|AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy||Gravity, black holes and physics|
GENERAL THEORY OF RELATIVITY
- PHYSICS OF GRAVITY
2.1. Acceleration and gravity from the point of view of special theory of relativity
2.2. Versatility - a basic property and the key to understanding the nature of gravity
2.3. The local principle of equivalence and its consequences
2.4. Physical laws in curved spacetime
2.5. Einstein's equations of the gravitational field
2.6. Deviation and focus of geodesics
2.7. Gravitational waves
2.8. Specific properties of gravitational energy
2.9.Geometrodynamic system of units
2.10. Experimental verification of the theory of relativity and gravity
2.5. Einstein's equations of the gravitational field
If we are building a field theory, we are basically interested in two areas of questions :
In this section we will
try to show how the gravitational field is generated
by its sources , ie to "derive" Einstein's
equations of the gravitational field *). We will first proceed in
an inductive manner similar to the way Einstein arrived at his
equations; this method is the most comprehensive heuristic. Some
other procedures leading to Einstein's equations , [ 1 66],
, which arose much later (and probably only thanks to the
knowledge of Einstein's equations and his general theory of
relativity) will be mentioned in the following.
*) Specifically, a new fundamental law of physics, such as the law of gravitation in GTR, cannot be derived in the mathematical sense of the word, ie as a direct consequence of some other previously known relations and laws (eg theorem 2.3 based on the principle of equivalence, because in the special theory of relativity we have no law of gravitation). Gravity law rather looking ( "constructs") on the basis of certain physical requirements and postulates.
universality of gravitational excitation
First, we extend the universality of the gravitational interaction . So far, we have understood by the universality of gravity the universal effect of the "already finished" gravitational field on all matter ~ energy. This allowed a geometric description of the gravitational field as a curved spacetime. However, the universality of gravity has a broader framework and also applies to the excitation of the gravitational field:
|Theorem 2.4 (universal excitation of gravity)|
gravitational field (curvature of spacetime) is excited
universally by all matter ~ energy, or
active gravitational mass = passive gravitational mass = inertial mass.
The simplest argument in favor of this statement is the law of action and reaction. If we have two bodies A and B placed so far apart that Newtow's law of gravitation applies to its mutual gravitational attraction, it must be
since this must apply to
bodies of any structure, the active and passive gravitational
masses are the same for each body (or both types of mass are
proportional to each other with the universal gravitational
Thus, the intensity of the gravitational field that excites a body around it does not depend at all on its composition and nature, it is given only by its total inertial mass (ie the resistance it would put to acceleration by non-gravitational forces). It does not matter whether they are solid bodies, the gas, cluster of elementary particles, or if only the electromagnetic field. Its own gravitational field also generate with electromagnetic waves (light, radio waves, X-rays), globally even gravitational waves (§2.7 " Gravitational waves ", see also Appendix B, §B.3 "Wheeler geometrodynamics. Gravity and topology. ").
If our task is to
determine the mass of a large body - such as a planet or a star,
we can proceed in two ways. The first method is based on
non-gravitational physics, where the mass of the body is given by
the volume integral M = ò T °° dV. However, in order to
accurately determine the energy-momentum tensor, it is necessary
to know in detail the internal structure of a star - what
particles it consists of, the nature of interactions between
them, the mechanisms of energy conversion and transfer, etc.
Figuratively speaking, we would have to either "¨to
count" all the particles the star contains, determine their
masses, make corrections to the mass difference caused by their
motions and bonds in the respective fields, and also gain the
mass of radiation, or calculate the energy-momentum tensor based
on certain assumptions arising from the internal construction of
a star. In this complex way, we could perhaps theoretically ( but
never practically!) determine the mass of a given star.
However, in practice we determine this mass precisely and at the same time incomparably more easily by means of gravity : we analyze the properties of the motion of other bodies in the external gravitational field of the observed stars, eg at a distance r we put a small test body in orbit around it and determine the mass of the star from Kepler's law M = w 2 .r 3 / G. This method is completely reliable, there is no danger (as in the first method) that we would "overlook" any contribution to the total weight. To determine the total mass of a star or other physical system, it is not necessary to know what is happening inside, just simple Newtonian measurements in a sufficiently distant region.
Exciting the gravitational field (gravitationally attracting other bodies) is a common universal property of all material formations, of every form of matter. Therefore most objective method of determining the mass pf some body is to measure how strong the gravitational field around this body excites. In the following, we will understand the mass of a system as this gravitationally measured mass .
Gravitational field excitation equations
In §1.4, where we followed the analogy between Newton's and Coulomb's law, we said that the principle of superposition applies to both Coulomb's electrostatics and Newton's gravity, so the corresponding equations are linear. But now we see that in this gravity applies only approximately, for weak fields within Newton's gravitational law . Even without knowledge of the exact form of the gravitational field equations, we can make the following statement as a direct consequence of theorem 2.4 (universality of excitation): the equations of the gravitational field must be fundamentally nonlinear .
