Empty space is the same everywhere and for everyone

The special theory of relativity deals with space devoid of all matter and with the propagation of light in such empty space. The subsequent general theory of relativity introduces matter as a distortion of space leading to a geometrical representation of the universal law of gravitation. We will confine ourselves to the special theory.

Many of the principles which led to the theory of relativity were recognised long before Einstein. In the 17th century Galileo had argued that any experiment carried out in the cabin of a moving ship would give exactly the same result as the same experiment in a stationary laboratory, and therefore the motion of the earth could not be detected by experiments with falling bodies. The Dutch physicist Hendrik Lorentz and the French mathematician Henri Poincare, who were contemporaries of Einstein, independently contributed to the mathematical formulation of special relativity. There can be no doubt, however, that the final distillation of these principles and the train of logic which led to observable predictions must be credited to Albert Einstein.

Just as quantum theory changed our understanding of the world around us, the theory of relativity 'wiped the slate clean of preconceived prejudices' and proposed new basic philosophical principles. The seemingly obvious concepts of space and time, energy, and matter are to be viewed from a different perspective. Light plays a central role in the new theory.

While working at the patent office in Zurich, Einstein spent his spare hours thinking about space and time. He was not trying to explain the results of particular experiments; this would come later as a by-product of his work. He tried to imagine how to construct the world in the most perfect way, as he would have done if he had been God! His basic criteria were symmetry and simplicity and elegance embodied in principles which apply equally to every aspect of the universe.

Einstein started with two basic postulates, which seem logical, if not obvious: empty space is absolutely uniform and symmetrical, and the speed of light is the same for everyone. Building on these postulates with one logical step after another, he came to conclusions which are not at all obvious.

We follow these steps slowly, one at a time. Surprisingly, they are not impossibly difficult. The relative nature of time is probably the most challenging, and strangest, concept to digest.

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