# Relativity

In the eighteenth century Isaac Newton proposed his famous three laws of motion and for the next two hundred years scientists were quite happy with this. Then in 1906 Albert Einstein published his Theory of Relativity (actually there are two theories – the Special theory for non accelerating objects and the General Theory for accelerating objects or ones in large gravitational fields).

Einstein made two basic assumptions in his special theory of relativity:
(a) physical laws are obeyed no matter where they are measured
(b) the velocity of light in free space is constant no matter where it is measured

This means that if you were to do an experiment to prove, say, Newton's second law in a train moving at a steady speed along a straight track you would get the same result as if you did it on the bank at the side of the track. Also an experiment to measure the speed of light would give the same result in both places.

This theory had three important consequences for an object moving at high speed (a significant fraction of the speed of light) relative to an observer:
(a) the length of the moving object appeared to get smaller
(b) the masses of the moving object appeared to get larger and
(c) time ran slower for the moving object compared with the observer

(a) Length contraction
The following two diagrams attempts to show the first of these. The first one shows everyday objects moving at speeds that we would expect – say some tens of km per hour in the case of the car and so on.

The next diagrams shows you what the world might look like if the traffic was moving at speeds close to that of light (300 000 km/s). The moving objects would all look as though they had shrunk – the ones that were moving faster would have shrunk by a bigger fraction than those moving a bit slower.

(b) The slowing down of time
The theory also suggested that time would run slower for a moving object than one that was at rest. This means that fast moving clocks would beat slower than stationary ones – this has actually been observed by checking very accurate clocks called atomic clocks. Some high energy particles called muons formed at the top of the atmosphere actually reach the ground. This is unexpected but can be explained by relativity. The muons are 'living longer' than predicted because of their high speed relative to the Earth.

(See: Muons time dilation)

(c) Mass increase
Relativity suggests that a fast moving object has a greater mass relative to its surroundings than the same object at rest. This means that it would be more and more difficult to accelerate and this has been proved in high energy nuclear accelerators where the particles are moving at speeds close to that of light. We don't observe that at slow speeds – it would be rather like pushing a shopping trolley and finding that as it moved faster it became more and more difficult to increase its speed.

One important result of the general theory was that a beam of light would bend in a gravitational field. This can really only be observed near very massive bodies like a neutron star or a black hole. Another way of looking at this idea is that space itself is bent by these huge gravitational fields.

The bending of light in gravitational fields has observed in an effect known as gravitational lensing.

(See: General relativity)

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