To clarify this important aspect of gravity, let's compare the situation with electrodynamics once again. The electromagnetic field is excited using Maxwell's equations
|F ik ;
k = - (4 p
/ c) j i .
á field á á source á
The source of the field
here are electric charges and their currents j i
, while the electromagnetic field itself does not transmit an
electric charge (it is uncharged) and is therefore not the source
of another electromagnetic field - Maxwell's equations are linear and the principle
of superposition applies .
In contrast, the source of the gravitational field is all matter (~ energy) and, because the gravity field itself is also a source of energy (and momentum), evokes some "complementary" gravitational field. This "self -gravity" leads to the principal nonlinearity of gravity , because the excited gravitational field contributes back to its own source (cf. also §2.1).
The equations (of generations) of each physical field have the character *) :
E.g. for the
electromagnetic field the object describing the field is the
tensor of the intensity of the electromagnetic field F ik , resp. its four-divergences F ik ; k . The source of the field is electric
charges and the object describing the source is a four-current j i
indicating the distribution and movement of electric charges. The
field generation is then given by equation (2.42), which is an
electromagnetic variant of general equation (2.43). Let us seek of a gravitational variant
*) We stand here on the classical position, according to which the field is excited by a certain external source (which is of a different character than the excited field) and we ask: " How is the field excited by its source?". The position of unitary field theory (whose equations have no external source) is the opposite - there is only a field with sufficiently rich internal properties and the question is solved:" How is what we considered to be a source, composed of its field? "- see Appendix B" Unitary field theory and quantum gravity ".
We know about the gravitational field that it is expressed by the geometry of spacetime and that it is excited universally by all matter (~ energy). Therefore, for the gravitational field, the general equation (2.43) should read :
whereas for weak
gravitational fields it must give the correct
Newtonian limit, ie the Poisson equation Dj = 4 pr G with the
potential j related to the metric tensor relation (2.27).
The object, which exhaustively and independently of the specific structure describes the distribution and motion of matter and energy in the physical system, is the energy-momentum tensor T ik (introduced in §1.6 "Four- dimensional spacetime and special theory of relativity ") . If we denote the left side describing the geometry of spacetime as G ik , the equations of the gravitational field should have the form
|G ik = K. T ik ,||(2.45)|
where K is a constant. In order to be able to specify the quantity G ik more precisely , we will impose several more or less reasonable physical requirements on it :
Requirements 1, 2 and 3 are quite clear and do not raise serious doubts. The least justified seems to be condition No. 5, which is to some extent a condition of simplicity in the sense of Einstein's credo. Its justification is given on the one hand by the fact that 2nd order equations predominate in physics (Cauchy's problem), and on the other hand by the transition to the Newtonian limit. For weak static gravitational field, which according to (2.27) related to the metric tensor potential j relation goo = - (1 + 2 j / c 2 ), the required gravitational equation must pass in to Newton's law of gravitation
|Dj = 4 p G r ,||(2.46)|
where r is the density of the mass distribution in
the source. Since the left-hand side of this equation contains
the second derivatives j , it is natural to require that G
ik also contain derivatives gik at most to the second order. Due to
the weakness of the gravitational field, a coordinate system can
be chosen at each point, which is approximately Galilean. In
order for equations (2.45) sought in such a system to pass into
the Newtonian limit (2.46), G ik must be
linear in ¶ 2 g / ¶ x i ¶ x k with coefficients that are functions only of g ik , not their first
Condition No. 4 looks physically very reasonable. Nevertheless, it was precisely this condition that Einstein temporarily gave up in connection with cosmological problems - see Chapter 5, §5.1 " Basic starting points and principles of cosmology ", §5.2 " Einstein's and deSitter's universe. Cosmological constant. " . Although the recently again some grounds for thinking cosmological element in the global cosmological problems in our text (except Section 5) currently cosmological member will not be used on the most discussed problems are not affected.
Conditions 1 to 5 allow unambiguous finding of the quantity G ik . In differential geometry it is shown ,  that a quantity satisfying the conditions 1., 2. and 5. must have the form G ik = AR ik + B.g ik R + Cg ik , where A, B, C are constants. According to requirement 4., C = 0 (if we omit condition 4, C would be equal to the cosmological constant).
