Einstein published his Theory of Special Relativity in 1905 and the General Theory of Relativity, covered in the next section, about ten years later. Special relativity describes how one's perception of space and time depends on the way one is moving. The notion of an absolute time, present in the Newtonian view of nature, turned out to be untenable. These fundamental differences become dramatic if one considers objects that move with relative velocities close to the speed of light.

The equations given here highlight three striking consequences. Firstly, the fact that velocities add up in a peculiar way such that one can never reach a velocity greater than the speed of light. This speed limit is universal, in the sense that it is the same for all observers. Secondly, the fact that moving clocks tick slower, quantifying the basic idea that time is relative. Finally, we highlight the most celebrated equation of twentieth-century physics, which expresses the fundamental equivalence of mass and energy.

Special relativity is based on two fundamental postulates concerning different observers who travel with a constant speed with respect to each other; so-called ‘inertial observers’. The first is that the laws of physics should be the same for all inertial observers – for all of them the equations should look exactly the same. The second is that for all inertial observers the velocity of light (in vacuum) is the same.

The first postulate may not be so surprising: it is the relativity postulate and in fact not so new. Galileo already formulated it in a rather explicit way. The postulate boils down to saying that if I do experiments to discover the laws of physics on the ground, and somebody else does the same experiment in a train that moves with constant speed, then we will obtain the same laws of physics. It entails that if you are traveling with a constant speed with respect to somebody else, there is no objective way to decide who is moving and who is not. This is something you may in fact have experienced while sitting in a slowly moving train. The notion of motion is relative.