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Speed of a photon relative to another photon

  1. Mar 27, 2007 #1
    Say a beam of light is cast somewhere - are the photons transmitted at a speed equal relative to one another, or would one photon still perceive the one behind it as moving at light speed?
     
  2. jcsd
  3. Mar 27, 2007 #2
    If you would see from the point of view of a photon...another photon moves at the speed of light relative to it....for a photon ...there is no conception of space and time
     
  4. Mar 27, 2007 #3
    Photons ALWAYS travel at the speed of light. So, this answers your question, no ?

    marlon
     
  5. Mar 27, 2007 #4

    pervect

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    Photons don't actually experience time. Technically, this means that the Lorentz interval along the worldline of a photon is zero (null), while the Lorentz interval along the worldline of any massive object is timelike.

    It is because the Lorentz interval along the worldline of a massive object is timelike that massive objects (such as human beings) exerience time, and paramaterize their worldlines by it.

    While photons can paramaterize their worldline by an affine parameter, this affine parameter cannot really be considered to be time.
     
  6. Apr 2, 2007 #5
    Do they now?.. I thought you could slow down a photon if you push it through a space occupied by other particles. Don't photons move slower through the atmosphere, or through a glass prism?
     
  7. Apr 2, 2007 #6

    ZapperZ

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    Please read the FAQ in the General Physics forum.

    Zz.
     
  8. Apr 2, 2007 #7
    That sort of makes sense, so the photon doesn't actually slow down, it just takes a longer route through the medium?

    ps. minor spelling/grammar error on the FAQ
     
  9. Jul 7, 2009 #8

    Fredrik

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    There no natural way to define the "photon's point of view". See my posts in this thread.

    In particular, there's no inertial frame that has the world line of a photon as its time axis.

    It doesn't. Photons have speed c in all inertial frames, but not in general coordinate systems.

    Edit: I didn't realize how old this thread is.
     
    Last edited: Jul 7, 2009
  10. Jan 19, 2010 #9
    if 2 photos are travelling, they would observe each other as 'stationary'.
    if a being on one photon fired another photon, the 2nd photon would travel the same speed as the 1st photon i.e the 1st photon will observe the 2nd photon travelling at the same speed, and the 2nd photon observing the being on the 1st photon trAvelling together. of course, this situation is possible if the being on the 1st photon was made of mc².
     
  11. Jan 20, 2010 #10

    HallsofIvy

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    First a photon cannot "observe" anything- as Fredrick said, it has NO "point of view".

    Also, throughout this you talk about speeds without saying what they are relative to. Third, I have no idea what you mean by "made of mc2".
     
  12. Jan 20, 2010 #11
    By "observe" I meant any form or method of detection. Obviously this won't be sight (using EM waves from the visible spectrum)

    Relativity of the speeds are to the photons in travel.

    "made of mc²" means an 'energy being'.
     
  13. Jan 20, 2010 #12
    In the theory of Millenium Relativity it shows that light-or any form of EM radiation, or c, is the only thing that is not relative, and is not affected by the motion of the source. Assuming that these photons are travelling through the same medium, then there would be no difference, but if there was, no matter the speed of the observer, the speed of light remains a constant.

    -Harry
     
  14. Jan 20, 2010 #13

    George Jones

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    Note that the Physics Forums rules,

    https://www.physicsforums.com/showthread.php?t=5374,

    to which all member agree when they register, state
     
  15. Jan 20, 2010 #14

    DaveC426913

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    Photons do not detect in any sense at all. Photons do not experience time. Photons do not have a valid frame of reference.

    Sorry, this is nonsense either way.
     
  16. Jan 20, 2010 #15

    HallsofIvy

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    I did not ask for a definition of "observe"- the point is that photons, having no "point of view" cannot observe anything.

    You cannot measure speed relative to a non-existant frame of reference and photons have NO frame of reference.

    In other words, it doesn't mean anything!
     
  17. Dec 27, 2010 #16
    I once heard Feynman say something like: ”c is just the mean value, some photons go slower, some faster”.
     
  18. Dec 27, 2010 #17

    DaveC426913

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    You met him? What was he like?
     
  19. Dec 27, 2010 #18
  20. Dec 27, 2010 #19

    Fredrik

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    Do you have a reason to think of those coordinates as the "reference frame of a photon"? And don't just say that a photon is stationary in it. (There are lots of coordinate systems with that property).
     
  21. Dec 27, 2010 #20

    pervect

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    If you stick to linear transformations of (x,t) where (x,t) are Minkowskian coordinates, I think there's only the one set that keeps light in both directions stationary.
     
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