From Requirement 3. with respect to Bianchi's identities (2.25), B = -A / 2. Thus A. (R ik - (1/2) g ik R) = KT ik . Constant A can be included in the constants K (refer to Kº K / A), so we get
|G ik º R ik - (1/2) g ik R = k. T ik .||(2.47)|
The tensor G ik is called the Einstein tensor (it was mentioned in the previous paragraph, equation (2.25b)). The constant k is determined from the condition that for weak fields the general equations change into Newton's law of gravitation. Therefore, in equations (2.47) we make a limit transition to nonrelativistic mechanics: we assume that the gravitational field is weak and the velocities of all bodies are small (both of these requirements are in fact related, because in order for velocities to remain small, the field must be weak). As the source of the gravitational field use incoherent material powder (which is most simplest classical model unstructured mass) with energy and momentum tensor T ik = r .c 2 V and V k . Assuming low velocities, we can set V i = (1,0,0,0), so the nonzero component T ik will be only T °° = r .c 2 . Einstein's field equations in the form R ik = k. ( T ik - (1/2) g ik T) (arising from equations (2.47) by a simple algebraic modification) are then reduced to one equation
|R oo = (1/2) k. r .c 2 ;||(2.48)|
the other equations are zero in our approximation. We calculate R °° from relation (2.18), omitting the second order members due to the weakness of the field. We get R °° = ¶G a oo / ¶ x a , which using the relation (2.27) g oo » - (1 + 2 j / c 2 ), valid for weak fields, gives R °° » (1 / c 2 ). Dj . The field equation (2.48) then reads Dj = (1/2) k. r .c 4 . This will Poisson equation (2.46) then , when we put
|k = 8 p G / c 4 ;||(2.49)|
Then Dj = 4 p G r and its solution will be j = -G. ò ( r / R)
dV. And this is Newton's law as a special case of Einstein's
gravitational equations for a very weak field.
Thus, Newton's gravitational constant G appeared in the general gravitational equations GTR . We get very important Einstein's equations of the gravitational field in the form
|Einstein's equations of gravitational field :|
By narrowing with the metric tensor g ik we get R - 2R = T.8 p G / c 4 , so Einstein's equations can also be written in equivalent form
|R ik = (8 p G / c 4 ). (T ik - 1 / 2 g ik T)||(2.50b)|
where T º T i i .
Einstein's equations of the gravitational field generally show that the presence of any matter (with a certain mass and thus energy) curves spacetime with everything in it. These nonlinear partial second order differential equations for the components of the metric tensor g ik determine the spatiotemporal distribution of the metric tensor, i.e. the spatial distribution and temporal evolution of the gravitational field excited by the system of sources described by the energy-momentum tensor T ik . They are also referred to as " geometrodynamic " equations because they describe the dynamics of space-time geometry.
Structure of the energy-momentum tensor T ik , which completely describes the distribution and motion of energy and momentum in a gravitational source, was described in §1.6 "Four- dimensional spacetime and special theory of relativity ", part " Energy-momentum tensor ". The most important and dominant component of the energy-momentum tensor is T 00 , representing the energy density ~ mass (it is also discussed in §2.6 " Deviation and focusing of geodesics ") . For a common substance, this component is given by the mass density r : T 00 = r .c 2 . The other three diagonal components T 11 , T22 , T 33 are equal to the pressure p for a common substance (modeled as an ideal liquid) . In such a case, Einstein's equations roughly say that the curvature of spacetime (sum of curvature values in three directions, scalar curvature) is proportional to the mass density r .c 2 + three times the pressure p of the substance: R ~ r .c 2 + 3.p.
Under normal conditions (on Earth, inside planets, sun and stars) the pressure of matter is negligible with respect to its density (quantity r .c 2 ) and therefore the pressure almost does not contribute to the curvature of space-time (gravity) - it is given almost exclusively weight of the source. Only inside neutron stars and during gravitational collapse (§4.2 " Final stages of stellar evolution. Gravitational collapse. Black hole formation. ") , or during cosmological evolution of the universe (§5.1 " Basic principles and principles of cosmology " and the following chapters) , pressure is important contributor to the gravitational curvature.
Einstein's equations generally describe how matter curves space-time in its surroundings , which manifests itself as gravity . Significant solutions of Einstein's equations, such as for a spherically symmetric body (§3.4 " Schwarzschild geometry "), show that the presence of a larger amount of matter ~ energy - a more massive object - curves space - time in its surroundings more markedly and in a greater range than a light object.
|A material gravitational body curves
space-time in its vicinity
(example of a spherically symmetric body)
In our terrestrial gravity conditions, it can be clearly illustrated by horizontally tensioned flexible curtain (which represents not curved space without gravity) , on which lay the material sphere M . The recess created by the load represents the curvature of the space. When we place a smaller ball m 1 (rest) here, it rolls into a depression and lands on a larger ball. However, if we give the ball m 2 a suitable circumferential speed, it will orbit a heavy sphere, which is the source of curvature, much like a planet orbits a gravitational star or a moon-satellite around a planet. At a higher ball speed m 3 , its path in the recess only curves and the ball continues to move at a different angle.
equations of field excitation express the global integral
behavior of the field
In field theory , the laws of its excitation by their sources, as well as its internal dynamic properties, are expressed by means of differential equations . These equations have a local character - they relate the local distribution of matter or electric charges with the derivatives of the field intensity or the curvature of spacetime in the same place . However, this does not mean that the sources determine the values ??of the quantities describing the field only locally or in the immediate vicinity of the field sources. By integrating the local differential equations of the field, we obtain the values of the field intensity or the curvature of spacetime in the whole space surrounding the source system. This happens so often through boundary conditions , as we will see, for example, in §3.4 " Schwarzschild geometry ".
