Post-Newtonian approximation of the field theory of gravitation

15. 16. 17. 18. 19. 20.

P. K. Rashevskii, Riemannian Geometry and Tensor Analysis [in Russian], Nauka, Moscow (1967). V. A. Fock, The Theory of Space, Time, and Gravitation, Pergamon (|964). L. P. Eisenhart, Continuous Groups of Transformations [Russian translation], Gostekhizdat, Moscow (1947). A. Einstein, Collection of Scientific Works [Russian translation], Vol. I, Nauka, Moscow (1965). J. Barnes, "Lagrangian theory for second-rank tensor fields," J. Math. Phys., 6, No. 5, 788-794 (1965). C. Fronsdal, "On the theory of higher spin fields," Nuovo Cimento, 2, No. 2, 4~6-443

(1958). 21. 22. 23. 24.

H. Minkowski, "Raum und Zeit," Phys. Z., 10, 104-118 (1909). V. I. Ogievetsky and I. V. Polubarinov, "Interacting fields of spin 2 and the Einstein equations," Ann. Phys., 35, No. 2, 167-208 (1965). N. Rosen, "A bimetric theory of gravitation," Gen. Rel. Gravit., 4, No. 6, 435-447 (1973). C. M. Will, "Experimental disproof of a class of linear theories of gravitation," Astrophys. J., 185, 31-42 (1973). CHAPTER 3 DESCRIPTION OF GRAVITATIONAL EFFECTS IN THE FIELD THEORY OF GRAVITAITON

16.

Post-Newtonian Approximation of the Field Theory of Gravitation

The first question which any theory of gravitation should answer is the question of the correspondence between its predictions and the results of available gravitational experiment. Until recently the requirements on possible theories of gravitation reduced to the necessity of obtaining Newton's law of gravitation in the weak-field limit and also the description of the three effects accessible to observation: the gravitational red shift in the field of the sun, the curving of a light ray passing near the sun, and the displacement of the perihelion of Mercury. Thus, the available requirements on possible theories of gravitation were clearly insufficient, since a large number of theories satisfied them. Formulation of qualitatively new experiments was required for further choice of gravitational theories. At the present time, in connection with the development of experimental technology, primarily cosmonautics, and the increase in the accuracy of measurements, new possibilities have appeared regarding more precise measurement of the orbital parameters of planets (primarily the moon), measurement of the retardation of radio signals in the gravitational field of the sun, and performance of new experiments within the solar system. These experiments make it possible to further restrict the circle of viable theories of gravitation~ Nordtvedt and Will [36] developed a formalism, called the parametrized post-Newtonian formalism to facilitate comparison of results of experiments performed within the solar system with predictions of various metric theories of gravitation (i.e., theories of gravitation according to which the action of a weak gravitational field on all physical process except gravitational processes is realized by a metric tensor of Riemannian space--time). In this formalism the metric of Riemannian space--time created by some body consisting of an ideal fluid is written as the sum of all possible generalized gravitational potentials with arbitrary coefficients called post-Newtonian parameters. Using these parameters of Will-Nordtvedt, the metric of Riemannian space--time can be written in the form goo = 1 -- 2U + 2~U 2 - (2~ + 2 + ~ + ~t) O1 + ~,A +

+ 2~+ (~

2 [(3v + 1 - 2~ + ~2) o2 + (1 + ~3)o~ + 3 (v + ~4)~d + - az - ~3) w ~U + ~ 2 w ~ w F u ~ - (2~3- ~ ) w ~ v ~ ;

( 16.1 )

1769

where w ~ are the spatial components of the velocity of the reference system relative to some universal rest system. For some theories of gravitation this is the velocity of the center of mass of the solar system relative to the rest system of the universe. The generalized gravitational potentials have the form

Po dV; i --~

U=

R=lr-r'l,

__>

~ po~dv'Ov

(16.2)

~---

Wo~= f

J

dV;

pov.vRVRo~ R~ dV;

~ i~ ( x ~ I r--r"

x'~)

[

~

__,

U~f~z I PoR~Rf~ R~ dV; . . . . I r'--r" I

> i d3r'd3r ", ] r--r" I

where 00 is the invariant density of the mass of the body; v ~ are the components of the three-dimensional velocity of elements of the ideal fluid; p is the isotropic pressure, P0H is the density of internal energy of the ideal fluid; R ~ = x ~ -- x '~. To each metric theory of gravitation there corresponds a collection of values of the 10 parameters

