light from distant galaxies is systematically shifted to red the more the galaxy is farther away.
   The first indication that compact bodies with strong gravitational fields may exist in space appeared in the 1930s. At that time, Chandrasekhar and Landau proved using Newton's theory of gravitation that the stars must be a certain maximum weight is to be at the end of their development achievable some state of equilibrium. Oppenheimer and Snyder then shortly (in 1939) used the previously found Schwarzschild's exact solution of Einstein's equations and came to the conclusion that a sufficiently massive object would collapse indefinitely under the influence of its own gravity. However, these conclusions did not arouse wider interest at the time.
   At the turn of the 1950s and 1960s, significant astronomical discoveries became a powerful stimulus for the development of the general theory of relativity and interest in it. Above all, it was the discovery of pulsars (see §4.2 "Final stages of stellar evolution. Gravitational collapse") and mainly quasars (see §4.8 "Astrophysical significance of black holes"), for which large concentrations of matter and strong gravitational fields could be expected. Furthermore, Fridman cosmological models were decisively supported by the discovery of microwave radiation of the thermal spectrum corresponding to a temperature of 2.7 °K, was interpreted as a remnant of a very hot and dense phase of the evolution of the universee - relic radiation. At this time, a likewise relatively well developed astrophysics of stars evolution, which showed that in the final phase of its development may star undergo gravitational collapse, that for sufficiently massive stars can be fully relativistic. This should create extraordinarily strange compact objects called collapsers or black holes. Thus, in the 1960s and 1970s, an important discipline of the general theory of relativity and relativistic astrophysics developed - the physics of black holes (Chapter 4 "Black Holes").
   The development of electronics, measuring and experimental techniques has made it possible to re -establish the contact of the general theory of relativity with experiments and astronomical observations at a much more accurate level. In 1960, Pound and Rebek accurately measured the gravitational redshift using the Mösbauer effect (see §2.4 "Physical Laws in Curved Spacetime", passage "Gravitational Frequency Shift"). Accurate measurements of subtle relativistic effects on the motion of planets and artificial satelits in the solar system are being performed or planned. With the help of sensitive experimental and electronic methods, the Ëtvös experiment, proving the equivalence of inertia and gravity, was repeated again and with high accuracy. Another interesting area of gravitational physics is the study of the properties of gravitational waves and attempts to experimentally confirm them (§2.7 "Gravitational waves"). In many places in this book, the relevant theoretical concepts and findings are supplemented by a brief description of their experimental verification. A brief systematic overview of experiments in the field of the theory of relativity and gravity is in §2.10 "Experimental verification of the theory of relativity and gravity", where there are references to the relevant passages in the individual chapters in which the experiments are discussed.

The path of knowing gravity is extremely interesting. It turns out to be the key to understanding the properties of space, time, matter and its motion, the key to the secret of the construction of the universe.

Gravity - "Foreword"   1.2. Newton's law of gravitation

Gravity, black holes and space-time physics :
Gravity in physics General theory of relativity Geometry and topology
Black holes Relativistic cosmology Unitary field theory
Anthropic principle or cosmic God
Nuclear physics and physics of ionizing radiation
AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy

Vojtech Ullmann