Editing Gravity Probe A (section)

===Time dilation===
{{main|Time dilation}}
Time dilation refers to the expansion or contraction in the rate at which time passes, and was the subject of the Gravity Probe A experiment. Under Einstein's theory of general relativity, matter distorts the surrounding [[spacetime]]. This distortion causes time to pass more slowly in the vicinity of a massive object, compared to the rate experienced by a distant observer. The [[Schwarzschild metric]], surrounding a spherically symmetric gravitating body, has a smaller coefficient at <math>dt^2</math> closer to the body, which means slower rate of time flow there. 

There is a similar idea of time dilation occurrence in Einstein's theory of [[special relativity]] (which involves neither gravity nor the idea of curved spacetime). Such time dilation appears in the [[Rindler coordinates]], attached to a uniformly accelerating particle in a flat spacetime. Such a particle would observe time passing faster on the side it is accelerating towards and more slowly on the opposite side. From this apparent variance in time, Einstein inferred that change in velocity affects the [[relativity of simultaneity]] for the particle. Einstein's equivalence principle generalizes this analogy, stating that an accelerating reference frame is locally indistinguishable from an inertial reference frame with a gravity force acting upon it. In this way, the Gravity Probe A was a test of the equivalence principle, matching the observations in the inertial reference frame (of special relativity) of the Earth's surface affected by gravity, with the predictions of special relativity for the same frame treated as being accelerating upwards with respect to free fall reference, which can thought of being inertial and gravity-less.