|AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy||Gravity, black holes and physics|
GRAVITATION AND ITS PLACE IN PHYSICS
1.1. Historical development of knowledge about 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
1.3. LeSage hypothesis about the mechanical origin of gravity
So Isaac Newton discovered his great law of gravity, but (perhaps in the spirit of his credentials "Hypotheses non fingo") did not comment on the causes of gravity. Why are all material bodies attracted to each other by the force given by the law (1.1)? Newton's followers naturally tried to explain the nature of gravity using the phenomena known at the time, ie using various mechanical or hydrodynamic models. From pre-relativistic (and of course unsuccessful) attempts to explain the cause of gravity, we will mention here the most remarkable of them - Le Sage's hypothesis dating from 1782 .
Fig.1.2. LeSage's hypothesis about the mechanical origin of gravity.
a) Ether particles hit the lone body from all sides with the same average intensity: the resultant of the reaction forces is zero.
b) Two bodies located in a space filled with chaotically moving ether particles form a "shadow" relative to each other. Fewer particles fall from the direction from one body to the other than from the other (unshaded) directions, which creates the resultant force pushing the two bodies towards each other.
LeSage assumed that all space (the whole universe) is filled with a huge amount of particles moving chaotically in all directions (ether). If there is only one body itself in such an environment (Fig. 1.2a), it is encountered by particles from all sides with an average of the same intensity (and frequency), so that the total resultant of forces is equal to zero. However, if in such an environment there are two bodies at a certain distance from each other (Fig.1.2b), the situation is different. Part corpuscles, which would otherwise collide with one of the bodies, will not reach it, because it encountered previously on the second body from which it was reflected or absorbed. The bodies thus form a mutual "shadow" against the ether particles. As a result, slightly fewer particles fall on each of the bodies from the direction of the other body than from the other (unshielded) directions. This creates a certain resultant of forcesa reaction that pushes both bodies together. If a quantitative analysis is performed for the case of sufficiently distant bodies (compared to their dimensions), it is based on the simple geometric idea that this force will be inversely proportional to the square of their distance, as in Newton's law of gravitation (1.1). Very small sizes LeSage particles render the corpus balls penetrate into the interior of bodies which interact independently with individual atoms, or "gaps" between the atoms freely pass out; the attractive force (reaction force) is then proportional to the number of atoms, ie the mass of both bodies.
At first glance, LeSage's hypothesis is very nice and tempting because it explains the mysterious gravitational forces using easily understandable mechanical phenomena. However, in a more detailed analysis, insurmountable shortcomings immediately arise, which exclude the possibility of explaining gravitational phenomena using the LeSage mechanism.
It is clear that the mechanistic nature of the original LeSage hypothesis is inapplicable in modern physics. Attempts have therefore been made to replace ether particles with some actually existing particles or radiation, such as neutrinos (see below).
Quantitative agreement with (perfectly verified) Newton's law (1.1) is achieved only for small and sufficiently distant bodies. If we have two objects located at a fixed distance from each other and increases the mass (density) of one of them will, according to Newton's law proportionally increase as gravitational force, this force does not have any finite limit and increases indefinitely (linearly) with the weight of gravitated bodies. According LeSageova mechanism should however gravity showed a condition of saturation : there exists a finite limit corresponding to the situation where all the ether particles, moving in the direction from a second body, already absorbed or reflected by this second body.
Another objection is that the zero resultant of forces acting on a free isolated body applies only to bodies at rest (relative to "ether"). However, if the body moves at a sufficient speed, the symmetry in the momentum distribution of the incident ether particles will break and a certain resultant of forces acting against the direction of motion will be created - a free fast moving body would be braked, contrary to the law of inertia.
However, the most
serious objection stems from the energy
balance of the LeSage process. During its interaction
with the body, part of the ether corpuscles can pass freely, part
is absorbed and part is reflected. The absorbed particles
transfer all their kinetic energy to the body; also the reflected
corpora transfer part of their kinetic energy to the body (unless
the collisions are perfectly elastic). Because the process takes
place at the microscopic level, this transferred energy will
change into heat: bodies gravitationally by the LeSage mechanism
would have to heat at the same time (and above
all!) . The same applies also for isolated bodies located
in a LeSage environment. To explain the observed gravitational
forces by the LeSage's "shielding" mechanism, the stream of
particles must have a very considerable intensity *), so that every body should thus be bombarded constantly
under the influence transmitted impact energy must in a short
time at a high temperature inflame, which is of course not observed.
*) Assuming absorption, the force F induced by the impacts of the particles will be approximately equal to F = e . p r 2 .sin 2 a , where e is the energy density of the absorbed particles [J/m3], r is the radius of one body and a is the angle of view at which the second gravitational body appears. The gravitational force between the Earth (r » 6.4.106 m) and the Sun is F » 4.1022 N, angle a » 17 ', so the density of LeSage particles would have to be roughly e» 1013 J / m 3 , which is 17 orders of magnitude higher the density of sunlight around the Earth!
This arguments shows that LeSage's theory has no hope of correctly explaining the cause of gravity. This is also true of the various modifications of the LeSage hypothesis if the actual particles or radiation (replacing hypothetical LeSage particles) are to have reasonable physical properties. Without the transfer of energy absorbed or reflected particles (or radiation), there would simply not be the factors causing s coveted "gravity" effect. If it were to be a neutrino, then moreover, due to the very low effective cross section of the interaction of neutrinos with matter, many orders of magnitude higher neutrino densities would be needed to explain the existing gravitational force than is observed in space.
It can be concluded that no mechanistic explanation of the nature of gravity, which would not lead to both non-existent phenomena and contradictions with reality, has been provided. A real explanation of the nature and cause of gravity must therefore be sought elsewhere: in the consistent relativistic field theory - in the general theory of relativity (Chapter 2) and the unitary field theories based on it (Appendix B). The usual question "Why do bodies around them excite a gravitational field?" in the approach of unitary field theory, it can even be completely upside down: the field is considered primary and the question is: "How is matter created from field?".
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