Special Relativity

History tells us that before Einstein arrived on the scene (1905) scientists used to believe that space was filled with an "aluminiferous ether" which was a transparent solid AND a perfect fluid through which objects could move without disturbing it.  Light was an electromagnetically induced vibration of this invisible elastic "goop".

The Michelson - Morley Experiment showed that the supposed ether wind (that would be created with the moving of our planet through the ether in space) did not exist.  

Special relativity involves the following ideas:  

Time is relative:   time flows differently in systems moving relative to each other.

Length of an object is not absolute:    the length of an object depends on the relative motion with regard to each particular observer.

Speed of light is constant:  c = 3 x 10⁸ m/s; gravitational and electromagnetic interactions happen at c rather than the previously supposed infinite velocity.

Energy & mass:   Einstein showed the energy mass equivalence with his equation E = mc². 

Einstein had two postulates:

1.  Principle of relativity:    All the laws of physics are the same for uniformly moving objects.

2.  Principle of constancy of light:  Light travels at c regardless of motion of source.