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Why do photons naturally travel at c?

  1. Nov 26, 2004 #1
    Why do photons naturally travel at c? Do photons have an actual size to them? If so, what is it? Im trying to figure out the nature of the photon.

    :zzz:

    There are 3 types of knowledge in the world, knowledge, understanding, imagination
     
  2. jcsd
  3. Nov 26, 2004 #2
    size is an extremely elusive thing i guess. The reason why photons travel at c is because that they are massless and they have infinite range in space as well (right?).
     
  4. Nov 26, 2004 #3

    russ_watters

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    Well, you can use the anthropic principle: light travels at the speed of light because it is light. That's pretty axiomatic.
     
  5. Nov 26, 2004 #4
    The question was not why light goes at speed of light, but why light goes at c?
    Although c is called "the speed of light" it is not only the speed of light but of other "things" also. Since, historically, light was the first thing to be discovered that goes at c, from this c got its name. A more appropriate name would be for example "velocity invariant under Lorentz transformation". As such the question of why light travels at c is a legitimate question.
     
  6. Nov 26, 2004 #5

    cepheid

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    Such as...?
     
  7. Nov 26, 2004 #6

    dextercioby

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    Try gravitons,if they're ever to be found.And any other massless representation of the restricted Poincaré group.Neutrinos,maybe...Though they're pretty much thought to have a mass...Anyway...
     
  8. Nov 26, 2004 #7

    Garth

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    This might help, my post #3 in the thread "space-time and the speed of light ".

    Why do photons naturally travel at c?
    Garth
     
    Last edited: Nov 26, 2004
  9. Nov 26, 2004 #8
    Hi,

    Photons travel at the velocity they do because of the properties of the space in which they are traveling. The two properties known today which determine the speed of light are electric permittivity and magnetic permeability.

    juju
     
  10. Nov 26, 2004 #9
    Maybe photons trabel at c because it is a "particle" which allows "space" to "freely" enter it, so we dont interpit photons to have mass. :wink:
     
  11. Nov 27, 2004 #10
    Gamish: "Im trying to figure out the nature of the photon."

    Near the end of his life Einstien wrote to his friend Besso "...all these years of conscious brooding about the photon have brought me no closer to the truth. Nowdays every Tom, Dick and Harry thinks he knows the answer, but he is mistaken."
     
  12. Nov 28, 2004 #11
    I guess your right on that one. In all of Einstines understanding, he did not know it all. There have been times in my life where I said "I understand", and then later on I find out that I didn't. But, "imagination is greater than knowledge", as Einstine said. :wink:

    Im the master at time!
     
  13. Nov 29, 2004 #12

    Chronos

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    The whole 'c' thing revolves around the very basic premise of causality. Without an invariant, background independent point of reference, such as invariant speed of light, causality breaks down. Which is to say the very concept of time itself vanishes, as Garth pointed out. To truly understand this [in my opinion], you also need to be familiar with the mathematics of Galilean and Lorentzian transformations. They are not as terribly complicated as you might think. In many ways, they are more elegant and logical than any verbal explanation.

    The only assumption required to make these models work is that light speed [time] is measurable and has a finite speed: which agrees well with observation. Try this link to see why:
    http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html#c2
     
    Last edited: Nov 29, 2004
  14. Dec 1, 2004 #13
    And if there are not enough explanations yet...here is an other version from the QFT people...photons travel at the speed of light because they have no restmass. They have no restmass because they do NOT interact with the Higgs-field. The reason for this : well, the U(1)-symmetry of the EM-interactions is never broken after the contineous breakdown of symmetry...(ie when nature has "chosen" one of the possible vacuum-states). This system is used to generate mass of elementary particles which are massless in QFT because a mass term mixes up the different chiralities which correspond to two different fundamental properties...To exemplify : left and right handed chiralities do NOT behave in the same way when they undergo weak interactions...

    regards
    marlon
     
  15. Dec 1, 2004 #14

    jcsd

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    As has been mentioned, given that it has no rest mass, it travels along a null geodesic (it's a fairly simple excerise to show this) and hence travels at c. Of course many will say this is just circular reasoning as we often use the assumption that it travels on a null geodesic to show that it has no rest mass!
     