T h e r e f o r e , f r o m t h e p o i n t o f v i e w o f e x p e r i m e n t s c a r r i e d o u t i n t h e s o l a r s y s t e m one theory of gravitation w i l l d i f f e r f r o m a n o t h e r o n l y by t h e v a l u e s o f t h e s e p a r a m e t e r s . To d e t e r m i n e t h e o r i e s o f g r a v i t a t i o n w h i c h i n t h e p o s t - N e w t o n i a n l i m i t make i t p o s s i b l e t o d e scribe experiments carried out in the solar system it suffices to determine from these experiments the values of the 10 post-Newtonian parameters and to select only those theories of gravitation whose post-Newtonian approximation leads to values of the parameters that coincide with those obtained from experiments. All such theories of gravitation will then be indistinguishable from the point of view of any experiments carried out with post-Newtonian accuracy. Further selection of a theory of gravitation adequate to reality is connected with an increase of the accuracy of measurements above the post-Newtonian level or with the search for possibilities of studying the properties of gravitational waves and also phenomena in strong gravitational fields. We shall determine the collection of values of the post-Newtonian parameters sponding to the field theory of gravitation. The equations of the gravitational

field in this theory have the form

E]2 f n m : - - 16~Jnm; If we use notation sion:

(13.11),

corre-

[] : a~Oi.

(16.3)

then for the tensor current jnm we obtain the following expres-

Jnm : ~]~nm ~ OnOihmi__ OmOihnt.Jf_ynmOiOlhli"

(16.4)

Following Fock [11], to construct post-Newtonian approximations valid in the solar system, we consider a problem of astronomic type. We shall assume that the components of the energy--momentum tensor of matter are equa ! to zero throughout space except for certain regions. Within each such region the energy--momentum tensor of matter should correspond to the model of an ideal fluid we adopt and should satisfy a covariant conservation equation in Riemannian space--time. Aside from the physical properties of the model of celestial bodies, the energy--momentum tensor of matter will also depend on the metric of Riemannian space--time. Therefore, construction of the energy--momentum tensor of matter and the determination of the metric tensor of Riemannian space--time must be accomplished together. We use the circumstance that within the limits of the solar system the maximal values of the gravitational potential, the square of the characteristic velocity v 2 (the velocity 1770

of celestial bodies relative to the center of mass of the solar system), the specific pressure p/P0, and the specific internal energy ~ have approximately the same order of smallness ~2 where ~ ~ 10 -3 is a dimensionless parameter. Therefore, in the solar system we have the following estimates:

U=O(~D; v~O(~), n = 0 &);

P = 0 p~

(16.5)

&-).

Moreover, we shall consider the field in the near zone, i.e., at distances from the sun considerably less than the length of a gravitational wave radiated by objects in the solar system which are moving with characteristic speed v ~ ~:R/% ~ R(3/3t) ~ g. In this case the change of all quantities with time is due primarily to the motion of matter. Therefore, the partial derivatives with respect to time are small as compared to the partial derivatives with respect to coordinates

~ =0(~),

o

(16.6)

Ox ~ 9

The problem of the joint determination of the energy--momentum tensor of matter and of the metric tensor of Riemannian space--time we shall solve in successive stages each of which corresponds to expansion of the exact equations of the problem in powers of the dimensionless parameter ~. We have the following exact relations: ideal fluid

the density of the energy-moment

tensor of the

T'~'~----] / - - g [(p--k $) t t ' ~ u ~ - - P g ~ m ] ,

(16.7)

the covariant equation of continuity

i

a

[f---fp0w]=0

(16 8)

and the conservation equation of the density of the energy--momentum tensor of matter in Riemannian space--time

FmT =0, where

$

(16.9)

is the total energy density of the ideal fluid.

For our purposes it is more convenient to write the equations of the gravitational (16.3) and the equation of minimal coupling (14.3) in the form

field (16.1'0)

[~2%nm=--16~Anm,

\

(16 11)

where we have introduced the notation

Zn==fnm--89 A~=D

% = % n n,

(16.12)

[h~-89y~m&~]-- a"Gh~z--o~aA~.

