1. Relative Motion

If you move in uniform motion relative to some surface and the surface is perfectly smooth, then you cannot make a physical experiment to find out whether you move or not.

2. Uniform Motion

If you do not have objects of reference, you cannot make a physical experiment to find out whether you move or not.

3. Special Relativity

If two events A and B take place so that A happens after the point in time necessary for light to reach from A one to B, but earlier than the point in time necessary for light to reach from B to A, then you cannot make a physical experiment to find out which event happened earlier and which later.

4. General Relativity

If you do not have objects of reference and experience inertial force, you cannot make a physical experiment to find out whether the force is caused by acceleration or gravitation.

These principles have so beautiful symmetry that it is difficult to imagine that they are not true. However the first one is obviously false because the surface must be made of atoms. Only in an imaginary universe where the matter can be continuous this principle holds. The second is the Galilean invariance. And the last two are Einstein’s.

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