The Michelson-Morley experiment had left only one way out, the most radical: giving up the Galilean transformations. In 1905 Einstein cut the Gordian knot not with an ad hoc hypothesis about the ether, but with two simple, extremely general postulates, elevating to a principle precisely the two facts that seemed to be in contradiction.
Principle — The postulates of special relativity
- Principle of relativity. The laws of physics (including Maxwell’s equations) are the same in all inertial reference frames.
- Constancy of the speed of light. The speed of light in vacuum, , is the same in all inertial reference frames, regardless of the motion of the source or the observer.
The stroke of genius lies in taking both statements as true at the same time, instead of sacrificing one of them. The first postulate saves the universality of Maxwell’s equations; the second takes seriously the consequence that is a constant, the same for everyone. The price to pay is not in the laws of electromagnetism, but in our idea of space and time.
If we accept these postulates, in fact, we must give up something very dear to intuition: absolute simultaneity and an absolute time interval between two events. Two identical clocks, in relative motion, no longer agree on “how much time” has passed. As we shall shortly show with the light clock, time itself becomes relative.
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Topics: Special relativity Concepts: Einstein’s postulates · Time dilation
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