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.My original considerations on the subject were based on two hypotheses:(1) There exists an average density of matter in the whole of space which is everywhere the sameand different from zero.(2) The magnitude (" radius ") of space is independent of time.Both these hypotheses proved to be consistent, according to the general theory of relativity, butonly after a hypothetical term was added to the field equations, a term which was not required bythe theory as such nor did it seem natural from a theoretical point of view (" cosmological term ofthe field equations ").Hypothesis (2) appeared unavoidable to me at the time, since I thought that one would get intobottomless speculations if one departed from it.However, already in the 'twenties, the Russian mathematician Friedman showed that a differenthypothesis was natural from a purely theoretical point of view.He realized that it was possible topreserve hypothesis (1) without introducing the less natural cosmological term into the fieldequations of gravitation, if one was ready to drop hypothesis (2).Namely, the original fieldequations admit a solution in which the " world radius " depends on time (expanding space).In thatsense one can say, according to Friedman, that the theory demands an expansion of space.A few years later Hubble showed, by a special investigation of the extra-galactic nebulae (" milkyways "), that the spectral lines emitted showed a red shift which increased regularly with thedistance of the nebulae.This can be interpreted in regard to our present knowledge only in thesense of Doppler's principle, as an expansive motion of the system of stars in the large asrequired, 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-shiftdiscovered by Hubble as an expansion (which can hardly be doubted from a theoretical point ofview), leads to an origin of this expansion which lies " only " about 109 years ago, while physicalastronomy makes it appear likely that the development of individual stars and systems of starstakes considerably longer.It is in no way known how this incongruity is to be overcome.83 Relativity: The Special and General TheoryI further want to rernark that the theory of expanding space, together with the empirical data ofastronomy, permit no decision to be reached about the finite or infinite character of(three-dimensional) space, while the original " static " hypothesis of space yielded the closure(finiteness) of space.Relativity: The Special and General Theory84 [ Pobierz całość w formacie PDF ] |
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