Fundamental postulates of relativity

Special relativity describes some surprising features about space and time, which can be stated vividly in terms of the instruments used to measure those concepts: moving clocks run slow and moving meter sticks are short (along the direction of motion), by amounts that have nothing to do with the materials those instruments are made of. This strange behavior has been proven by a lot of experiments. It can also be understood as a logical consequence of two innocent-looking fundamental postulates:

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

  2. 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. It is also consistent with experience: you can't tell how fast your vehicle is travelling without looking out the window. The second postulate follows from the laws of electricity and magnetism (c =1/sqrt(e0 m0)= 2.998 x 108m/s), and was first tested by the Michelson-Morley experiment.

In the late 19th century, there was a great debate regarding the "aether" --- not the anesthetic, but a hypothetical medium that "carried" light waves in the same sense that sound waves are carried by air, or ocean waves are carried by the surface of the water. Without a medium such as air, sound waves cannot exist; but light and other electromagnetic waves travel easily through vacuum. When there is a medium, one can assign a velocity to the medium. If there were an aether, one could ask whether Earth or our solar system was at rest with respect to the aether.

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 aether, 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 aether was moving at the same speed as Earth, certainly it would not be 6 months later. Conclusion: There ain't no aether, and uniform absolute motion can't be observed.

Two consequences of the two postulates 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.

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