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**Special Relativity**

Special Relativity is a physical theory developed by Albert Einstein in1905, drawing on previous work by Lorentz. It was born as an attempt to reconcile Maxwell’s equations of electromagnetism with Newtonian physics.

Einstein’s theory has only two postulates, with the aid of which one can deduce the rest of its far from intuitive consequences. The two postulates are:

- The principle of relativity, which states that the laws of physics have to be the same for any observer, regardless of their motion.
- The constancy of the speed of light, which states that any observer will measure the same speed of light, regardless of their motion.

These two seemingly harmless postulates lead to a myriad of puzzling consequences. Basically, in order to keep the speed of light as a constant for any observer, one is forced to accept that space and time are relative to the observer. Moreover, together they act as a single entity, space-time, the properties of which are independent of the observer.

Space and time being relative means that, if two observers are moving relative to each other, they will obtain different values for lengths and time intervals measured in each of the systems, even if those measurements refer to the same object. For example, a rocket seen from the Earth will have a different length than this same rocket when seen from the International Space Station. These difference are, at normal speeds, very little. However, they become appreciable as the speed gets closer to the speed of light.

These seemingly absurd effects have been observed systematically in all kinds of macroscopic and microscopic systems, from satellites to cosmic rays. As of today, Special Relativity has not failed a single experimental test.

For an accessible but rigorous deduction of the time dilation and space contraction, visit Andrew Hamilton’s webpage.

Short video illustrating Special Relativity.

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