# What is the true cause of time dilation? Velocity, momentum or energy?

When learning about relativity, it is said that velocity (an object or a particle traveling close to the speed of light) results in time moving slower for the object moving.

However, there is also gravitational time dilation and the mass-energy equivalence as given by E^2 = (pc)^2 + (m0C^2)^2 which takes momentum into account.

So what is the true cause of time dilation? Is it is velocity? Momentum? Perhaps energy?

As an example, let's say we have a metallic ball with a lot of momentum. It has so much momentum that it would travel at 10% the speed of light. But it is restricted by an equally powerful magnet so that it does not move at all. Would that system have time dilation? My understanding is that it would because there is so much energy in the system, so it would warp spacetime.

• It is energy in all cases, because time and energy are Fourier conjugates. A great insight is that gravitational time dilation equals relativistic time dilation at the escape velocity: en.m.wikipedia.org/wiki/… – safesphere May 20 at 7:49

Time dilation can be caused by as you mention relative speed (as per SR) or gravitational time dilation (as per GR).

According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field.

1. Relative speed SR

SR indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to him, will be measured to tick slower relative to the clock that is at rest.

1. Gravitational time dilation GR