go straight instead; it is
impossible to distinguish
between a reference
frame that is moving at a
constant velocity and one
that is not moving, and
clearly, in a motionless
elevator, light would not
bend.In almost all circumstances, both Newton’s and Einstein’s descriptions of gravity
give similar results. However, very precise measurements, especially in close
proximity to a very massive object, show deviations and agree with Einstein’s
general relativity exactly. One field in which general relativity has had particularly
special importance is astrophysics. Since stars, galaxies, and other astronomical
objects often have very large amounts of mass, the bending of spacetime is
significant in the vicinity of these objects. For example, the orbit of Mercury, the
closest planet to the Sun, deviates measurably (if slightly) from the ellipse
predicted by Newtonian gravitation. The orbit precesses, supporting Einstein’s
theory. Light passing near the Sun bends measurably, as predicted by Einstein and
measured originally by Arthur Eddington in 1919. Very precise clocks have
measured gravitational time dilation by detecting differences in time near the
surface of Earth and far above it.
Furthermore, very massive objects may have so much gravitational pull that the
velocity required to escape from their attraction is greater than the speed of light. In
general relativity, because light is subject to the curvature of space that affects
matter as well, light cannot escape from these objects either, and as a result, they
are called black holes. They are black because they emit no light, nor can they even
reflect light. They cannot be seen directly, but their gravitational effects on nearby
objects can be quite dramatic and they can be detected thereby. For example,
quasars are extremely bright but extremely distant objects that were unexplained in
astronomy for a very long time, but the best explanation now is that they are caused
by black holes that are pulling matter in at extremely high speeds. The matter rubs
against other falling in matter, and the frictional effects generate light. They
eventually attract all the nearby matter and run out of fuel, in effect, so that they stop
generating light, and this is the reason that the only ones that we can see are very
distant: Very distant in astronomy means that it takes the light a very long time to
reach the earth, so astronomers are seeing light emitted a long time ago, and
therefore they have a picture of the quasar in the very distant past. By now, quasars
have likely stopped emitting light, but they won’t wink out in our sky until many
years from now, because light they emitted long ago is still traveling to us.
Another landmark discovery in astronomy in the 20th century, made by Edwin
Hubble, was that galaxies outside our own send light to us that appears redshifted;