GENERAL THEORY OF RELATIVITY
INTRODUCTION

Relativity theory was first studied by Albert Einstein. The general theory of relativity was published in 1915. It is the current description of gravitation in modern physics. It unifies special theory of relativity and Newton’s law of gravitation. It describes gravity as a geometrical property of space and time or space - time. The speed of light in a vacuum is constant and an absolute physical boundary for motion. This does not have much effect on person’s day to day life since we travel much lower speed than light. For objects traveling near light speed, however, the theory of relativity states that objects will move slower and shorten in length from the point of view of an observer on Earth. Einstein also derived the famous equation, E = mc2, which reveals the equivalence of mass and energy.

THEORY OF RELATIVITY

Theory of relativity is of two types. Special Theory of Relativity and General Theory of Relativity.

Special Theory of Relativity.

The first is the Special Theory of Relativity, which essentially deals with the question of whether rest and motion are relative or absolute, and with the consequences of Einstein’s conjecture that they are relative.

General Theory of Relativity.

The second is the General Theory of Relativity, which primarily applies to particles as they accelerate, particularly due to gravitation, and acts as a radical revision of Newton’s theory, predicting important new results for fast-moving and/or very massive bodies. The General Theory of Relativity correctly reproduces all validated predictions of Newton’s theory, but expands on our understanding of some of the key principles. Newtonian physics had previously hypothesized that gravity operated through empty space, but the theory lacked explanatory power as far as how the distance and mass of a given object could be transmitted through space. General relativity irons out this paradox, for it shows that objects continue to move in a straight line in space-time, but we observe the motion as acceleration because of the curved nature of space-time.The development of general relativity began with the equivalence principle, under which the state of accelerated motion and being at rest in a gravitational field are physically identical. The upshot of this is that free fall is inertial motion; an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as in the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects can not accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first propose that space – time is curved. Some of the consequences of general relativity are
:i) Time goes slower in higher gravitational fields. This is called gravitational time dilation.
ii) Orbits precess in a way unexpected in Newton’s theory of gravity.
iii) Rays of light bend in the presence of a gravitational field.
iv) Frame – dragging in which, a rotating mass “ drags along” the space time around it.
v) The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light.

In particular, the curvature of spacetime is directly related to the four momentum (mass energy momentum ) of whatever matter and radation are present. The relation is specified by the Einstein field equations , a system of partial differential equations .
Many predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall , and the propagation of light . Examples of such differences include gravitational time dialation, the gravitational redshift of light, and the gravitational time delay . General relativity's predictions have been confirmed in all observations and experiments to date. Although general relativity is not the only relativistic theory of gravity, it is the simplest theory that is consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity.

(A simulated black hole of ten solar masses as seen from a distance of 600 kilometers with the Milky Way in the background.)

Einstein's theory has important astrophysical implications. It points towards the existence of black holes —regions of space in which space and time are distorted in such a way that nothing, not even light, can escape—as an end-state for massive stars . There is evidence that such stellar black holes as well as more massive varieties of black hole are responsible for the intense radiation emitted by certain types of astronomical objects such as active galactic nuclei or . The bending of light by gravity can lead to the phenomenon of gravitational lensing , where multiple images of the same distant astronomical object are visible in the sky. General relativity also predicts the existence of gravitational waves , which have since been measured indirectly; a direct measurement is the aim of projects such as LIGO. In addition, general relativity is the basis of current cosmological models of a consistently expanding universe.

Dragging space and time

The results of two studies announced in early November 1997 provide unprecedented support for “frame-dragging,” a concept predicted by physicist Albert Einstein's general theory of relativity. Frame-dragging describes how massive objects actually distort space and time around themselves as they rotate. One of the studies examined frame-dragging around black holes, an example of which is shown here in an artist's conception.