The equations of motion of
resources as a consequence of the field equations
Covariant 4-divergence left part G ik Einstein's equations (2.50) is identically equal to zero. This is due Bianchi identities (2.25) for the curvature tensor, a manifestation of the principle of geometric-topological "boundaries boundary equals zero" - in this case oriented two-dimensional boundary-dimensional four-dimensional region of space boundaries   (see also §3.1 " Geometrical -topological properties of spacetime ") . Taking therefore Einstein equation (2.50) as a basis, it follows them automatically T also k = 0 - local Act ofn conservation of energy and momentum of the source *). This law of conservation leads to the equations of motion of the source system described by the corresponding energy-momentum tensor T ik , so in this sense "the equations of motion flow from the Einstein equations of the gravitational field ". Einstein gravitation equation determine not only the gravitational field of the mass distribution, but also determines the movement of the source.
*) It is an expression of the general regularity between the source and the field it evokes: "the source creates a field around itself so that it preserves itself " - an array of such properties from which the laws of conservation of this source automatically follow . E.g. for the electromagnetic field from the Maxwell equations F ik ; k = 4 p j i / c due to the antisymetry of the electromagnetic field tensor F ik, the relation j i ; i = 0 follows identically , which is the equation of current continuity or the law of conservation of electric charge. Maxwell's equations thus limit the "freedom" of resources only in terms of electrical (not eg mechanical), while Einstein's gravitational equations affect all forms of resource motion.
Unlike all other field theories, the general theory of relativity has the specific property that it is not necessary to postulate separately (enter "from outside") equation the motion of test particles in a given gravitational field - these equations of motion can be obtained as a consequence of the field equations. Indeed, the application of the law of conservation of energy and momentum (2.6) of a test particle, resulting from Einstein's equations (2.50a), leads to its geodetic motion  described by equation (2.5). Gravity and mechanics are inextricably linked ("unification of gravity and mechanics") in contrast to Newton's theory, where they are completely independent. Similarly, if we have a free electromagnetic field in a vacuum, then from the Einstein equations (on the right side of which is the energy-momentum tensor of the electromagnetic field) of the gravitational field excited by this electromagnetic field, result the Maxwell's equations F ik ; k = 0 of free electromagnetic field; an interesting interpretation of this fact for unitary field theory is in the geometrodynamics of Wheeler and Misner, see Appendix B, §B.3 " Wheeler's Geodynamics. Gravity and Topology. ".
Let us now turn to the question of how to determine the structure of spacetime in given physical situations, ie how to solve Einstein's gravitational equations . The straightforward use of Einstein's equations to determine the evolution of a physical system based on suitable initial conditions - the so-called Cauchy problem - will be outlined in §3.3 " Cauchy problem, causation and horizons " . Einstein's gravitational equations (2.50) are tensor nonlinear partial differential equations of second order, so their general solution is very difficult and in most cases can not be found analytically. There are several ways to simplify the solution :
Weak gravitational field - linearized theory
Appropriate approximations of the general theory of relativity - the so-called linearized theory of gravity - can be used for weak gravitational fields . The linearized gravity theory the Einstein equation and the equation of motion formulate and solve as if spacetime is substantially planar, with only a very small curvature characterizing gravitational phenomena. For sufficiently weak fields and in a suitable (almost inertial) frame of reference, the metric tensor can be expressed in the form
|g ik = h ik + h ik ; | h ik | << 1,||(2.51)|
where h ik is the Minkowski flat space-time
tensor and h ik is a small
"correction" expressing a weak gravitational field. The
components h ik are proportional to the
Newtonian gravitational potential, as we determined for h oo in §2.4, relation (2.27), and for the
other components h we find it below.
We then decompose the nonlinear Einstein's gravitational equations in a series according to the powers h ik, and due to their smallness we can neglect their products and powers without much loss of accuracy and leave only the members of the first order; thus we get the sought linearized equations . Linearized connection coefficients for metric (2.51) are G i kl = (1/2) (h i k, l + h i l, k - h , i kl ) and Ricci's curvature tensor with 1st order precision in h ik has the form R ik = (1/2) (h l i, kl + h l k, il - h l ik, l - h , ik ), where h º h i i = h ik h ik ; while the indices at h ik are "raised" and "triggered" by h ik , not the whole g ik (contributions from h ikthey are second order and neglected). Einstein's equations in this approximation are then
h il, k l + h kl, i l - h ik, l l - h , ik - h ik ( h lm , lm - h , l l ) = (16 p G / c 4 ). T ik .
