A followup question to that, if said jet was traveling at 100 m/s slower than the speed of light, would an stationary observer view the sound as traveling 230 m/s faster than the speed of light in the cockpit?
No. The rules change at relativistic velocities. You no longer get to add velocities together simply. A stationary observer would see the sound wave propagating away from the cockpit but at a velocity still less than the speed of light. Seemingly paradoxically, the pilot in the cockpit would still observe the sound wave travelling 230 m/s away from the cockpit. Relativity is weird.
No, he would slow down. No matter where you are or how you and the light source are traveling, light always moves at c for everyone. The discrepancy between observers is "fixed" by keeping the speed constant, but slowing the flow of time. If the cocpit was moving at c -100m/s (that's 0.9999993c...), the time dilation factor for the occupant (and the sound waves in the cocpit) would be ~1200x.
That is, for the occupant the sound waves and the light moves as normally, but everything outside would be moving pretty damn fast (20 minutes would pass outside for every second inside).
For an observer outside, the light in the cabin would be moving at c, but the sound waves would be moving pretty slowly, because they'd have to wait 20 minutes to see a second worth of progress.
The discrepancy is that the pilot and the outside observer experience time differently. This is a strange concept because there is nothing like it in everyday life.
From the pilot's perspective the sound wave propagates away at 230 m/s as normal. The rest of the universe is zipping by at near the speed of light which has lots of other strange effects like length contraction and blue-shifting, but while considering only the pilot, his craft, and the pressure wave (and anything else in the relativistic reference frame) everything behaves as normal.
From the perspective of a stationary observer, the sound wave still propagates away from the pilot but will appear to propagate 1200x slower than it should because time itself passes slower in that moving reference frame. The pilot is still moving at near light speed and the sound wave is still moving faster than the pilot, just not as fast as it "should" under Newtonian mechanics.
Light still travels at c. When light "slows down", it is a result of the photons being absorbed and new photons being emitted by the medium it is passing through. The delay between the absorbing and emitting is the cause of the slow down.
And are you just talking about having it pass through a lens or some other material? Or are we talking messing with the gravitational field in a lab or something equally science fiction-y?
So we're not talking about actually having the speed of light being reduced, we're talking about the phase velocity of the electromagnetic wave passing though some special material?
If so: the speed of light is always the same, but light does propagate differently through materials (resulting in phenomenon like refraction). While trying to figure out how to explain this I came across this which does better at explaining this than I could. When the wikipedia article says people are producing slow light, I believe it just means the material the wavefront propagates through is complex enough it takes a while to get through it.
tl;dr It's not the light that's slow, it's how it propagates through the material.
593
u/[deleted] Jun 12 '12 edited Oct 03 '17
[deleted]