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The assumptions on which Einstein's special theory of relativity (1905) depends are (i) all inertial frameworks are equivalent for the description of all physical phenomena, and (ii) the speed of light in empty space is constant for every observer, regardless of the motion of the observer or the light source. (Although the second assumption may seem plausible in the light of the Michelson–Morley experiment of 1887, which failed to find any difference in the speed of light when measured in the direction of the earth's rotation or when measured perpendicular to it, it seems likely that Einstein was not influenced by the experiment, and may not even have known the result.) As a consequence of the second postulate, no matter how fast she travels, an observer can never overtake a ray of light, and see it as stationary beside her. However near her speed approaches to that of light, light still retreats at its classical speed. The consequences are that space, time, and mass become relative to the observer. Measurements made of quantities in an inertial system moving relative to one's own reveal slower clocks, contracted lengths, and heavier masses, with the effect increasing as the relative speed of the systems approaches the speed of light. Events deemed simultaneous as measured within one such system will not be simultaneous as measured from the other: time and space thus lose their separate identity, and become parts of a single space-time . The special theory also has the famous consequence (E = mc₂) of the equivalence of energy and mass.

Einstein's general theory of relativity (1916) treats of non-inertial systems, i.e. those accelerating relative to each other. The leading idea is that the laws of motion in an accelerating frame are equivalent to those in a gravitational field. The theory treats gravity not as a Newtonian force acting in an unknown way across distance, but as a metrical property of a space-time continuum that is curved in the vicinity of matter. Gravity can be thought of as a field described by the metric tensor (see geometry ) at every point. The classic analogy is with a rock sitting on a bed. If a ball-bearing is thrown across the bed, it is deflected towards the rock not by a mysterious force, but by the deformation of the space, i.e. the depression in the sheet around the rock. Interestingly, the general theory lends some credit to a version of the Newtonian absolute theory of space, in the sense that space itself is regarded as a thing with metrical properties of its own (see also clock paradox ). The search for a unified field theory is the attempt to show that just as gravity is explicable as a consequence of the nature of space-time, so are the other three fundamental physical forces: the strong and the weak nuclear forces, and the electromagnetic force (see also physics, philosophy of ). The theory of relativity is the most radical challenge to the ‘common-sense’ view of space and time as fundamentally distinct from each other, with time as an absolute linear flow in which events are fixed in objective relationships.