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Should the speed of light be slightly uncertain?

  1. Apr 8, 2013 #1
    The position and momentum of a photon is uncertain. If that is the case, then shouldn't the speed of light be slight uncertain?
     
  2. jcsd
  3. Apr 8, 2013 #2

    strangerep

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    Have you ever tried to construct a good position operator for the photon in the relativistic context? Afaik, no one has done it in a fully satisfactory way.
     
  4. Apr 8, 2013 #3

    Integral

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    We do not need to measure the location of a photon when finding the speed of light.

    Read this about the definition of the meter.

    By definition the speed of light is an integer, therefore no uncertinatiy
     
  5. Apr 8, 2013 #4
    Don't know what you mean by integer. Also didn't understand why you wanted me to read the wiki article on meter.
     
  6. Apr 9, 2013 #5

    Integral

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    Because you have to read to learn. NOW READ IT.
     
  7. Apr 9, 2013 #6
    I read it before you advised me to read it of course and I still don't understand what you're getting at. If what you're getting at is the speed of light is relative to the observer, well, I already know that but I still don't see why it's speed cannot be uncertain.
     
  8. Apr 9, 2013 #7

    Integral

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    If you had read all of it you would know why the speed of light is an integer.
     
  9. Apr 9, 2013 #8
    Never mind, I give up
     
  10. Apr 9, 2013 #9

    Dale

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    After 2 hours and 40 minutes. Really?

    c is a defined constant, as such its uncertainty is 0. What is uncertain is the length of a meter. That is the point to take from the reading.
     
  11. Apr 9, 2013 #10
    Thanks. I appreciate your help.
     
  12. Apr 9, 2013 #11
    My god, you guys, what a bunch of rude and pointless arguing of semantics you're giving out here! Those answers completely ignore the intent of the question. I have no idea what you were trying to accomplish with that.

    g.lemaitre: The speed of light is a consequence of relativity, not of quantum mechanics, so the types of uncertainty that pop up in studies of quantum behavior don't really apply to it. Lorentz invariance implies that there is a maximum speed that things asymptotically tend towards as you try to accelerate them, and the principles of Quantum Field Theory tell you that a massless particle can't propagate at any velocity except for that speed, so the speed of light is thus completely fixed.
     
    Last edited: Apr 9, 2013
  13. Apr 9, 2013 #12

    Integral

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    I will spoon feed you some more. From the article I was trying to get you to read.

     
  14. Apr 9, 2013 #13

    Integral

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    This is a bit backwards, Relativity is a consequence of the constancy of the speed of light. The constancy of the speed of light is due to the nature of the universe. Lorentz invariance was COOKED to model the constancy of the speed of light. So naturally it has a max speed.
     
  15. Apr 9, 2013 #14
    Except that that wasn't at all what the question was asking. The question was: since the Heisenberg Uncertainty principle limits our ability to simultaneously know the position and momentum of a particle, is it possible EVEN IN PRINCIPLE to know the speed of light exactly? A satisfactory answer to that question must involve some reference to the physics of relativity and quantum mechanics, not a history lesson.
     
  16. Apr 9, 2013 #15
    Right, except for one detail. Lorentz invariance implies there is a maximum speed that anything can go, but it doesn't necessarily tell you that light must go at that speed. In order to show that, you have to use Quantum Field Theory, where you show that massless particles have four-momenta with magnitude zero, meaning that they travel at exactly that top speed. Then Lorentz invariance tells you that they must travel at that speed in all frames of reference.
     
  17. Apr 9, 2013 #16

    Integral

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    Seems to me in post #10 the OP was happy with the being spoon fed the contends of the article I pointed him at. So maybe he really didn't understand his original question as well as you do.

    My entire effort was pointed at getting the OP to think for himself just a bit.
     
  18. Apr 10, 2013 #17
    Well the easy way of telling this maybe would be that yes we don't know the exact position of a particle but the speed of light doesn't depend on this one particle to measure , we can just have a beam of light from one point traveling down a certain distance (assume a large one as the speed is very high ) and then take a measurement on a very precise clock to see how many "ticks" or "vibrations" of an cesium atom (in a atomic clock) have passed.
    Now the rest is elementary maths so remember that we have measured light speed on the macro scale and to do that doesn't require to know where a particular photon is at a given time as they usually come many not one.
     
  19. Apr 10, 2013 #18
    Saying that we now DEFINE light to have a certain 'exact' speed doesn't really address whether it HAS that speed or not, and if it does, can we measure it....'exactly'....

    OP:

    Why do you think it is 'uncertain' ??

    from the current discussion....
    first part: Well, as has been discussed in numerous threads in these forums, that is NOT what HUP says.....just briefly from those discussions:





    One of the lengthy discussions is here:

    https://www.physicsforums.com/showthread.php?t=516224


    Note there are likely conflicting viewpoints along the way....so read along before getting 'all fired up' [lol]

    And of course this IS an issue, but has nothing to do with HUP....this is a problem with measurement apparatus.


    Another discussion is here:

    Do particles have well defined positions at all times
    https://www.physicsforums.com/showthread.php?t=499976

    But BEWARE: THE BALLENTINE paper seems to draw some questionable conclusions!!
     
  20. Apr 10, 2013 #19
    I forgot to post:

    http://en.wikipedia.org/wiki/Speed_of_light#Measurement

    ......doesn't seem like photons are routinely used to 'measure the speed of light' anyway....


    edit: so to answer the original question, it seems like we are limited only via our ingenuity in developing ever more accurate measurement devices.
     
    Last edited: Apr 10, 2013
  21. Apr 10, 2013 #20

    phinds

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    And you did it in a way that I think reflects how good this forum is, that you take the time to figure out what might be helpful to an OP and you nudge him/her towards it. It's hardly your fault that some people resist nudging and thinking.
     
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