Theory of Relativity- A critique.

Relativity 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. One consequence of the theory is that space and time are no longer viewed as separate, independent entities but rather are seen to form a four-dimensional continuum called space-time . Full comprehension of the mathematical formulation of the theory can be attained only through a study of certain branches of mathematics, e.g., tensor calculus. Both the special and general theories have been established and accepted into the structure of physics. Einstein also sought unsuccessfully for many years to incorporate the theory into a unified field theory valid also for subatomic and electromagnetic phenomena.
Evolution from Classical Theory

The modern theory is an extension of the simpler Galilean or Newtonian concept of relativity, which holds that the laws of mechanics are the same in one system as in another system in uniform motion relative to it. Thus, it is impossible to detect the motion of a system by measurements made within the system, and such motion can be observed only in relation to other systems in uniform motion. The older concept of relativity assumes that space and time are correctly measured separately and regards them as absolute and independent realities. The system of relativity and mechanics of Galileo and Newton is perfectly self-consistent, but the addition of Maxwell's theory of electricity and magnetism to the system leads to fundamental theoretical difficulties related to the problem of absolute motion.

It seemed for a time that the ether , an elastic medium thought to be present throughout space, would provide a method for the measurement of absolute motion, but certain experiments in the late 19th cent. gave results unexplained by or contradicting Newtonian physics. Notable among these were the attempts of A. A. Michelson and E. W. Morley (1887) to measure the velocity of the earth through the supposed ether as one might measure the speed of a ship through the sea. The null result of this measurement caused great confusion among physicists, who made various unsuccessful attempts to explain the result within the context of classical theory.

The Special Theory of Relativity

The validity of the classical concepts of absolute and independent time and space was challenged by H. A. Lorentz and others. Since absolute motion cannot be confirmed by objective measurement, Einstein suggested that it be discarded from physical reasoning; he explained the results of the Michelson-Morley experiment by means of the special relativity theory, which he enunciated in 1905. This theory accepts the hypothesis that the laws of nature are the same in different moving systems applies also 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 the light. Einstein deduced from these hypotheses the full logical consequences and reformulated the mathematical equations of physics, basing them in part on equations of H. A. Lorentz (see Lorentz contraction ) by which measurements made in one uniformly moving system can be correlated with measurements in another system if the velocity of one relative to the other is known.

The theory resolves the conflict between Newton's mechanics and Maxwell's electrodynamics by introducing fundamental changes in Newton's theory. In most phenomena of ordinary experience the results obtained from the application of the special theory approximate those based on Newtonian dynamics, but the results deviate greatly for phenomena occurring at velocities approaching the speed of light. In innumerable cases where the results predicted by these theories are incompatible, experimental evidence supports the Einstein theory. Among its assertions and consequences are the propositions that the maximum velocity attainable in the universe is that of light; that mass and energy are equivalent and interchangeable properties (this is spectacularly confirmed by nuclear fission, on which the atomic bomb is based); 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; and that, since absolute time is excluded from physical reasoning because it cannot be measured, the results of observers in different systems are equally correct.

The General Theory of Relativity

Einstein expanded the special theory of relativity into a general theory (completed c.1916) that applies to systems in nonuniform (accelerated) motion as well as to systems in uniform motion. The general theory is principally concerned with the large-scale effects of gravitation and therefore is an essential ingredient in theories of the universe as a whole, or cosmology . The theory recognizes the equivalence of gravitational and inertial mass . It asserts that material bodies produce curvatures in space-time that form a gravitational field and that the path of a body in the field is determined by this curvature. The geometry of a given region of space and the motion in the field can be predicted from the equations of the general theory.

Details of the motions of the planet Mercury had long puzzled astronomers; Einstein's computations explained them. He stated that the path of a ray of light is deflected by a gravitational field; observations of starlight passing near the sun, first made by A. S. Eddington during an eclipse of the sun in 1919, confirmed this. He predicted that in a gravitational field spectral lines of substances would be shifted toward the red end of the spectrum. This has been confirmed by observation of light from white dwarf stars. Further confirmation has been obtained in recent years from precision measurements using artificial satellites and the Viking lander on Mars, and from detailed observations of pulsars.

If the main starting point of special relativity is that you can not tell constant velocity from standing still, then the starting point for general relativity is that you can not tell acceleration from a gravitational field.

When a system is accelerated forces spring up due to the inertia of the body being accelerated. Think of trying to stand up in a coach or train as it speeds up. What you feel is the same as if the coach was stopped facing up a slope. Similarly a slowing coach feels much like one stopped facing down a slope. What Einstein set out to do was to produce a set of equations from which all the phenomena of gravity and mechanics could be derived. His approach was very much that of Maxwell who used vector calculus to produce a set of four equations for electricity and magnetism.

The equations summarised what was known about stationary charge and the electrostatic field, and moving charge, current, and the magnetic field, but additionally other solutions yielded te laws of optics and predicted electromagnetic waves, corresponding to accelerated charge. Whereas Maxwell's equations needed only four parameters, Einstein's equations needed ten parameters and a branch of mathematics called tensor calculus. In many situations these equations have not been solved exactly. The most famous solution is the one by Schwarzschild, for an isolated spherical mass.

The Newtonian theory of gravity is the first approximation to general relativity, corresponding to static mass, a second term corresponds to a field caused by moving mass, but in addition to this there is a third 'non-linear' effect. Around a mass is a region of gravitational potential energy, which is, by E = mc?, in turn equivalent to mass. This mass acts as the source of an additional gravitational field, and an additional potential, and so on. This means that general relativity makes some subtle alterations to the predictions of Newtonian gravity which can be tested experimentally.