How Could Michelson-Morley Work?

Einstein says all observers will measure light speed to be the same. Michelson/Morley concluded beams arrived back at the source in phase so that must be true. However, if light is traveling @ say one foot in about one millionth of a sec., how could any human determine that it was perfectly in phase? Also, if an observer were traveling @ light speed alongside a beam, wouldn't he say the light was not moving at all?
- john watson (age 75)
camden, sc usa

The M-M experiment looked at interference fringes of the light that had traveled the two pathways. A path-length difference  change of even a fraction of a wavelength of light would have produced a noticeable shift in the fringe positions. Even though light travels about a foot per nanosecond, the apparatus was big enough to pick up any ether effects from the Earth's motion, if they had existed. The point is that the paths contained a very large number of wavelengths, so a small change in relative lengths would give a fringe shift.

We don't have anything to say about observers traveling at light speed as seen by another observer. Special Relativity doesn't allow their existence, for several reasons including that they would have infinite energy.

Mike W.

(published on 01/28/2014)