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Time stops at light speed?

  1. May 14, 2008 #1
    Okay, basic question regarding time and photons and I have a little thought experiment to illustrate my confusion.

    From what I understand, light waves do not experience the passage of time since they are travelling at light speeds, but suppose you are a hyopthetical conscious observer from the point-of-view of a photon travelling through space.

    If you are not experiencing the passage of time, then what do you experience? For example, how do you account for your past experience in the beginning if you were launched from, say, a flashlight?

    If a photon possessed consciousness, does it possess knowledge of ALL its experiences--past, present, and future--simultaneously? And futhermore, what does it "see" as it whizzes through space?
     
  2. jcsd
  3. May 14, 2008 #2

    mathman

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    The best way to look at it is to consider something (X) going at almost light speed. X experiences time as anything else. However distances along its trajectory are greatly forshortened. This is the essence of the twin paradox. For example going to alpha centauri (4.3 light years away) could be almost instantaneous, since in X's frame it would be a very short distance away.
     
  4. May 14, 2008 #3
    I have always thought that the photon exist on its entire path at one instance. Therefore it has not experiences, no past, no future, just now. And since at light speed the entire universe has a width of 0 the photon would see a flatened universe. So when we combine those 2 ideas the photon might "see" a flatened universe with objects everwhere they should be between when it was created till when it dies.
     
  5. May 14, 2008 #4

    DaveC426913

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    This is why the speed of light is not a valid frame of reference. You cannot look at the universe from a photon's PoV, even in principle.

    For an idea of how to visualize a photon's not experiencing the passage of time, read this.
     
  6. May 14, 2008 #5
    At all…?

    Couldn’t you, at least /most, have a tunnel vision of your world?

    I mean, as you are moving in any direction, couldn’t you see the reflected or emitted light of anything that is, at best, within your normal 180 degree range ?

    Regards,

    VE
     
  7. May 14, 2008 #6
    isn't time always the same to the observer? Meaning that you'll never notice time "stopping" or slowing down, as long as you're the one who's in that particular time frame.
     
  8. May 14, 2008 #7

    DaveC426913

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    At all.
     
  9. May 14, 2008 #8
    Dave…

    Agreed with no movement or ‘time’ but no agreement with the fact that the string has not experienced any emission or absorption… it’s just normal that being bathed in daylight or any artificial light, the string would, of course, absorb and emit photons…


    regards,

    VE
     
  10. May 14, 2008 #9
    Ok,

    there has to be something I'm not grasping correctly here... I'm a photon , and hypothetically, I have a functionning sense of sight...

    I agree that any light that is emited in back of me or parallel to me , I will not 'see', but couldn't I resolve any light that has been emited / reflected in my direction ?

    What's the flaw in this thought ?

    VE
     
  11. May 15, 2008 #10

    jtbell

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    Questions related to the "perspective" or "point of view" of a photon have come up repeatedly here, as the following Google searches show:

    photon perspective site:physicsforums.com

    photon point of view site:physicsforums.com

    photon reference frame site:physicsforums.com

    In order to be able to say what things "look like" from the point of view of an object that is moving at constant speed with respect to something else, you have to be able to define an inertial reference frame (system of space and time coordinates) in which that object is stationary, and there has to be a Lorentz transformation between your inertial reference frame and the object's inertial reference frame. However, there is no such transformation where the relative velocity equals c. Try plugging v = c into the Lorentz transformation equations. You'll find that the result is mathematically undefined. Therefore, relativity theory cannot answer questions about the "reference frame of a photon."
     
    Last edited: May 15, 2008
  12. May 15, 2008 #11

    DaveC426913

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    You're missing the point of the analogy. The piece of yarn IS a photon.

    The elevator is moving through time (the vertical dimension in this model).
    The yarn is not moving at all.
     
  13. May 15, 2008 #12

    DaveC426913

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    Nope. A sense of sight requires an experience of time. You have neither.

