Reference: Einstein’s 1920 Book
Section XXII (Part 2)
A Few Inferences from the General Theory of Relativity
Please see Section XXII at the link above.
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Summary
Gravitational field exists along with the fact of acceleration. We may now study its influence theoretically. We notice that a body at rest, or with uniform motion otherwise, would be accelerating in this gravitational field. In general it would have a curvilinear motion instead of a straight line motion. This would then apply to a ray of light as well when it passes through a gravitational field.
Theoretical calculations show that the light from stars that reach earth would acquire a curvature of 1.7 seconds of arc as they pass the sun at a grazing incidence. This would mean that the velocity of light must also change as it passes through a gravitational field. We then conclude that the results of the special theory of relativity hold only so long as we are able to disregard the influences of gravitational fields on light.
We may consider the special theory of relativity to be contained within the general theory of relativity as a limiting case. The general theory of relativity also tells us about the laws satisfied by the gravitational field itself.
We may visualize space-time domains where no gravitational fields exist as Galilean domains. The general theory of relativity considers those space-time domains where the gravitational field is present. We hope that the general theory of relativity leads to laws that are applicable to all gravitational fields. This will extend our ideas of the space-time continuum still farther.
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Final Comments
The most natural condition in this universe is the presence of gravitational fields. The general theory of relativity considers the effect of this condition on the space-time continuum.
The space-time continuum essentially describes the continuum of energy in which the matter is floating. Our knowledge of this energy extends to the electromagnetic spectrum. The flexible structure of this electromagnetic spectrum manifests in the form of gravitational fields. The gravitational fields are expected to be made up of curvilinear motions in the sea of energy.
The gravitational fields may be visualized as rotating motions much like whirlpools in the sea of energy. The substance in gravitational fields accelerates toward a center, where it collects and condenses.
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