RELATIVITY!  (Invariance Theory!)

Inertial frames of reference!

In 1905 Albert Einstein (1879 - 1955) published 4 papers on 4 totally different topics, each of which broke new ground in physics, and each of which deserved (but didn't get) a separate Nobel Prize!

Like most physicists at the dawn of the 20th Century, Einstein was aware that Newton's Laws and Maxwell's Equations were inconsistent. Something had to give, but what? He decided to make two postulates that seemed to be well justified by both theory and experiment, and to see where those postulates led him.

(1) All the laws of physics should have the same form in all inertial frames of reference.
(2) The speed of light in vacuum is a universal constant, independent of the motion of source or observer, or frame of reference.

Immediate consequence: two events or processes which occur simultaneously as seen by one observer will not be seen to occur simultaneously as seen by any other observer in any other inertial frame of reference!

This means we will have to be very careful in thinking about how to write laws of physics that have the same form as seen by all inertial observers... the laws can't depend on expecting all observers to see the same processes happen the same way!!