VACUUM PROBLEM and YOUR IDEA "AIR slown down light "
Ok let me expain this
When a photon encounters a media particle what it does is get absorbed into the media molecule a short time later the molecule pops back out and continues on its way. The momentum is always conserved in the absorb and re-emission but there is a problem we will talk about.
So a basic photon travel in media looks like this
---- c ----- delay ----- c ----- delay ----- c ---->
So the light varies between the speed of light and delays in the absorbing and re-emission by media molecules.
So lets talk about your laser in your tests moving thru air:
While the light is absorbed inside the air molecule sometimes the molecule gets knocked by another air molecule and it changes direction. When it re-emits it goes in the wrong direction because its momentum was relative to the molecules when it entered. This is the process of how you can see the beam moving thru the air even though the beam starts out completely arranged along its path like all lasers do. That is also why its very dull side on but bright looking end on into it. If you increased the air pressure the beam will get brighter and brighter to you from side on because that process is more and more likely to happen. Besides from that effect you also have dust and pollution etc in the air that are reflecting and scattering the beam.
So in a vacuum you should be able to guess what happens if you understood all that ... the laser becomes invisible side on.
You can also make light spin by the same absorbed molecule trick ... here
http://physicsworld.com/cws/article/news/2011/jul/05/rotating-cylinder-puts-a-new-spin-on-slow-light
It has long been known that a moving medium can shift the position of passing light. The reason is that the light's photons can be absorbed by the medium's atoms, which jump into a higher energy state as a result. A moment later, these atoms return to their original state, re-emitting the photons. But by this time the atoms have moved slightly, so the photons continue their path from that new position.
The important things to note.
- Light doesn't really travel at any speed other than 'c' even in a media
- The slowing of light in a media is created by the absorption and re-emission delays
- The light momentum is preserved relative to the molecule at absorption if the molecule moves while absorbed the re-emission relative to the new moved position.
Those effects have big implications for what you are trying to test because any media on earth spins with the earth and the lights movement gets affected by the earths movement.
The effect is really not noticeable to us because we also spin with the earth but it does foul up what you are try to test
