postulates of special relativity

In 1905 Einstein rebuilt physics on just two simple assumptions. The first, the principle of relativity, says the laws of physics are the same for everyone moving at constant velocity; no inertial observer can claim to be the one who is 'really' at rest. The second says the speed of light in empty space has the same value for every such observer, no matter how the source or the observer moves.

The second postulate is the radical one. Common sense says that if you chase a beam of light, you should catch up a little and measure it moving more slowly past you. Einstein insisted instead that you always measure the same speed, about 300,000 kilometres per second. To keep that single fact true, our older ideas about absolute time and absolute distance have to give way.

Everything else in special relativity follows from these two statements as logical consequences. Out of them tumble the slowing of moving clocks, the shrinking of moving rulers, the disagreement over which events happen at the same time, and the famous link between mass and energy. The theory is not a grab-bag of strange effects but a tight chain of reasoning from two innocent-looking premises.

Picture two travellers passing each other in deep space, each with identical equipment. Neither can perform any experiment, with light or anything else, that reveals who is 'moving' and who is 'still'. That democratic refusal to grant anyone a privileged standpoint, plus the stubborn constancy of light, is the whole foundation.