If we introduce quantities
|y ik = h ik - 1 / 2 h ik h,||(2.52)|
these equations have the form
- y ik, l l - h ik y lm , lm + y il, k l + y ik, l l = (16 p G / c 4 ). T ik .
One can show that, without loss of generality it is possible for the variable yik introduce calibration conditions 
|y ik , k = 0||(2.53)|
analogous to Lorentz calibration conditions A ik , k = 0 in electrodynamics. The linearized Einstein equations then take on a simple form
|- y ik, l l º o y ik = (16 p G / c 4 ). T ik ,||(2.54)|
where oº ¶ 2 / ¶ x 2 - (1 / c 2 ) ¶ 2 / ¶ t 2 is the d'Alembert operator . The general solution of these linearized gravitational equations in Lorentz calibration (2.53) can be expressed in the form of retarded potentials
similarly to electrodynamics, where R = Ö [ a = 1 S 3 (x a - x ' a ) 2 ] is the distance from individual places x' and the source system to the investigated point x a (similar to Fig.1.4a). The retarded solution (2.55) shows that changes in the gravitational field propagate at the speed of light . The significance of this solution for gravitational waves will be discussed in §2.7 " Gravitational waves " (where in the passage " How fast is gravity? ", general questions of the speed of propagation of changes in the gravitational field will be briefly discussed) .
Here we will consider a situation where the gravitational field is excited by a source for which Newtonian physics applies with sufficient accuracy, ie the velocities are small and T oo << | T i a |. Moreover, if found close enough to the source, or if the resource is static (i.e. located in the "inductive t Participative zone" r << CT, where T is a characteristic time changes in the distribution of mass in the source) can be neglected and the retardation solution (2.55 ) has the shape
y oo = - 4 j / c 4 , y o a = 0 , y ab = 0,
where j (t, x a ) = -G. ò (T oo (t, x ' a ) / R) dx' 1 dx ' 2 dx' 3 is the ordinary Newton's potential. In this case, the metric (2.51) is
|ds 2 » - c 2 (1 + 2 j / c 2 ) dt 2 + (1- 2 j / c 2 ) (dx 2 + dy 2 + dz 2 ),||(2.56a)|
The expression (2.27) for the time component of the metric tensor for weak fields is thus supplemented by other components. At distances r substantially greater than the dimensions of the source, approximately R » r can be laid down (2.56a) can be expressed by the total mass M = ò T °° d 3 x of the source system:
This metric is an approximate expression of the Schwarzschild geometry (3.13) derived in §3.4 " Schwarzschild geometry ".
In the more general case, when the velocities at the source of the gravitational field can be large and the components of the stress tensor T ab and the momentum density T ° and can be comparable to the mass-energy density T °°, a weak gravitational field at a sufficient distance from the source can be approximately determine by distributing the retarded potentials (2.55) into a Taylor series according to the powers of x '/ R. In the rest frame of reference with the origin in the center of gravity (ie P a = ò T ° and d 3 x = 0, ò x and T °° d 3 x = 0) then after suitable calibration we get with an accuracy of 1 / r:
where J a = ò e abg x b T g ° d 3 x is the
momentum of the source body.
We will not consider gravitational-wave terms in metrics here, their meaning and properties will be discussed in §2.7 " Gravitational waves ". Here we will mention some gravidynamic effects . In polar coordinates with the polar axis oriented in the direction of the momentum vector J , the external gravitational field of the rotating body will be described by an approximate metric
which is a special case
of general Kerr geometry
(3.37) for a small momentum J (§3.6
" Kerr and Kerr-Newman geometry ") .
In Newton's theory, the gravitational field is given only by the distribution of matter and does not depend at all on the instantaneous velocity of individual parts of the source or on its possible rotation (unless, of course, this leads to changes in the distribution of matter). In GTR, however, the rotation of the source leaves characteristic "traces" in the form of non-diagonal terms *) on the external gravitational field (ie on the space-time metric).
*) This metric cannot be diagonalised without the explicit dependence of the components of the metric tensor on the time t .
