Fundamental postulate of relativity

Relativity has a lot of surprising consequences: Moving clocks run slow and moving meter sticks are short. This bizarre behavior all follows from two fundamental postulates. The first postulate is implicit in Newtonian mechanics, but the second is all together new.

  1. The laws of physics are independent of an observer's reference frame

  2. The speed of light, c, is the same in all reference frames.

In the late 19th century, there was a great debate regarding the ether, an assumed medium that carried light waves. Other waves, such as sound or water waves move in a medium, such as air or water. Without the medium, in a vacuum, sound could not travel, whereas light traveled easily through a vacuum. If there was a medium, than one would assume that one could assign a velocity to the medium. If there were such a ether, one might ask whether Earth or our solar system was at rest in the frame of the ether.

In 1887, Michelson and Morley performed an experiment that demonstrated that light moved with the same velocity at all times during the year. If there were an ether, Earth's velocity around the sun would alter the speed of light which would disrupt a carefully adjusted interference pattern in a device known as the Michelson-Morley interferometer. If, by coincidence, the ether was moving at the same speed as Earth, certainly it would not be 6 months later.

Two consequences of the postulate which we will discuss in future pages are:

Time dilation: The time measured between two events, t0 measured in the frame where both events occur at the same location, appears longer to a moving observer. Thus a moving clock appears to run slow.

Length contraction: The length of an object, L0 when measured in the rest frame of the object, appears shorter to an observer who is moving relative to them object.

Examples     Relativity's index