In formulating his special or restricted
theory of relativity, Einstein considered only inertial observers. 

To generalize his ideas to include all
observers took him another ten years of work, finally arriving at
the (much more mathematically complicated) general theory of
relativity 

Einstein based his special theory of
relativity on a principle, already accepted by Galileo, Newton and
others, the principle of relativity. 



This principle can be stated as follows: 

The laws of physics
must be the same for all (inertial) observers. 



Hopefully the reader will find this
perfectly reasonable, however, Einstein realized that this principle
must be extended, in the light (pun intended) of Maxwell's work, to
cover the propagation of electromagnetic radiation. 



To emphasize this he added to the principle the idea that the
velocity of light (in empty space) must be the same for all
(inertial) observers. 



It could be argued that
the principle of relativity already implies this because the
velocity of light depends on the behaviour of electric and magnetic
fields, ie on the "laws of electromagnetism". 

It is perhaps
nevertheless worth stating separately as it is somewhat
counterintuitive. 

We are saying that, if you make a
measurement of the speed of a given beam of light and a somebody
else makes the same measurement on the same beam of light then you
will both find (about) 3×10^{8}ms^{1}
even if you and the other observer are moving relative to each
other at (say) 100ms^{1}! 



Einstein was convinced that this simply
must be true and went on to deduce that time and space are
intimately linked (as "spacetime") and that measurements of
distance, time, force, mass etc must all be stated as being
relative to the observer making the measurements. 



The physicists Michelson and Morley designed
an experiment to see if the velocity of light is a constant, and
found... guess what... it is. 


