Physical consequences

In 1907, Einstein was still eight
years away from completing the
general theory of relativity.
Nonetheless, he was able to
make a number of novel,
testable predictions that were
based on his starting point for
developing his new theory: the
equivalence principle.[7]
The gravitational redshift of a
light wave as it moves upwards
against a gravitational field
(caused by the yellow star below)
The first new effect is the
gravitational frequency shift of
light. Consider two observers
aboard an accelerating rocket-
ship. Aboard such a ship, there is
a natural concept of "up" and
"down": the direction in which the
ship accelerates is "up", and
unattached objects accelerate in
the opposite direction, falling
"downward". Assume that one of
the observers is "higher up" than
the other. When the lower
observer sends a light signal to
the higher observer, the
acceleration causes the light to
be red-shifted, as may be
calculated from special
relativity; the second observer
will measure a lower frequency
for the light than the first.
Conversely, light sent from the
higher observer to the lower is
blue-shifted, that is, shifted
towards higher frequencies.[8]
Einstein argued that such
frequency shifts must also be
observed in a gravitational field.
This is illustrated in the figure at
left, which shows a light wave
that is gradually red-shifted as it
works its way upwards against
the gravitational acceleration.
This effect has been confirmed
experimentally, as described
below.
This gravitational frequency
shift corresponds to a
gravitational time dilation: Since
the "higher" observer measures
the same light wave to have a
lower frequency than the "lower"
observer, time must be passing
faster for the higher observer.
Thus, time runs more slowly for
observers who are lower in a
gravitational field.
It is important to stress that,
for each observer, there are no
observable changes of the flow
of time for events or processes
that are at rest in his or her
reference frame. Five-minute-
eggs as timed by each
observer's clock have the same
consistency; as one year passes
on each clock, each observer
ages by that amount; each
clock, in short, is in perfect
agreement with all processes
happening in its immediate
vicinity. It is only when the clocks
are compared between separate
observers that one can notice
that time runs more slowly for
the lower observer than for the
higher.[9] This effect is minute,
but it too has been confirmed
experimentally in multiple
experiments, as described below.
In a similar way, Einstein
predicted the gravitational
deflection of light: in a
gravitational field, light is
deflected downward.
Quantitatively, his results were
off by a factor of two; the
correct derivation requires a
more complete formulation of
the theory of general relativity,
not just the equivalence principle.