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Speed of light being constant

  1. Sep 15, 2013 #1
    Speed of light is constant because photon is massless. If this is wrong please correct me.
    So is it that any massless particle will move at speed of light? Do we know any such particles?

    There is this analogy that explains how particles get mass. Basically the higgs field gather around heavy particles and not around lighter particles. So when a photon moves there is no resistance from the higgs field and hence it moves fastest.
    But considering this analogy if another particle moves towards a photon with velocity u, speed of light will increase.
    What is wrong with this analogy?

    Is there any explanation why speed of light remains constant or is it just another fact, another one of those things that we have to accept?
  2. jcsd
  3. Sep 15, 2013 #2


    Staff: Mentor

    Gluons are also massless. Gravitons are hypothesized to be massless also.
  4. Sep 15, 2013 #3
    A massless particle always travels at speed of light. This can be proved but i can't :(.
    I see it this way: if you have a massless particle not traveling at the speed of light, there exists a frame of reference in wich the particle is at rest, but because the rest mass is zero, it's energy would be zero and it does not exist. This is not a good reasoning but it should give you a good feeling about it.
  5. Sep 15, 2013 #4
    The reason that the speed of light is constant is this: there is no preferred frame of reference. For all non accelerating observers physics is the same. This is a very beautifull and actually intuitive symmetry. If you know maxwell's equations of electromagnetism, you should conclude with the above symmetry that the speed of light is constant and the same for all observers.
  6. Sep 16, 2013 #5
    So do gluons move at speed of light?
    I don't understand how you can conclude that speed of light is same for all observers.
  7. Sep 16, 2013 #6
    No one knows why the propagation speed of light in space is constant, or why it has the specific value of c. It would seem to depend on the structure of space, since permittivity (electric constant) and permeabliity, measured and defined values for properties of space(vacuum) determine c. Space cannot be 'nothing' if it has properties, can exhibit quantum fluctuations of virtual particles, and be deformed by the presence of large masses.

    The perceived speed of light is constant because of length contraction and time dilation. These are complementary effects resulting from motion induced phenomena, that preserves a constant relation of space/time.
  8. Sep 17, 2013 #7
    It's another fact you have to accept. In fact, it goes deeper than physics. It's an axiom of geometry. This has nothing to do with massless particles. In a mathematical structure called Minkowski space or spacetime, there is a velocity which is constant for all observers. If you look at it geometrically, velocity is an angle between a world line of an observed particle and a world line of an observer. In Minkowski spacetime, there exists a special angle that is always constant for all world lines. In other words, there exist such straight line that is always at the same angle to all other lines sharing a point with it. With some imagination, you can assign a value 45 degrees to that angle. This may sound odd, but this is the axiom. A mathematical statement, which has nothing to do with physics yet.

    Now for some reason the universe we live in has the Minkowski geometry. This is a physical falsifiable statement, an observational fact that has been tested and the tests turned out positive. There is a limit velocity in our universe, just like in Minkowski spacetime.

    Now there is a certain kind of particles called photons, that travel at that speed. If they travel at that speed to one of the observers, then they have that speed relative to any observer. Photons are what light is made of, so we call it the speed of light, but that's a bit misleading. The limit velocity has nothing to do with light. It's a coincidence that light happens to travel at that speed.

    Now some other laws of dynamics (which you have to believe as they are) state that if something travels at the speed of light, then it has zero rest mass.
  9. Sep 17, 2013 #8


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    Staff: Mentor

    From our Relativity FAQ (at the top of this forum):

    https://www.physicsforums.com/showthread.php?t=534862 [Broken]
    Last edited by a moderator: May 6, 2017
  10. Sep 17, 2013 #9
    At first you seem to refer to the second postulate: relativity theory simply postulates that light speed is a constant, as a law of physics. One explanation (the oldest) is that it is a property of space. More commonly it is held to be a property of spacetime.

    But at the end you make a system transformation: you "jumped" to a different reference frame. There you used assumptions of classical relativity (Newtonian, also called "Galilean") that have been discarded with special relativity (SR). SR applies the first postulate to the speed of light: all laws are invariant with frame transformations (second postulate). Frame transformations have nothing to do with Higgs fields. But as you correctly remark, those two conditions (postulates) are contradictory according to classical mechanics. If you abandon classical mechanics then they are, as Einstein put it, "only apparently irreconcilable". - http://www.fourmilab.ch/etexts/einstein/specrel/www/
    The oldest explanation is that length contraction and time dilation assure invariance. A more popular explanation is that it is a property of spacetime. But again, being a postulate it is not explained by SR.

    Part of the confusion may be caused by some text books that don't clearly distinguish between a velocity difference and a system transformation. For more, see older threads on similar topics, at the bottom of this page.
    Last edited: Sep 17, 2013
  11. Sep 17, 2013 #10


    Staff: Mentor

  12. Sep 18, 2013 #11
    Oops I had rearranged a phrase and forgot to delete "(second postulate)" - that may be confusing.
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