Hafele – Keating – Experiment

At the beginning of the 20^th century Albert Einstein created the theory of relativity.
One of his theory said, that gravity expands space and time. 
As a result, two very accurate watches should measure a difference, if one watch is closer to a gravity source than the other one.

In the experiment two atomic watches were used to approve Einstein´s theory. Originally the two atomic watches were used, which worked very similar. During the experiment, one watch was placed on the ground an the other one was carried on a plane. 
After the experiment the two watches had difference of 39ns. So, the first approve of Einstein´s theory was realised.