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Questions about photons. Their interaction with time and their life-cycle.

  1. Oct 25, 2011 #1
    Do photons experience time? If they move at the speed of light then special relativity states that they do not, but then does that mean they do not move forward in time which means the distance they travel is instantaneous (thus it is infinitely fast), which we know is not true (as they move at the speed c).

    Furthermore, what happens when a photon ceases to exist? How does it cease to exist? Does it hit my body and 'poof' it is gone? what is process by which photons end their life.
     
  2. jcsd
  3. Oct 25, 2011 #2
    There was recently an ample thread about a similar subject, about 12 pages long, that was unfortunately deleted.
    So now we'll probably need 12 pages again to answer this.

    When they cease to exist they collide with another particle (which can also be another photon) and they create other particles (which can also be other photons).
     
  4. Oct 25, 2011 #3
    About experiencing time, a short answer would be "they don't".

    Speed of light travel is actually "instantaneous (thus it is infinitely fast)", in the reference frame of the photon, if you can define such a frame.

    Edit: And I make an appeal to bring the other 12 pages long thread back. Maybe trim it a bit, but a lot of science advisors spent a lot of their time and put a lot of effort in those posts. Just trim the crackpot posts and leave the others, don't delete the whole thread.
     
  5. Oct 25, 2011 #4

    atyy

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    One has to distinguish between coordinate time and proper time.

    If you choose a particular Lorentz inertial coordinate system and use that to draw a spacetime diagram, a photon's wordline is a 45 degree line in spacetime, along which a photon does advance in coordinate space and coordinate time.

    However, there is also proper time (or its analogue for null vectors). For a massive particle travelling less than the speed of light, the proper time is the time elapsed as read by an ideal clock travelling with the particle. However, for photons, this quantity is zero or null. This quantity being zero could be loosely said to mean that time does not pass for photons.

    A property that is equivalent to photons travelling on null paths is that their speed is the same regardless of their wavelength. Hence two light waves travelling in the same direction with different frequencies will be "frozen" relative to each other. This could also be loosely said to mean that time does not pass for photons.
     
    Last edited: Oct 25, 2011
  6. Oct 25, 2011 #5

    ZapperZ

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    Please start by reading the FAQ sub-forum in the Relativity forum.

    Zz.
     
  7. Oct 25, 2011 #6

    jtbell

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  8. Oct 26, 2011 #7
    Very sweet explanation atyy.
    If I may add.

    'Proper time' is just the time 'any/one/thing' finds itself to have locally as measured by its wrist watch. And for a 'photon' that 'time passed' should be null intrinsically, but not for us measuring its 'propagation'.
     
  9. Oct 26, 2011 #8
    "If they move at the speed of light then special relativity states that they do not ...": this is not true.
    At any speed any object "experiences time".
    The singularity of the Lorentz transformation only reflects how clocks can be compared, not the flow of time.
     
  10. Oct 26, 2011 #9
    That's debatable. It might be about semantics and what exactly a particular person means by "experiences time".
    Any object traveling with the speed of light (a massless particle) does not experience the flow of time.
     
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