These off-diagonal members d j. dt lead to the fact that the body exerts a certain additional force acts (in the geodesic equation to d 2 j / dt 2 ¹ 0 becomes non-zero), which causes entrainment of the local inertial system ( frame dragging ) - entrainment loose bodies rotating the gravitational field in the direction of rotation of the source. It is similar to a sphere rotating in a viscous liquid entraining a liquid near its surface. This phenomenon is called the Lense-Thirring effect according to the authors who first studied it [ 248 ]. For common rotating bodies (macroscopic objects, planets, shining stars, etc.) the effect of entrainment is very small, but it can be crucial for rotating black holes in the so-called ergosphere , as will be shown in §4.4 " Rotating and electrically charged Kerr-Newman black holes " .
|Hydrodynamic analogy of the
influence of the rotation of the source body on the
properties of the excited gravitational field.
Left: In Newton's theory, the gravitational field of a body is given only by the distribution of matter and does not depend at all on its possible rotation (unless it leads to changes in the distribution of matter). Similarly, a smooth and symmetrical body (such as a sphere) rotating in an ideal liquid without viscosity does not cause the liquid to move around it.
Right: In the general theory of relativity, however, the rotation of the source leaves characteristic "traces" on the external gravitational field (on the space-time metric) - local inertial systems are entrained - free bodies are entrained by the rotating gravitational field in the direction of the source rotation.
Similarly, a body rotating in a viscous liquid entrains the liquid near its surface.
For these gravidynamic effects , a certain analogy can be traced with magnetism in electrodynamics. In §1.4 " Analogy between gravity and electrostatics " we have shown that the Newtonian gravitational field is, from the formal point of view of the mathematical description, completely analogous to the electric field. More generally, it can be shown that there are formal analogies between the equations of electromagnetism (Maxwell's equations) and special approximations of Einstein's gravitational equations in GTR. This analogy is called gravitoelectromagnetism - some specific kinematic effects of gravity are analogous to the magnetic effects of moving charges. This is mainly the above-mentioned effect of entraining bodies in the direction of rotation of the source of the gravitational field ( Lense-Thirring effect ) , which is somewhat reminiscent of magnetism. Using special "purpose" transformations, Einstein's gravitational equations can be modified into the form of electromagnetism equations.
From an objective point of view, however, these analogies are only formal , with little physical significance. Phenomena seemingly reminiscent of magnetism are of the second and higher order in comparison with the primary gravitational ("gravistatic") action. Real physical magnetism caused by the interaction of moving "charges" - the sources of field - the gravity is not included ...
Note 1: For the magnetism of gravity could be considered well-known Coriolis force F c = -2 m. [ v ´ w ] , which resemble the magnetic Lorentz force F m = (1 / c) .q. [ v ´ B ] applied perpendicularly with electric charge q moving speed of the magnetic field intensity (induction) B . However, these forces are in fact only a kinematic effect in a rotating frame of reference (angular velocity w ), occurring even in classical Newtonian mechanics ...
In the field of electricity, magnetic phenomena are well manifested in normal laboratory conditions because the electric force effects of positive and negative electric charges are annulled on average, so they do not overlap dynamic magnetic effects. A metal wire (conductor) is generally electrically neutral even when a stream of charged electrons passes through it; the resulting magnetic field can thus exert an undisturbed force on the second (also uncharged) conductor with an electric current.
In the field of gravity, however, the attractive "gravistatic" forces add up, so that in this strong static field, the subtle gravidynamic effects are normally completelycovered by static gravity. Ordinary macroscopic bodies, planets and stars can never move or rotate at high (relativistic) speeds in bound systems, so they can excite strong gravity, but only minimal dynamic effects. Only in compact gravitationally collapsed objects, neutron stars and especially black holes , the rotational motion can be relativistic, as a result of which gravidynamic effects can almost equal gravitational forces and can manifest significant astrophysical effects - §4.4 " Rotating and electrically charged Kerr-Newman black holes "and §4.8" Astrophysical significance of black holes ".
Note 2: In this context, we can also mentionanalogy of electromagnetic and gravitational waves - §2.7 " Gravitational waves ". It is also interesting to note that even in "empty" space without material sources, a source of the global gravitational field appears on the right-hand side of Einstein's equations - Isaacson's tensor of " effective distributed " energy-momentum of gravitational waves (2.76). This is somewhat analogous to how the Maxwell shear current (cf. §1.5, equation (1.34)) excites the magnetic field as well as the current of real electric charges , even in a vacuum without currents for a non-stationary electromagnetic field . However, even this analogy is formal, without a direct connection with magnetism ...
