Einstein's Special Relativity

Inertial Frame of Reference

An inertial frame of reference is one in which

 have no acceleration when no net forces act on it.

The Speed of Light

It is quite natural to ask whether the principle of Galilean relativity also applies to

electricity, magnetism, and optics. Experiments indicate that the answer is no. Recall

that Maxwell showed that the speed of light in free space is 3x10^8 m/s. 

Physicists of the late 1800s thought that light waves moved 

through a medium called the ether and that the speed of light was c only in a special, 

absolute frame at rest with respect to the ether.