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)