Special Theory of Relativity

relativity (rèl´e-tîv¹î-tê), physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference. Space and time are no longer viewed as separate, independent entities but rather as forming a four-dimensional continuum called space-time. In 1905 Einstein enunciated the special theory of relativity, in which the hypothesis that the laws of nature are the same in different moving systems also applies to the propagation of light, so that the measured speed of light is constant for all observers regardless of the motion of the observer or of the source of light. From these hypotheses Einstein reformulated the mathematical equations of physics. In most phenomena of ordinary experience the results from the special theory approximate those based on Newtonian dynamics, but the results deviate greatly for phenomena occurring at velocities approaching the speed of light. Among the assertions and consequences of the special theory are the propositions:

• the maximum velocity attainable in the universe is that of light (c)
• mass increases with velocity
• mass and energy are equivalent (E=mc²)
• objects appear to contract in the direction of motion (Lorentz contraction)
• the rate of a moving clock seems to decrease as its velocity increases (time dilation)
• events that appear simultaneous to an observer in one system may not appear simultaneous to an observer in another system.

The special theory became the foundation of the study of elementary particles and of quantum mechanics.