This has been demonstrated by noting that
atomic clocks at differing
altitudes (and thus different gravitational potential) will eventually show different times. The effects detected in such Earth-bound experiments are extremely small, with differences being measured in
nanoseconds. Relative to Earth's age in billions of years, Earth's core is effectively 2.5 years younger than its surface. Demonstrating larger effects would require greater distances from the Earth or a larger gravitational source.
Gravitational time dilation can also equivalently be interpreted as
gravitational redshift: if two
oscillators (attached to
transmitters producing electromagnetic radiation) are operating at different gravitational potentials, the oscillator at the higher gravitational potential (farther from the attracting body) will seem to ‘tick’ faster; that is, when observed from the same location, it will have a higher measured frequency than the oscillator at the lower gravitational potential (closer to the attracting body).
Clocks that are far from massive bodies (or at higher gravitational potentials) run more quickly, and clocks close to massive bodies (or at lower gravitational potentials) run more slowly. For example, considered over the total time-span of Earth (4.6 billion years), a clock set in a geostationary position at an altitude of 9,000 meters above sea level, such as perhaps at the top of
Mount Everest (
prominence 8,848m), would be about 39 hours ahead of a clock set at sea level. This is because gravitational time dilation is manifested in accelerated
frames of reference or, by virtue of the
equivalence principle, in the gravitational field of massive objects.
Consider a family of observers along a straight "vertical" line, each of whom experiences a distinct constant
g-force directed along this line (e.g., a long accelerating spacecraft, a skyscraper, a shaft on a planet). Let be the dependence of g-force on "height", a coordinate along the aforementioned line. The equation with respect to a base observer at is
where is the total time dilation at a distant position , is the dependence of g-force on "height" , is the
speed of light, and denotes
On the other hand, when is nearly constant and is much smaller than , the linear "weak field" approximation can also be used.
Ehrenfest paradox for application of the same formula to a rotating reference frame in flat spacetime.
Outside a non-rotating sphere
A common equation used to determine gravitational time dilation is derived from the
Schwarzschild metric, which describes spacetime in the vicinity of a non-rotating massive
spherically symmetric object. The equation is
is the proper time between two events for an observer close to the massive sphere, i.e. deep within the gravitational field
is the coordinate time between the events for an observer at an arbitrarily large distance from the massive object (this assumes the far-away observer is using
Schwarzschild coordinates, a coordinate system where a clock at infinite distance from the massive sphere would tick at one second per second of coordinate time, while closer clocks would tick at less than that rate),
mass of the object creating the gravitational field,
is the radial coordinate of the observer within the gravitational field (this coordinate is analogous to the classical distance from the center of the object, but is actually a Schwarzschild coordinate; the equation in this form has real solutions for ),
is the escape velocity, expressed as a fraction of the speed of light c.
To illustrate then, without accounting for the effects of rotation, proximity to Earth's gravitational well will cause a clock on the planet's surface to accumulate around 0.0219 fewer seconds over a period of one year than would a distant observer's clock. In comparison, a clock on the surface of the sun will accumulate around 66.4 fewer seconds in one year.
In the Schwarzschild metric, free-falling objects can be in circular orbits if the orbital radius is larger than (the radius of the
photon sphere). The formula for a clock at rest is given above; the formula below gives the general relativistic time dilation for a clock in a circular orbit:
The speed of light in a locale is always equal to c according to the observer who is there. That is, every infinitesimal region of space time may be assigned its own proper time and the speed of light according to the proper time at that region is always c. This is the case whether or not a given region is occupied by an observer. A
time delay can be measured for photons which are emitted from Earth, bend near the Sun, travel to Venus, and then return to Earth along a similar path. There is no violation of the constancy of the speed of light here, as any observer observing the speed of photons in their region will find the speed of those photons to be c, while the speed at which we observe light travel finite distances in the vicinity of the Sun will differ from c.
If an observer is able to track the light in a remote, distant locale which intercepts a remote, time dilated observer nearer to a more massive body, that first observer tracks that both the remote light and that remote time dilated observer have a slower time clock than other light which is coming to the first observer at c, like all other light the first observer really can observe (at their own location). If the other, remote light eventually intercepts the first observer, it too will be measured at c by the first observer.
Gravitational time dilation in a gravitational well is equal to the
velocity time dilation for a speed that is needed to escape that gravitational well (given that the metric is of the form , i. e. it is time invariant and there are no "movement" terms ). To show that, one can apply
Noether's theorem to a body that freely falls into the well from infinity. Then the time invariance of the metric implies conservation of the quantity , where is the time component of the
4-velocity of the body. At the infinity , so , or, in coordinates adjusted to the local time dilation, ; that is, time dilation due to acquired velocity (as measured at the falling body's position) equals to the gravitational time dilation in the well the body fell into. Applying this argument more generally one gets that (under the same assumptions on the metric) the relative gravitational time dilation between two points equals to the time dilation due to velocity needed to climb from the lower point to the higher.
^A. Einstein, "Über das Relativitätsprinzip und die aus demselben gezogenen Folgerungen", Jahrbuch der Radioaktivität und Elektronik 4, 411–462 (1907); English translation, in "On the relativity principle and the conclusions drawn from it", in "The Collected Papers", v.2, 433–484 (1989); also in H M Schwartz, "Einstein's comprehensive 1907 essay on relativity, part I", American Journal of Physics vol.45,no.6 (1977) pp.512–517; Part II in American Journal of Physics vol.45 no.9 (1977), pp.811–817; Part III in American Journal of Physics vol.45 no.10 (1977), pp.899–902, see
parts I, II and III.