# Can anyone explain the theory of relativity in simple terms?

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Relativity completely changed our physics?

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We used to think that the universe was three dimensional space. We also thought that time ticked at a universal rate. Relativity changed all that. It can be described in simple terms by a series of thought experiments.

Think of two spacecraft travelling apart at a constant velocity which is say 90% of the speed of light. An observer on either spacecraft can say that the other spacecraft is moving away at 90% of the speed of light. There is no concept of absolute velocity. Everything is moving relative to something else.

Each observer sees time as ticking at the same rate. Each observer sees time passing more slowly on the other spacecraft. Each observer sees the other spacecraft's length contracted in the direction of travel. Also, each observer sees the other spacecraft getting more massive the faster it moves away.

Einstein showed that gravity and acceleration are indistinguishable. In fact gravity slow time.

Relativity says that space and time are not separate. We live in four dimensional spacetime. Matter makes spacetime curve. Time doesn't exist as such. We move along paths in spacetime always moving forward in the time dimension.

General relativity can be summed up in two sentences. Mass, energy and momentum tell spacetime how to curve. Curved spacetime tells matter how to move. Gravity is not a force, it is simply to effects of curved spacetime.

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