What is Time Dilation?

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  • Written By: M. Walker
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 19 January 2020
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Time dilation is a scientific phenomenon that occurs according to the theory of relativity. When two observers are present, each will experience time differently than the other, leading each to believe that the other’s experience of time is in error. Time dilation can be created through either a relative difference in velocity between the two observers, or their difference in distance in relation to a large mass. The first situation is referred to as relative velocity time dilation, and the second is known as gravitational time dilation.

Any time dilation caused by a relative difference in velocity will lead one observer to believe that time is moving slower for the second observer. In this setup, the two observers must be far away from any gravitational masses, and they must be moving at significant velocities for the effect to be significant enough to detect. Higher velocities increase the amount of time dilation experienced for the observers.

Each observer carries a clock and watches the clock of the other observer in relation to his or her own clock. Due to the relative velocities, time appears to dilate, or slow down, in the other clock relative to the local clock. This phenomenon will be experienced by both individuals, and both observers will feel that the other clock is moving slower relative to their own clocks.


Gravitational time dilation is the second type described by relativity. During this situation, two observers are at rest with respect to each other and a gravitational mass. Both observers are situated at different distances from the mass, which are significant enough to allow for the experience of time dilation. The observer who is closer to the mass is experiencing a stronger gravitational pull from the mass relative to the observer who is farther away. The gravitational force of the mass is sometimes referred to as a gravity well, and the first observer is described as deeper in the well than the second observer.

The two observers each carry a clock with them to record time. The first observer, who is nearer to the gravitational mass, will experience the other observer’s clock as faster than his or her own clock, while the second observer will see the first observer’s clock as slower than his or her own clock. Each observer still experiences the local clock to be the correct record of time. Unlike in the relative velocity situation, the observers in this situation agree that one clock is the slower one, while the other is the faster one.


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