It is not possible therefore, to expect that gravidynamic phenomena could allow, even in principle, some " gravitronics " - the gravitational equivalent of electronics on a cosmic scale!
of verification of the effects of rotation
Despite the slight effect of rotation on the excited gravitational field, however, in the 1960s several experts from the University of Stanford (L. Schiff, G. Pugh, R. Conannon, W. Fairbank, F. Everitt, N. Roman) proposed , but not yet implemented *) , highly sensitive experiments that could demonstrate this effect and measure even in the weak gravitational field of the Earth by accurately monitoring changes in the direction of the rotational axis of the gyroscope - precession, orbiting in polar orbit. Axis of rotation of the flywheel during such a revolution will change due to two effects GTR :
a) The gyroscope orbits in the curved spacetime of the Earth's gravitational field - geodetic effect, connections, change of vector direction during parallel transmission - see §2.4. This effect geodetic precession should be dominant and lead to a change in the axis of rotation in the direction of movement of the probe in orbit by about 6 '' / year.
b) Due to the entrainment by the Earth's rotational momentum, the rotational axis of the gyroscope should show a slight "anomalous" precession - twisting in the Earth's direction of rotation (for a polar orbit perpendicular to the plane of orbit) at an angular velocity proportional to the Earth's momentum and indirectly proportional to the cube radius. The expected value of this anomalous Lense-Thirring precession is only a few hundredths of an angular second per year (approx. 0.04 '' / year for the proposed orbit approx. 600 km above the Earth).
Both of these deviations a), b) are perpendicular to each other. To objectively demonstrate the effect, it is necessary to compare the rotational axes of at least two gyroscopes rotating in opposite directions.
Another possibility is an accurate analysis of the orbit of special satellites **) and an analysis of the orbital dynamics of tight binary pulsars ***).
*) Gravity Probe B
This experiment was at the project stage for a long time. After overcoming a number of technical difficulties and long-term testing (team led by F.Everitt), the Gravity Probe B satellite was launched on April 20, 2004 , carrying in polar orbit at a height of 640 km carrying 4 precision gyroscopes with a diameter of 3.8 cm . Two rotate in one direction, two in the opposite. Their surface is coated with a superconducting layer of niobium. During its rotation, this superconducting layer generates a magnetic field, which is monitored by electromagnetic induction in the so-called SQUID (Superconducting QUantum Interference Device) electronic device, which with high sensitivity (10 -4'') detects the deviation of the axis of rotation of the gyroscope. The case with the gyroscopes is connected to a small pointing telescope focused on the star IM Pegasi, which ensures the reference direction of the rotational axes of the gyroscopes. To increase the sensitivity of the measurement (signal-to-noise ratio), the entire measuring system is built inside a Dewar vessel with 2400 liters of liquid helium, which cools the measuring box to 1.8 °K during an operating time of more than 1,5 year. Data transmitted by the satellite were collected until February 2006.
The high sensitivity of the installed equipment gave hope that during the planned approximately 18 months of measurements, the peculiar very fine dynamic-kinematic effects of the general theory of relativity will be verified with high accuracy. During operation the Gravity Probe B , encoutered difficulties. Disorders from the solar protuberances caused disturbing deviations in the positions of the rotational axes of the gyroscopes, and other noises appeared. From the native data it was not possible to accurately prove the analyzed effects (especially the Lense-Thirring effect). The whole two years took a complicated processing of results - data filtering and removal of various types of faults and noise. The resulting measured values after this treatment were finally: geodetic precession (6601.1 ± 18.3) x10-3 ''/year, compared to the GTR predicted value 6606.1x10-3 ''/year; Lense-Thirring precession (37.2 ± 7.2) x10-3 ''/year, compared to the GTR predicted value of 39.2x10-3 ''/year. However, the excessive complexity of data processing somewhat reduced the validity of the results ..?..
**) Satellite orbit analysis
Both of these subtle "gravidynamic" GTR effects can also be detected with comparable accuracy by measuring the orbit of the Laser Geodynamics Satellite (LAGEOS). This satellite consists of a metal sphere with a diameter of 60 cm, equipped with 426 passive laser mirror reflectors (so-called retro-reflectors). It orbits the Earth in low orbit at an altitude of 5,900 km. Measurements are made using reflections of laser pulses from many ground stations - the time intervals between sending the beam and receiving the reflected pulse are evaluated, which allows you to measure instantaneous distances very accurately. These measurements of the exact positions of the satellite relative to different places on the earth's surface make it possible to measure the shape of the earth's geoid and to study the movements of tectonic plates and terrestrial continents.
However, by measuring the differences between the orbits of many successive orbits, the contribution of the Lense-Thirring effect - the relativistic precession caused by the rotational momentum of the Earth's momentum - can also be determined here. This gravitomagnetic action will change the point of the closest approach of the satellite to the Earth (perigee) by about 3 meters / year. However, there are a number of disturbing influences such as the pressure of solar radiation, the gravitational action of the Moon, tides, geological inhomogeneities, ....
