relativity

relativity Theory, proposed by German-born US physicist Albert Einstein, based on the postulate that the motion of one body can be defined only with respect to that of a second body. Mass, space and time are interdependent. This theory led to the concept of a four-dimensional space-time continuum in which the three space dimensions and time are treated on an equal footing. Einstein devised two theories concerning relativity. The special theory, put forward in 1905, is limited to the description of events as they appear to observers in a state of uniform relative motion. The more important consequences of the theory are: (1) that the velocity of light is absolute, that is, not relative to the velocity of the observer; (2) that the mass of a body increases with its velocity, although appreciably only at velocities approaching that of light; (3) that mass (m) and energy (E) are equivalent, that is, E = mc², where c is the velocity of light (this shows that when mass converts to energy, a small mass gives rise to large energy); (4) the Lorentz-Fitzgerald contraction, that is, bodies contract as their velocity increases, again only appreciably near the velocity of light; and (5) an object's sense of elapsed time expands – ‘time dilation’. The general theory of relativity, completed in 1915, is applicable to observers not in uniform relative motion. This showed the relation of space and gravitation. The presence of matter in space causes space to ‘curve’, forming gravitational fields; thus gravitation becomes a property of space itself. The existence of black holes, where not even light can escape extreme gravity, is postulated as a consequence of this.