The Postulates of Special Relativity

The Special Theory of Relativity is founded on two postulates:

1. All the laws of mechanics and electrodynamics are the same in any inertial reference frame .

2. The speed of light is independent of the velocity of the source.

What does it mean?

Imagine you are in a room with no windows. Roughly speaking, the first postulate means that you can't tell if you are moving at a constant velocity without looking outside the room. Both the observer in the "lab" and the one in the rocket will get the same results when they perform experiments. This is true even if the experiments involve light.

The second postulate means that a moving source of light doesn't make the light move faster that a source at rest (with respect to any observer). In other words, the speed of the source doesn't add to the speed of light.

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