Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light waves

  1. Aug 20, 2012 #1
    Can anyone tell me how light waves propogate. Will a photon have a trailing cone?
     
  2. jcsd
  3. Aug 20, 2012 #2

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    If a photon left a trailing cone, like a wake behind a boat or any other body moving through a medium, then we would be able to track its progress. But we can't.

    We can't track the progress of light. If we could, we could identify a medium in which the light waves propagate. This is what scientists prior to Einstein (and a few afterwards) were trying to do.

    Einstein showed that we could just assume that light propagates at a speed of c in any inertial state we choose and it would be just like if we really were at rest in a medium that propagates light.
     
  4. Aug 20, 2012 #3
    The way to test this would be to have two spheres one inside the other. The inner sphere would contain the slits as in the two slit experiment with a photon fired through. Detecting a 360 degree interference pattern around the sphere would be a proof.
     
  5. Aug 20, 2012 #4
    You could also have two cones one within the other at a set angle.
     
  6. Aug 20, 2012 #5
    Interestingly if this worked you may actually measure the size of a photon.
     
  7. Aug 20, 2012 #6

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Any experiment that you can devise in an arbitrary inertial state that is supposed to identify the propagation of light will result in a false positive. So you better only do the experiment one time since on the surface of the earth, we are continually changing our approximately inertial state (due to the rotation of the earth, its revolving around the sun and the motion of the sun through our galaxy) and you apparently want to ignore all the other experiments that already confirm that we cannot identify the propagation of light.
     
  8. Aug 20, 2012 #7
    I haven't read the experimental data. Do you have any links?
     
  9. Aug 20, 2012 #8

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

  10. Aug 20, 2012 #9
    I have read it and digested it. The interference patterns are very interesting. In particular the spacing and width of the dark band. This doesn't, however preclude what I was saying in the first instance. Light speed is constant to all observer's. Whatever frame you are in. There is a peculiarity to light that no one seems to have noticed. If it were possible to film a photon from directly behind as it was fired, what would you record? The photon is traveling away from the observer. Would you still record it? Would you record it's entire journey to its target?
     
  11. Aug 20, 2012 #10

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    How do you expect to film a photon? It's not a bullet that you can shine a light on and see it from reflected light. A photon is light traveling away from you or on some path that doesn't intersect you. If it does intersect you, particularly in the eye, that's the end of the photon and you see it when it arrives at your eye but it doesn't provide a clue about when and where it was on its way to you, does it?

    You cannot record, measure, observe or track the progress of a photon or even vast collections of photons making up light waves. What you can do is what Einstein suggested, which is to assign a propagation to light, but remember, when you then say that light propagates at c, that's because you defined time and distance in such a way for it to do just that.
     
  12. Aug 20, 2012 #11
    How do you know you are right? Have you tried the suggested experiment?
     
  13. Aug 20, 2012 #12

    russ_watters

    User Avatar

    Staff: Mentor

    You have not suggested an experiment. All you did was say something that is gibberish and assume it means something. The phrase "...film a photon from directly behind as it was fired..." is meaningless at best. Scientists (and not just scientists!) film photons all the time. How photons are "filmed" is understood exquisitely well. You "film" a photon by colliding a detector with it. Period.
     
  14. Aug 20, 2012 #13
    OK how about this. If you had two spacecraft outside the earth's atmosphere that shine a wide beam of light directly from one to the other so that the photons are absorbed completely at the target, would you see the light? There is no medium through which it travels so reflection would not be an issue. Would you see it through a telescope? It is not travelling in your direction. Working from your argument the light beam would be invisible.
     
  15. Aug 20, 2012 #14
    I also suggested the sphere and cone experiments.
     
  16. Aug 20, 2012 #15

    russ_watters

    User Avatar

    Staff: Mentor

    "Will it leave a trailing cone?" Is not an experiment either. Your "sphere experiment" is just the double slit experiment and it doesn't produce the results you suggest.
     
  17. Aug 20, 2012 #16

    russ_watters

    User Avatar

    Staff: Mentor

    Of course. Spacecraft do this. Heck, you can test this yourself by looking at satellites from your backyard.
    No, you would not see the beam. Yes, it is invisible.
     
  18. Aug 20, 2012 #17
    Thank you. That's all I wanted to know.
     
  19. Aug 20, 2012 #18

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    No, you would not see the light. How could you, you just said "the photons are absorbed completely at the target".
    If you want to see the beam, then you need to have some kind of an atmosphere to reflect part of the light back to you.
     
  20. Aug 20, 2012 #19
  21. Aug 20, 2012 #20

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    But the particle that is traveling faster than the speed of light in the material is not a photon, it's a charged particle. Why does this make you think of a photon leaving a "wake" in the shape of a cone? If that were the case, then there would be cones all over the place in all directions in the example of the cherenkov radiation, wouldn't there?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Light waves
  1. Light Waves (Replies: 13)

Loading...