Anyone who’s ever dabbled in physics is familiar with Albert Einstein’s Theory of General Relativity . This famous theory describes gravity as the effect of massive objects on the geometry of space, saying that the space around a massive body is distorted in the same way that the surface of a trampoline is distorted by a bowling ball. The theory includes an interesting concept called space-time. This concept combines the three dimensions of familiar Euclidean space, but it also includes time as a fourth dimension.

Amateur physicists may also be familiar with Einstein’s Theory of Special Relativity. In essence, this theory explains how massive objects traveling near the speed of light would appear to stationary observers. One point of the theory says that the accelerating object is distorted by acceleration. This is known as a Lorentz transformation. The concept of space-time was originally developed to describe the Lorentz transformation in the terms of three-dimensional Euclidean geometry. An imaginary fourth coordinate was needed to quantify the distortion caused by acceleration. This coordinate would represent the dimension of time, and its inclusion meant that the rate of time slowed, or dilated, for accelerating objects.

Hermann Minkowski extended the concept of space-time to show how the equations of Maxwell remained constant under the Lorentz transformation in his 1907 publication entitled "Space and Time." He suggested that space and time should be treated equally, and that events occurred in a single space-time continuum. At this time, Einstein had already begun work on his Theory of General Relativity. However, he still needed a way to describe acceleration inside a gravitational field. He needed to answer the question of how a beam of light would be affected as it passed near a massive object. Furthermore, his explanation would have to appeal to the scientific community, meaning that it would have to be mathematical in nature. Einstein decided to incorporate Minkowski’s space-time concept into his theory. The inclusion of a time coordinate meant that the rate of the passage of time became dilated in the presence of gravity, just as it dilates with acceleration. Additionally, the dilation of time increases with proximity to the gravitational center.

Time dilation has since been experimentally verified. It has been demonstrated that clocks at the base of a mountain run slightly slower than clocks carried to the top, even when the clocks are synchronized beforehand. Perhaps the most famous experiment in time dilation was the Hafele-Keating experiment performed in 1971, in which time dilation was measured not only due to proximity to the center of the Earth but also to motion relative to the Earth’s surface.