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Thought Experiment on the speed of light

  1. Nov 4, 2011 #1
    OK, so I am not a physics major, but I think that I understand that matter or energy cannot travel faster than the speed of light. Today I was using a laser pointer, and I noticed that it traveled rather fast, and it traveled faster as it went farther away from me. So if you imagined the laser beam as a lever arm, I could use velocity=rw to determine the velocity of the point on a laser pointer. So say I built a chamber with a radius of 40,000 meters. Then, I stood in the center and shot a beam of light at the wall of my chamber. I rotated the laser pointer in the center at 10 m/s. The dot would move at 40,000(10 m/s)=400,000 m/s. The speed of light is appr. 300,000 m/s. So that would mean that the photons at the end of the laser pointer would be traveling 100,000 m/s faster than the speed of light...isn't there something wrong here. My guess was that the beam of light would bend once it hit 300,000 m/s so that the dot was only travelling at 300,000 m/s. How would you analyze this?
  2. jcsd
  3. Nov 4, 2011 #2


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    As somebody pointed out recently on this forum, you could do the same thing with a machine gun, poking holes in a wall far enough away that the holes would travel at faster than the speed of light, right?
  4. Nov 4, 2011 #3


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    This is possible because no physical "thing" is traveling faster than the speed of light in that scenario. Each individual photon from the laser always travels at the speed of light.

    Imagine you had a laser pointer that shot out photons 1 at a time. If you shot one out towards one location on the moon, and then shot one out half a second later towards a totally different location on the moon, then the first photon would hit one part of the moon and half a second later another photon would hit another part of the moon. Even if the two locations on the moon are far enough such that d/.5s > c (d being the distance between them), nothing weird is happening here, as it's easy to see. Now just make that laser shoot many many photons at once, it's still nothing special.

    BTW, the speed of light is more like 300,000,000m/s not 300,000m/s.
  5. Nov 6, 2011 #4
    Thanks, sorry about messing up the speed of light, pretty dumb mistake, i guess change meters to kilometers and now it works, OK, so nothing is actually travelling faster than the speed of light, because it is not the same photon hitting the wall in one direction as it is when i turn the laser pointer. Right?
  6. Nov 6, 2011 #5
    You got it :)

    You can also rotate your head 'faster than light', relative the stars you see as you do it.
    At night I mean.
  7. Nov 9, 2011 #6
    Haha, in fact, i guess you are always travelling "faster than light" if you assumed that we were still, and the stars just revolved around us every day. Of course that's not true, but it would seem that way.
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