(16.13)

We shall expand all quantities contained in Eqs. (16.9)-(16.12) in series in the small parameter ~. If we neglect energy losses to the radiation of gravitational waves, then these expansions must also be valid when the sign of the time is reversed. In reversing the sign of the time, i.e., under the coordinate transformation x '~ = --x~ the components v ~, • A0~, g0~, 3/3x ~ change their sign to the opposite sign. Since v ~ g and 3/3x ~ ~ e, when the sign of the time is reversed the dimensionless parameter ~ also changes sign. From this it follows that if energy losses to the radiation of gravitational waves are neglected the expansions of the components v ~, X ~ T~ A ~ contain only odd powers of the parameter e. The expansions of the tensor current A nm and the field X nm we write O)

(2)

x " ~ = x ~ + x ~ + 999 (o)

(i)

A=~ = A ~ ' } - A ~ i f - . . -

in the form

(16.14) ( 16.15 )

1771

(o)

0)

where the components of the zeroth A n~ the following order of smallness:

~o~=0(0;

,

first

A n~

(o)

AOO=O(1);

(1)

(2) ,

and second

approximations have

O)

~"~=-0(1);

(1)

A n~

A~

(2)

(16.16)

A*~ = O (e~); AOO=O (e2); A~ = O (e~); (2) (2) A o o = O (~4); A=~--~ O (~4).

With consideration of expansions (16.14), (16.15) and estimate (16.6) we can rewrite the equations of the gravitational field (16.10) in the form of a series of successive approximat ions :

(t) (o) A2%n,n= __ 16nA~m; (2) (1) ~2 0) A2%n m = - - 1 6 h A n m + 2 ~ t ~AXrim. From expressions

(16.17)

(16.18)

(13.11) and (16.11) we obtain

h"m=Tnm+~lT~izi~+T~izt~

b, + b= ^.Tn,n + 4

]

4

(16.19)

ynmTslYsi"

For the tensor current A nm we then have

A(o)nm -~- - - A A"n'O)

= ~.a* [ r [(o),nz - - ~-1

(0) ] F(O) ;j [T.m-- 21yn,nTstysi-

ra(~

1 n FmTsiO) 1 0). b, (0) (1) (0) O) (16 .T v,,]+o ( ~,(0) )+o_m (rO)nj (0)s,..)--A~[(~)nm---fy'mTs'v,,+-y-(T'nx,~+T"%/9-

b,+b2 ~) (I)

(o) (1) Y ntnTs~

(1) (o).

/b~

(0)

.20)

] J

where A = - - 3 a ~ a. To find the post-Newtonian parameters it suffices for us to determine the coefficients gas to accuracy g2, the components g0~ to accuracy ~3, and the component g00 to accuracy ~4. It follows from the coupling equations (16.11) that for this it is necessary to determine the components of the field XaB up to E 2, X ~ up to ~3, and X ~176up to e 4. In the initial approximation we assume that the metric tensor of Riemannian space--time coincides with the metric tensor of pseudo-Euclidean space--time, i.e., we neglect gravitational forces. Equations (16.8) and (16.9) then take the form

o-~ (og ') =O (82), OnT"O = 0 (e3), Consider

the

estimates

(16.5),

from these

T ~ = p 0 [ 1 +O(82)1, Therefore,

the

components

of the

tensor

O,~T"~ = 0 (82).

equations

we h a v e

T(~=poO(e2), current

T~

~[1 +O(e~)].

Anm i n z e r o t h

approximation

c a n be w r i t t e n

in the form

(o) 1 AOO = - - ~ A9o; From Eq.

(,~oo~

- - - - A (poV~);

1 __o. A " =-~- y""a9o.

(~

( 1 6.21 )

(16.17) we then obtain

(16.22) As a result, the components of the metric tensor of Riemannian space--time (16.11) in first approximation can be written in the form

1772

goo = 1 - - i U + O (84); g0~ = 4 V ~ [1 + O (sz)];

(16.23)

Knowledge o f t h e m e t r i c i n t h i s a p p r o x i m a t i o n makes i t p o s s i b l e t o d e t e r m i n e t h e comp o n e n t s o f t h e energy-momentum t e n s o r o f m a t t e r i n t h e n e x t a p p r o x i m a t i o n . Using t h e e x p r e s s i o n s ( 1 6 . 2 3 ) , we f i n d t h a t - - g = 1 + 2 U -t- 0

@4),

/~0 = 1 + U - - v i, =~ v

Fo

OU . 00=--~-+o