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The Question
(Submitted March 08, 1997)
Why does time appear to run slower near a gravitational field? I was told that
because light loses energy when it travels away from a gravitational field, so
its frequency goes down, and hence time runs quicker. The part I don't
understand is: WHY does lower frequency of light mean that time runs quicker?
The Answer
General Relativity does indeed predict that time should run slower near a
massive body. This is because there is a relation between the energy of
light and its frequency: the greater the energy, the higher the frequency.
As light travels upward in a gravitational field, it loses energy and so
its frequency goes down. This means that the length of time between one
wave crest and the next goes up. To an observer high up, it would appear that
everything below was taking longer to happen.
This prediction was first tested in 1962 using a pair of very accurate clocks
mounted at the top and bottom of a water tower. The clock at the bottom, i.e.
nearer the Earth, was found to run slower, in exact agreement with General
Relativity.
Hope this answers your question.
Regards,
Laura Whitlock
for Ask a High-Energy Astronomer
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