From the conceptual stand-point there are several important consequences
of the Generalised or Gravitational Theory of Relativity. Physical
space-time is perceived to be intimately connected with the actual local
distribution of matter. Euclid-Newtonian space-time is _not_ the actual
space-time of Physics, simply because the former completely neglects the
actual presence of matter. Euclid-Newtonian continuum is merely an
abstraction, while physical space-*time is the actual framework which
has some definite curvature due to the presence of matter. Gravitational
Theory of Relativity thus brings out clearly the fundamental distinction
between actual physical space-time (which is non-isotropic and
non-Euclid-Newtonian) on one hand and the abstract Euclid-Newtonian
continuum (which is homogeneous, isotropic and a purely intellectual
construction) on the other.
The measurements of the rotation of the earth reveals a fundamental
framework which may be called the “inertial framework.” This constitutes
the actual physical universe. This universe approaches Galilean
space-time at a great distance from matter.
The properties of this physical universe may be referred to some
world-distribution of matter or the “inertial framework” may be
constructed by a suitable modification of the law of gravitation itself.
In Einstein’s theory the actual curvature of the “inertial framework” is
referred to vast quantities of undetected world-matter. It has
interesting consequences. The dimensions of Einsteinian universe would
depend on the quantity of matter in it; it would vanish to a point in
the total absence of matter. Then again curvature depends on the
quantity of matter, and hence in the presence of a sufficient quantity
of matter space-time may curve round and close up. Einsteinian universe
will then reduce to a finite system without boundaries, like the surface
of a sphere. In this “closed up” system, light rays will come to a focus
after travelling round the universe and we should see an “anti-sun”
(corresponding to the back surface of the sun) at a point in the sky
opposite to the real sun. This anti-sun would of course be equally large
and equally bright if there is no absorption of light in free space.
In de Sitter’s theory, the existence of vast quantities of world-matter
is not required. But beyond a definite distance from an observer, time
itself stands still, so that to the observer nothing can ever “happen”
there. All these theories are still highly speculative in character, but
they have certainly extended the scope of theoretical physics to the
central problem of the ultimate nature of the universe itself.
One outstanding peculiarity still attaches to the concept of electric
force—it is not amenable to any process of being “transformed away” by a
suitable change of framework. H. Weyl, it seems, has developed a
geometrical theory (in hyper-space) in which no fundamental distinction
is made between gravitational and electrical forces.
Einstein’s theory connects up the law of gravitation with the laws of
motion, and serves to establish a very intimate relationship between
matter and physical space-*time. Space, time and matter (or energy) were
considered to be the three ultimate elements in Physics. The restricted
theory fused space-time into one indissoluble whole. The generalised
theory has further synthesised space-time and matter into one
fundamental physical reality. Space, time and matter taken separately
are more abstractions. Physical reality consists of a synthesis of all
three.