Gravitational time dilation is the relative slowing down of time as a result of being in a stronger gravitational field.
This is described in Einstein's theory of general relativity.
Just like in velocity time dilation under Einstein's theory of special relativity, it's not only clocks that tick slower but the actual passage of time as well. For the person undergoing time dilation thus, things appear normal and he does not perceive that time runs slower in his particular situation.
As for experimental proof of gravitational time dilation, Wikipedia notes:
'Gravitational time dilation has been experimentally measured using atomic clocks on airplanes. The clocks that traveled aboard the airplanes upon return were slightly fast with respect to clocks on the ground. The effect is significant enough that the Global Positioning System needs to correct for its effect on clocks aboard artificial satellites, providing a further experimental confirmation of the effect. Gravitational time dilation has also been confirmed by the Pound-Rebka experiment and by observations of the spectra of the white dwarf Sirius B.'
An astronaut floating high up above the earth is aging faster thus than the rest of us.
The lower one is in the gravity well, the slower his passage of time is. 'The lower, the slower' is a mnemonic some people use.
A non-mathematical proof of gravitational time dilation can be found here.
[Image credit: Wikimedia Commons]
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