***) Dynamics of binary pulsars
Another possibility is the observation of binary pulsars, here especially PSR J0737 + 3039 and PSR J1757-1854. These two orbiting neutron stars are rapidly rotating massive compact objects at a short distance from each other in a massive gravitational field, so that all generally relativistic effects are much stronger here than in the faint gravitational field around Earth. Measurement of the periastron shift in relation to the moment of inertia of the double pulsar can assess the contribution of the Lense-Thirring effect (gravito-magnetic spin-orbital precession of the periaston). ....... .......
aspects of space-time curvature
In the general theory of relativity, three basic methods of space-time curvature are applied due to different mass-energy distribution and its temporal dynamics :
× Space curvature
× Time deformation
× Rotational motion of space
Variational Derivation of Einstein's Equations of
the Gravitational Field
The culmination of the mathematical structure of every physical theory is the formulation of its laws using Hamilton's variational principle of least action , . This approach consists in constructing a Lagrange function (Lagrangian) L for the investigated physical system such that its integral - action - is extreme for real motion (trajectory, evolution), ie the variation of the action is zero. From this follows the basic Lagrange equations of motion of a given physical systems. The main benefit of the variational method is that it helps to clarify some structural laws of the theory, such as the relationship between the principles of symmetry and conservation laws, the uniqueness of equations of motion and the like.
For the system [source
bodies + excited field], the total quantity of the action can be
considered as the sum of three terms: S = S m + S f + S mf , where S m is the action of the source
bodies (particles), S f is the action of the field
itself and S mf expresses the mutual interaction
of particles with the field. For a physical field in the theory
of relativity, the action is given by the integral of the
Lagrangian L f , which is a function of the
field potentials and their derivatives over the investigated
4-dimensional spacetime region W : S f
= ò L f ( j , j , i ) d W ,, d W
= dtdxdydz. In
relativistic physics, it is also advantageous to write the
quantities S m and S mf in the form of integrals (S m
= ò L m d W over
a 4-dimensional space-time region, ie to use the approach of
continuum physics. When determining the
possible shapes of the Lagrangian, resp. integral of the action,
is based on certain general physical requirements for the
resulting field equations, such as relativistic invariance (general
covariance), resp. linearity (superposition principle), symmetry,
the degree and the like. From the group of
possible Lagrangians thus defined, it is then often selected
according to "aesthetic" criteria of simplicity.
For example, for an electromagnetic field, the described vectors of electric and magnetic field intensities, it follows from the requirement of linearity of field equations (superposition principle) that the Lagrangian must be a quadratic function of the field intensities; the simplest scalar (relativistic invariance) of these properties, created from components of electric and magnetic intensities, is the summation product F ik F ik of the electromagnetic field tensor component (1.114), so the integral of the action for the electromagnetic field has the form S e = k. ò F ik Fik d W . The total Lagrangian of the charged particle system and the electromagnetic field is then  L = - r m c2 + (1/16 p c) F ik F ik + (1 / c 2 ) A i j i . From the variational principle dS = d òL dW = 0, then we can get both zhe equations of motion of particles in the electromagnetic field (if the electromagnetic field is considered as a given and variances particle trajectories) partly Maxwell's electromagnetic field equations (while varying the components 4-pote n tial at the specified distribution and movement of charges).
For the gravitational field in GTR, when the investigated physical system consists of a system of source (material) bodies and an excited gravitational field, the total action will be given by the sum of S = S m + S g , where S m = ò L m (q a , q a , i ) Ö ( -g) d W is the integral of the action of the source part described by the generalized coordinates q a (a = 1,2, ..., N is serial number of the generalized coordinate) a S g = ò L g (g ik ) Ö ( -g) d W is the action of the gravitational field itself described by the components of the metric tensor g ik . The factor Ö ( -g) comes from curvilinear coordinates - it guarantees that the product Ö ( -g) d W behaves as an invariant when integrated over a 4-dimensional volume. The interaction term is not here, because it is implicitly contained in the term S m (by describing the source by physical laws in the curvilinear coordinates of curved spacetime, its interaction with the gravitational field is also expressed). The Lagrangian L g must be a scalar function of the metric tensor g ik and its derivative so that the variations of the resulting field equation contain derivatives not higher than the 1st order. The simples such scalar is a scalar curvature R spacetime (2.24); if we also admit the presence of the constant term C = const., the Lagrangian of the gravitational field could have the shape L g = K + C. In order to obtain the Einstein equation directly with the usual form of constant factors, writes this Lagrangian in the form L g = (c 3 /8 p G) (R - 2 .L ) where G is Newton's gravitational constant and L is the cosmological s constant. The variation principle d S = d (S g + S m ) = 0 at complete variation then after adjustments gives the relation 
is a tensor of energy - momentum of a source. The variation of the metric g ik gives Einstein's equations of the gravitational field
R ik - (1/2) g ik R - L .g ik = (8 p G / c 4 ) T ik ,
while the variation of the source variables qa leads to the equations of motion of the source system (non-gravitational fields):
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