How do we know gravity actually affects time?
In: Physics
You’ve already got a good answer here, but you might also be interested in the very first experimental test.
See in the early 20th century Einstein’s theory of relativity was controversial; at that time Newton’s theory of gravity still dominated the scientific field, and it produced different (mathematical) results from Einstein’s. But the instrumentation of the time was not advanced enough to allow the type of minute measurements terrestrial clocks can make today which made testing impossible… until nature provided the means.
On May 29, 1919 a total solar eclipse took place; Britain’s Royal Astronomer had realized that light from certain, known stars would have to pass through the sun’s gravitational field and that during the eclipse they would actually be visible. Since the positions of stars would be known before and after the eclipse, it would be possible to tell whether the sun’s gravitational field was bending light the way Einstein predicted, or not.
So off he sailed to the island of Principe off the western coast of Africa, a part of the world where the sun would be obscured most thoroughly (a sister expedition was dispatched to Brazil.) They measured the light from those stars and returned to England with the results, which conformed to Einstein’s prediction.
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