Book · Relativity
Page 175 of 212
Author: Albert Einstein
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Suppose we draw lines or stretch strings in all directions from a
point, and mark off from each of these the distance r with a
measuring-rod. All the free end-points of these lengths lie on a
spherical surface. We can specially measure up the area (F) of
this surface by means of a square made up of measuring-rods. If
the universe is Euclidean, then F = 4πr2; if it is spherical,
then F is always less than 4πr2. With increasing values of r, F
increases from zero up to a maximum value which is determined by
the "world-radius," but for still further increasing values of r,
the area gradually diminishes to zero. At first, the straight
lines which radiate from the starting point diverge farther and
farther from one another, but later they approach each other, and
finally they run together again at a "counter-point" to the
starting point. Under such conditions they have traversed the
whole spherical space.
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