Relativity theory sought to eliminate from physics the idea of absolute space and time – that measurements of space and time were fixed and quite independent of the person measuring them. To Isaac Newton, absolute space and time existed as a backdrop against which he could formulate general laws about such quantities as acceleration and force. It was the genius of Einstein that through the special and general theories of relativity, show that such absolutes did not exist and that Newton's laws were not universally true.
The special theory of relativityEinstein's special theory of relativity (1905) was based on the idea that all uniform motion is relative – that is, an object can be seen to move uniformly only in relation to some stationary frame of reference. The classic experiment made by Albert Michelson and Edward Morley (1838–1923) determined that the speed of light is always the same in a given vacuum regardless of the speed of the source of light, of anyone observing it or of its wavelength. From these results Einstein deduced an astonishing set of conclusions. They showed that the mass, length and time interval of an object begins to move relative to an observer.
If say, an astronomer were to observe an extremely fast-moving spaceship, then his instruments would indicate that the mass of the spaceship had increased, that all lengths in the direction of the spaceship's motion had decreased, and time aboard was slower. Yet in the spaceship itself nothing would appear to have changed, although if the pilot looked back at the astronomer – who would be in the same motion relative to him – he would observe that mass, length and time there had changed in exactly the same way.
The light clock shows why time varies with motion and by how much. Normally the effects of special relativity are undetectable in an object until it is traveling at nearly the speed of light (300,000km [186,000 miles] a second), although very sensitive atomic clocks have been used to detect clocks "going slow" on aircraft in flight. The effects do become large for subatomic particles moving at close to the speed of light. Thus, because of their high speed, very fast unstable particles in cosmic rays live longer in the Earth's atmosphere than would otherwise be expected. Sub-atomic particles can be so speeded artificially that their masses are increased many thousandfold; particle accelerators have to be specially designed to allow for this effect.
It is Einstein's famous equation "E=mc2", relating the energy E and mass m of a moving particle with c in the velocity of light that shows ever greater energy will increase its mass. Because c2 is so large, only a small amount of mass is equivalent to a vast amount of energy. The conversion of mass into energy takes place in nuclear reactors, in atomic power stations, in nuclear weapons and in the Sun and other stars.
As the speed of a particle approaches that of light its energy increases indefinitely. But there is a limit to the amount of energy available to any particle and so it can never travel faster than light. The light barrier cannot be crossed, but there may exist particles that are always traveling faster than light. These particles, called tachyons, have been looked for but not yet found.
The general theory of relativityTo take account of acceleration and of the force of gravity, Einstein's general theory of relativity (1915) incorporated the fact that all bodies fall equally fast at the surface of the Earth. In other words, the effect of the Earth's gravitational field is an intrinsic feature of the space around the Earth. Einstein described this feature in terms of curvature of space: the greater the distortion the greater the gravitational force. If time is included with space in this distortion it is possible to incorporate the idea that all motion is relative. The amount of space-time distortion caused by massive bodies can be quantified and it was Einstein's genius that showed how the amount of this curvature depends on nearby massive bodies.
Experimental observations, for example of small deviations in motions of planets from those predicted by Newton, make the general theory of relativity the most satisfactory of a whole range of similar theories. Confirmation also comes from the bending of the path of a ray of light near a massive body. Light has energy – and hence mass – and therefore moves in a curved path in the distorted space around the body. Such bending of light by the Sun was confirmed at an eclipse.
Holes in the heavensAll these effects involve weak gravitational fields and cannot put general relativity through the most searching test. When stars have used up their nuclear fuel they may evolve into extremely condensed objects in which strong gravitational fields occur and so they are good testing grounds for general relativity. It is postulated that very heavy stars collapse in on themselves so completely that the escape velocity on their surface is greater than the speed of light. As a result, nothing can ever escape them again – not even light – and so they are known as "black holes". Good candidates for black holes in our own Galaxy are the variable X-ray stars such as Cygnus X-1.
Related category• SPACE AND TIME
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