Speed of Light in Various Directions

This is almost a political question. I have spent some time lately on some creationist Facebook pages. I am a Christian but find no conflict with science. I came across an interesting one the other day that has been bugging me. Creationists have been trying hard to explain how star light got to us in 6000 years. This guy says when we measure the speed of light we send a beam one way to a mirror you need to reflect it back to the starting point to measure the speed. In other words you are measuring the average speed. He says why couldn't the light travel much faster one way than the other. So in his view the light is arriving here from stars virtually instantaneously. He says his theory cannot be disproved because if we try to measure the speed one way we would need to synchronize two clocks then move them apart. The relativistic effects of moving them apart would mean they weren't synchronized anymore. Is there anyway to disprove this?
- Russ Harris (age 68)
Canada

Hello Russ,

People worried about this possibility for a long time.   If light traveled in a medium, like sound waves in water, then the speed of the wave would depend on the direction of the light with respect to the speed of the medium.  The name people gave to this fictitious medium was the stationary lumineferous aether.  If the earth were moving in this medium then the velocity of light would depend on the direction the earth is moving with respect to it.   In 1887 Michelson and Morley performed an experiment that proved that this was not true.
They constructed a light interference experiment mounted on a stone block floating in a bath of liquid mercury.  As the apparatus was slowly rotated they expected the interference fringes to change as the light beams alternatively were parallel or anti-parallel to the earth's motion with respect to a presumed stationary ether.  Their results were null.   No evidence was seen.    Since then this experiment has been improved by many orders of magnitude. The conclusion is that light travels with the exact same speed in all directions.

https://en.wikipedia.org/wiki/Michelson%E2%8%93Morley_experiment#Most_famous_.22failed.22_experiment

LeeH

(published on 10/08/2016)