SIDELIGHTS ON RELATIVITY

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of magnitude.

It appears less problematical to extend the ideas of practical
geometry to spaces of cosmic order of magnitude. It might, of course,
be objected that a construction composed of solid rods departs more
and more from ideal rigidity in proportion as its spatial extent
becomes greater. But it will hardly be possible, I think, to assign
fundamental significance to this objection. Therefore the question
whether the universe is spatially finite or not seems to me
decidedly a pregnant question in the sense of practical geometry.
I do not even consider it impossible that this question will be
answered before long by astronomy. Let us call to mind what the
general theory of relativity teaches in this respect. It offers
two possibilities:--

1. The universe is spatially infinite. This can be so only if the
average spatial density of the matter in universal space, concentrated