Time Dilation

Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity, a consequence of special relativity, or a difference in gravitational potential between their locations due to gravitational time dilation. When unspecified, "time dilation" usually refers to the effect due to velocity. The dilation compares co-moving clock readings between events measured in different inertial frames and is not observed by visual comparison of clocks across moving frames.
These predictions of the theory of relativity have been repeatedly confirmed by experiment, and they are of practical concern, for instance in the operation of satellite navigation systems such as GPS and Galileo.

Invisibility
Time dilation is a relationship between clock readings. Visually observed clock readings involve delays due to the propagation speed of light from the clock to the observer. Thus there is no direct way to observe time dilation. As an example of time dilation, two experimenters measuring a passing train traveling at .86 light speed may see a 2 second difference on their clocks while on the train the engineer reports only one second elapsed when the experimenters went by. Observations of a clock on the front of the train would give completely different results: the light from the train would not reach the second experimenter only 0.27s before the train passed. This effect of moving objects on observations is associated with the Doppler effect.

History

Time dilation by the Lorentz factor was predicted by several authors at the turn of the 20th century. Joseph Larmor (1897) wrote that, at least for those orbiting a nucleus, individual electrons describe corresponding parts of their orbits in times shorter for the [rest] system in the ratio:





1
−



v

2



c

2








{\textstyle {\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}

. Emil Cohn (1904) specifically related this formula to the rate of clocks. In the context of special relativity it was shown by Albert Einstein (1905) that this effect concerns the nature of time itself, and he was also the first to point out its reciprocity or symmetry. Subsequently, Hermann Minkowski (1907) introduced the concept of proper time which further clarified the meaning of time dilation.

Time dilation caused by a relative velocity…