Relativity - The Special and General Theory Part 12

Notes

*) Especially since the next planet Venus has an orbit that is almost an exact circle, which makes it more difficult to locate the perihelion with precision.

The displacentent of spectral lines towards the red end of the spectrum was definitely established by Adams in 1924, by observations on the dense companion of Sirius, for which the effect is about thirty times greater than for the Sun. R.W.L. -- translator

APPENDIX IV

THE STRUCTURE OF s.p.a.cE ACCORDING TO THE GENERAL THEORY OF RELATIVITY (SUPPLEMENTARY TO SECTION 32)

Since the publication of the first edition of this little book, our knowledge about the structure of s.p.a.ce in the large (" cosmological problem ") has had an important development, which ought to be mentioned even in a popular presentation of the subject.

My original considerations on the subject were based on two hypotheses:

(1) There exists an average density of matter in the whole of s.p.a.ce which is everywhere the same and different from zero.

(2) The magnitude (" radius ") of s.p.a.ce is independent of time.

Both these hypotheses proved to be consistent, according to the general theory of relativity, but only after a hypothetical term was added to the field equations, a term which was not required by the theory as such nor did it seem natural from a theoretical point of view (" cosmological term of the field equations ").

Hypothesis (2) appeared unavoidable to me at the time, since I thought that one would get into bottomless speculations if one departed from it.

However, already in the 'twenties, the Russian mathematician Friedman showed that a different hypothesis was natural from a purely theoretical point of view. He realized that it was possible to preserve hypothesis (1) without introducing the less natural cosmological term into the field equations of gravitation, if one was ready to drop hypothesis (2). Namely, the original field equations admit a solution in which the " world radius " depends on time (expanding s.p.a.ce). In that sense one can say, according to Friedman, that the theory demands an expansion of s.p.a.ce.

A few years later Hubble showed, by a special investigation of the extra-galactic nebulae (" milky ways "), that the spectral lines emitted showed a red s.h.i.+ft which increased regularly with the distance of the nebulae. This can be interpreted in regard to our present knowledge only in the sense of Doppler's principle, as an expansive motion of the system of stars in the large -- as required, according to Friedman, by the field equations of gravitation. Hubble's discovery can, therefore, be considered to some extent as a confirmation of the theory.

There does arise, however, a strange difficulty. The interpretation of the galactic line-s.h.i.+ft discovered by Hubble as an expansion (which can hardly be doubted from a theoretical point of view), leads to an origin of this expansion which lies " only " about 10^9 years ago, while physical astronomy makes it appear likely that the development of individual stars and systems of stars takes considerably longer. It is in no way known how this incongruity is to be overcome.

I further want to rernark that the theory of expanding s.p.a.ce, together with the empirical data of astronomy, permit no decision to be reached about the finite or infinite character of (three-dimensional) s.p.a.ce, while the original " static " hypothesis of s.p.a.ce yielded the closure (finiteness) of s.p.a.ce.

K = co-ordinate system x, y = two-dimensional co-ordinates x, y, z = three-dimensional co-ordinates x, y, z, t = four-dimensional co-ordinates

t = time I = distance v = velocity

F = force G = gravitational field