Time dilation is an observed difference of elapsed time between two observers which are moving relative to each other, or being differently situated from nearby gravitational masses. An observer will see the other observer's clock ticking at slower rate than his/hers. This effect doesn't arise from technical aspects of the clock or the fact that any signal needs time to propagate, but from the nature of space-time described by theory of relativity.
There are two types:
1. Relative velocity time dilation
When two observers are in relative uniform motion and uninfluenced by any gravitational mass, the point of view of each will be that the other's clock is ticking at a slower rate than the local clock. The faster the relative velocity, the greater the magnitude of time dilation. This case is sometimes called special relativistic time dilation. It is often interpreted as time "slowing down" for the other clock. But that is only true from the physical point of view of the local observer, and of others at relative rest . The point of view of the other observer will be that again the local clock is correct and it is the distant moving one that is slow. From a local perspective, time registered by clocks that are at rest with respect to the local frame of reference always appears to pass at the same rate.
2. Gravitational time dilation
There is another case of time dilation, where both observers are differently situated in their distance from a significant gravitational mass, such as the Earth or the Sun. One may suppose for simplicity that the observers are at relative rest. In the simplified case, the general theory of relativity describes how, for both observers, the clock that is closer to the gravitational mass, i.e. deeper in its "gravity well", appears to go slower than the clock that is more distant from the mass. They agree at least that the clock nearer the mass is slower in rate and on the ratio of the difference.
There are two types:
1. Relative velocity time dilation
When two observers are in relative uniform motion and uninfluenced by any gravitational mass, the point of view of each will be that the other's clock is ticking at a slower rate than the local clock. The faster the relative velocity, the greater the magnitude of time dilation. This case is sometimes called special relativistic time dilation. It is often interpreted as time "slowing down" for the other clock. But that is only true from the physical point of view of the local observer, and of others at relative rest . The point of view of the other observer will be that again the local clock is correct and it is the distant moving one that is slow. From a local perspective, time registered by clocks that are at rest with respect to the local frame of reference always appears to pass at the same rate.
2. Gravitational time dilation
There is another case of time dilation, where both observers are differently situated in their distance from a significant gravitational mass, such as the Earth or the Sun. One may suppose for simplicity that the observers are at relative rest. In the simplified case, the general theory of relativity describes how, for both observers, the clock that is closer to the gravitational mass, i.e. deeper in its "gravity well", appears to go slower than the clock that is more distant from the mass. They agree at least that the clock nearer the mass is slower in rate and on the ratio of the difference.
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