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 relativity theory, 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 that the maximum velocity attainable in the universe is
that of light; that Mass increases with velocity; that mass and energy are equivalent; that objects
appear to contract in the direction of motion; that the rate of a moving clock
seems to decrease as its velocity increases; that events that appear simultaneous
to an observer in one system may not appear simultaneous to an observer in
another system. Einstein expanded the special theory of relativity into a general theory (completed in 1915) that is principally concerned with the large-scale
effects of Gravitation (see also COSMOLOGY). The general theory recognizes the equivalence of
gravitational and Inertial mass, and asserts that material bodies produce the curvature of the
space-time continuum and that the path of a body is determined by this curvature. The
theory predicts that a ray of light is deflected by a gravitational Field; observations of Starlight passing near the Sun, first made by Arthur Eddington and colleagues during a 1919 eclipse of the
sun, confirmed this. The theory also predicts a Red Shift of spectral lines of
substances in a gravitational field, a result confirmed by observation of light
from white dwarf stars. Finally, the theory also accounts for the entire
observed perihelion motion of the planet Mercury, only part of which could be
explained by Newtonian (Newton, Sir Isaac) Celestial Mechanics.