The beam of light from a laser on a rotating turntable casts into space. At some distance, the beam moves across space faster than c. Why does this not contradict relativity?
2007-05-03 05:25:03 · 4 answers · asked by LostinPhysics 1 in Science & Mathematics ➔ Physics
A beam dot isn't a physical entity. It doesn't carry information. So it's free to go just as fast as it pleases and not be bothered by relativity. The photons moving from the laser along the beam to the dot, of course, will travel at c.
2007-05-03 06:10:56 · answer #1 · answered by Anonymous · 0⤊ 0⤋
The beam will bend and lag behind as it sweeps across space. The photons from that laser gun can only travel at light speed (c); no faster. So, depending on your point (d) in space, it will take t = d/c seconds for the photons to arrive at d. Meanwhile, back in your lab, the laser source has rotated theta = w t degrees on to a different advanced angle. w is the angular velocity of your turntable.
It's something like taking a water hose and swinging it back a forth. The water coming out of the nozzle is always ahead of the water landing in the grass some yards away. That's because it takes time for the water molecules to get to the grass. Meanwhile the nozzle has moved on.
Bottom line, no c is still c. There is no contradiction here.
2007-05-03 05:52:03 · answer #2 · answered by oldprof 7 · 1⤊ 0⤋
If what you write is true, it is the beam front (or wave front) which moves and the limit of velocity of light in vacuum is for the wave, not the wave front.
2007-05-03 05:44:56 · answer #3 · answered by Swamy 7 · 0⤊ 1⤋
It won't!
2007-05-03 05:33:20 · answer #4 · answered by Anonymous · 0⤊ 2⤋