Fundamental postulates of relativity
Relativity has a lot of surprising
consequences: e.g., moving clocks are observed to run slow and moving meter
sticks appear shorter. This nearly unbelievable behavior is proven (in part
indirectly) by a lot of experiments. It can also be understood as following
from two innocent-looking fundamental postulates:
The laws of physics are
independent of the observer's reference frame The speed of light,
c, is the same in all reference frames.
The first postulate is implicit in Newtonian mechanics -- recall what you
learned in Physics 231 about describing motion from different reference
frames that differ by moving at a constant velocity relative to each other.
The second postulate follows from the laws of electricity and magnetism
(c
=1/sqrt(e0
m
0)=
2.998 x 108m/s),
and has been tested by the Michelson-Morley experiment.
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 travels easily through a vacuum. When there is a medium, one can assign a velocity to the medium. If there were an 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 interval two events that occur at the same point in space appears longer to a moving observer. Thus a moving clock appears to run slow.
Length contraction: The length of an object appears shorter to an observer who is moving relative to the object.