This theory comprises of two theories, relativity and general relativity.
- measurement  of various quantities in relation to velocities of observers.
- time and space are considered together in relation .
- the speed of light is same for all observers.
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The mass and dimensions of physical objects depends on the relative velocities of objects.  Even time duration depends on the velocity of the objects.
 The mass of an object at rest is called inertial mass.  As the velocity of an object increases its mass also increases.

These changes do not become significant unless they move with a speed close to the speed of light, like 1/100th or 1/10th of c.

The speed of an object cannot be more than the speed of light.  Speed of light is same in any reference system , inertial or moving.

These principles are very much applicable for small particles like electrons, photons, protons, neutrons etc., as they move with  high speeds.

Theory of relativity is proposed by A Eistein.  There is a general theory of relativity and special theory of relativity.

According to special theory,  moving clocks tick slower, no object can move faster than speed of light, objects become shorter in the direction of object's velocity.
mass energy equivalence is also from the theory of relativity.

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