  16. Dec 2, 2004 #15

    Andrew Mason

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    Asking why the speed of light is c is a little like asking 'why is the lowest temperature -273 C? Our concept of temperature relates to another concept: kinetic energy of atoms. -273C or 0 Kelvin is the natural limit of kinetic energy of matter.

    The 'photon escape' phenomenon necessarily defines time and distance for matter. No object having rest mass can attain the state of a photon. As matter tries to approach the photon state, the photon continues to escape. Thus c represents a natural limit on the rate at which matter can interact. As matter approaches the photon state, time and distance approach 0.

    c, which defines the photon state, is the absolute zero of space and time.

    AM
     
  17. Dec 6, 2004 #16
    I think that the question is a bit more profound than may be immediately apparent. I am intrigued that Special Rel is basically determined by the behavior of a quantum entity (the photon, of course)-- the disconnect between GR and quantum may ultimately hinge on the fact that there is a subtle connection between quantum properties and relativity that we are not quite getting.

    To say that light is on the "null geodesic" is, I think, just a tautology. The speed of light is not like absolute zero -- a minimum temp is predicted by the definition of heat. What in the universe predicts that EM travels at a specific vel? It is just a discovery.
     
    Last edited: Dec 6, 2004
  18. Dec 6, 2004 #17

    jcsd

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    The fact that the original formulation of SR depends on the behaviour of light is merely an accident of history, you should recognize that it is NOT necessary to formulate it in such a way.

    A geodesic can be thought of as a (locally) distance (using the word distance in the sense of the metric function) minimizing path, a null geodesic is simply a geodesic where the distance between all points on the path is zero, any massless particle can be shown to travel along a null geodesic.
     
  19. Dec 6, 2004 #18
    OK - I know that you can get to SR from several directions; but, really, what is a null geodisic other than a path where (ct)^2 - x^2 - y^2 - z^2= 0 --- i.e., where something is travelling the (constant) speed of light. Again, all the formulations of SR end up equavalent to "c is constant" (e.g., the British textbook approach that the frequency shift follows a constant K or 1/K depending on direction of v, regardless of which emitter is taken to be "at rest").

    And even if the particular formulation "the speed of light is constant" is not in itself important, some sort of interaction between quantum and SR is happening at a very basic level.

    Now maybe that is merely a banal trusim and I am not "getting it" somehow --- but I'm still not convinced that this speed limit donen't contain a bit more truth than we have yet.
     
    Last edited: Dec 6, 2004
  20. Dec 6, 2004 #19

    jcsd

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    Yes, but your equating the constant c with the speed of light so of course the speed of light will be constant!

    Infact some prefer not to call 'c' the speed of light for precisely this reason, i.e. it can lead to the confusion that light has some special almost magical role to play in relativity.
     
  21. Dec 6, 2004 #20

    Andrew Mason

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    If one were to attain the speed of light, time and space would collapse. As an observer approaches the speed of light (relative to the geometric centre of the universe, say), the size of the breadth of the universe approaches 0 and time slows so that the observer takes foreever to travel even that small distance.

    If there was no matter in the universe there would be nothing to define a frame of reference. Location (ie. position in relation to another point in space) and time would have no meaning as these can only have meaning in reference frame. So a piece of matter, in effect, sucks back a frame of reference from an otherwise dimensionless universe. In this sense, light isn't moving at all (ie. to a matterless but illuminated universe). Matter simply defines location and time. Light will appear to move from one point to another. But that is just a consequence of defining a reference frame. To the photon, there is no such thing as motion, distance or time.

    AM
     
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