SIDELIGHTS ON RELATIVITY

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reduced to action and reaction between bodies.  From the equations
of the general theory of relativity it can be deduced that this
total reduction of inertia to reciprocal action between masses--as
required by E. Mach, for example--is possible only if the universe
is spatially finite.

On many physicists and astronomers this argument makes no impression.
Experience alone can finally decide which of the two possibilities
is realised in nature. How can experience furnish an answer? At first
it might seem possible to determine the mean density of matter by
observation of that part of the universe which is accessible to our
perception. This hope is illusory. The distribution of the visible
stars is extremely irregular, so that we on no account may venture
to set down the mean density of star-matter in the universe as
equal, let us say, to the mean density in the Milky Way. In any
case, however great the space examined may be, we could not feel
convinced that there were no more stars beyond that space. So it