Michelson Morley

In Michelson-Morley, if instead of light the experiment had used, say, tennis balls fired in perpendicular directions and reflected off backboards (ignoring friction,air resistance, and the inability of tennis balls to occupy the same space), we wouldn't expect a rotation of the apparatus to result in any change of the relative times for the tennis balls to return to their original positions. This is because the tennis balls begin the experiment with a momentum caused by the velocity of their inertial frame, the earth. What's the difference using light? Doesn't light have momentum?
- Greg Jordan (age 72)
Macon, GA

Nice question. Yes, light does have momentum. If your tennis-ball analogy applied to light, then the speed at which light traveled past us would depend on the speed of the source. That sounds reasonable, except that Maxwell's equations provide a beautiful and accurate description of all electromagnetic phenomena including light and that description says that light waves travel at a fixed speed regardless of the source.  So the question became, which do you throw out:

1) Maxwell's equations    or
2) the idea (supported by the Michelson-Morley experiment) that the laws of physics look the same to differently moving observers

Einstein's answer was:

3) Neither.
Instead you throw out your instinctive ideas about which features of the world (e.g. time intervals) look the same to those different observers.

Mike W.

(published on 12/09/2016)