The Special Theory of Relativity is used to describe a moving frame of reference at constant velocity. Put simply; imagine you are on a train. This train is travelling in a straight line at a constant speed and there are no bumps on the track. Inside this train you can do everything you could normally do whilst stationary. If you dropped a ball on the train it would fall in an equally straight line as it would if dropped in a stationary frame of reference. If the train had no windows or doors you could not tell by any scientific test if the train was moving or not.

" If two systems move uniformly relative to each other (i.e. the train and the ground), then all the laws of mechanics are the same in both systems", (Einstein). This is one of the laws of the Theory of Special Relativity.

Michelson and Morley carried out an experiment in 1887 to see if light travelled at a uniform velocity. Their measurements of the speed of light showed that the motion of the earth in relation to the sun had no influence on the speed of light. This means that light travels at a constant velocity in all frames of reference. Using this Einstein made some deductions. Firstly he concluded that no object could travel faster than the speed of light because the lights’ speed remained constant no matter how fast the earth travelled. Secondly, the speed of electromagnetic waves, or light, in empty space is constant, and furthermore is independent of the source or observer.

The second part of Einstein’s theory of Special Relativity describes the strange effects of high velocities on time. When an object travels at high speed, for example an aeroplane, it experiences an effect known as time dilation. Time dilation is the slowing down of time due to great speed. So in essence, clocks moving at great speed run slower.