SIDELIGHTS ON RELATIVITY

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seems impossible to estimate the mean density. But there is another
road, which seems to me more practicable, although it also presents
great difficulties. For if we inquire into the deviations shown
by the consequences of the general theory of relativity which are
accessible to experience, when these are compared with the consequences
of the Newtonian theory, we first of all find a deviation which
shows itself in close proximity to gravitating mass, and has been
confirmed in the case of the planet Mercury. But if the universe
is spatially finite there is a second deviation from the Newtonian
theory, which, in the language of the Newtonian theory, may be
expressed thus:--The gravitational field is in its nature such as
if it were produced, not only by the ponderable masses, but also by
a mass-density of negative sign, distributed uniformly throughout
space. Since this factitious mass-density would have to be enormously
small, it could make its presence felt only in gravitating systems
of very great extent.