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The Special (or Restricted) Theory of Relativity
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 electro-magnetic 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 counter-intuitive.   
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) 3108ms-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 "space-time") 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.  
   
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