    Go back to the elevator analogy. The passenger on the elevatgor sees time pass as he rises through floors in the building. The peice of yarn does not experience different floors in the building, it just spans several of them, unmoving.
     
    Last edited: May 15, 2008
  14. May 15, 2008 #13
    Thanks for the responses, guys. As you can probably tell, I am not a science major but I have always been fascinated by theoretical physics, especially in subjects related to relativity and string/M theory.
     
  15. May 15, 2008 #14


    Actually, I think your previous post on the thread, in which you said this:


    ...answers the photon's POV question quite nicely. From what I understand then is that a photon experiences all of its past, present, and future moments simultateneouly, correct?
     
  16. May 15, 2008 #15
    No, I think a photon would only experience the point in the observer's time corresponding to the photon's emission (what the observer would call the past). The photon wouldn't know anything about what the observer calls the present or the future.

    Let's take a particle emitted from somewhere far away that travels at 99.999% of the speed of light and that is absorbed by the eyeball of a human observer on earth. If the particle was emitted in what the observer calls Jan 1, 1960 and absorbed by his eyeball in what he thinks is 2008, the particle will think that everything in the universe is almost frozen in Jan 1, 1960 ("almost frozen" because its speed is slightly less than the speed of light). If the particle's velocity is constant (i.e. no acceleration), instead of seeing the human that its about to bash into, it'll see whatever was occupying the point in space where the observer's standing back in Jan 1, 1960 (of course, once it actually bashes into the guy's eyeball, it will decelerate to zero very quickly, thus screwing up the picture).

    I like to use this model: imagine you have a globe. Now imagine a stationary observer as someone who is on the equator of the globe, and imagine the particle moving at 99.999% of the speed of light as being almost at the north pole. Now imagine both are moving east along their respective line of latitude (the lines that move horizontally across the globe) from the Americas to Europe/Africa. Let this eastward motion represent movement through time. Let "1 second" be represented by the chunk between two neighbouring lines of longitude (the lines that run vertically up and down the globe). As you go up the globe, that chunk that we call 1 second will get narrower and narrower. For the guy at the equator, more stuff will happen during each "1 second". For the particle near the north pole, very little will happen for each "1 second". So the guy at the equator sees the particle close to the north pole hardly changing each second - i.e. almost frozen in time. Think of light as being AT the north pole, and therefore not changing at all during each "1 second".

    Meanwhile, the particle close to the north pole thinks it's really at the equator and the guy on earth is the one close to the north pole (which is a perfectly valid assumption since our designation of north pole and equator is entirely arbitrary) - so it's like spinning the globe around 90 degrees, and the whole thing gets reversed. Now movement from the Americas towards Europe/Africa no longer represents time - instead, time is represented by movement from say Canada down to Antarctica. With this new definition of time, it looks like the guy hardly changes for each "1 second", but the particle changes a lot.

    This is why each sees the other as frozen in time while still feeling normal passage of time in its own frame of reference.
     
  17. May 15, 2008 #16

    DaveC426913

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    No. This is totally wrong. The photon does not experience time.

    If one must bend physics to the breaking point, one might pretend that the photon is an unchanging line with one end attached to a star very far away and the other end attached to an eyeball.
     
  18. May 15, 2008 #17
    First, a particle that moves at 99.999% of the speed of light experiences time. What was wrong with my description of what that particle would see?

    Second, as the word "Relativity" suggests, something moving at some constant velocity relative to an observer appears to that observer to age slower than the observer. The thing that's moving, however, feels that it's aging normally but thinks that the observer is aging more slowly. This was the whole point of the twin paradox. The hypothetical of something moving at the speed of light (such as light itself) is simply the limit of this phenomenon, i.e. that the moving thing would continue to experience time as normal in its own frame of reference but think that everything else is frozen in time. Again, where is the error in this (other than the usual objection to the hypothetical of something moving at the speed of light)?

    Are you saying that if moving at 99.99999999999999999999999999999% of the speed of light, something experiences time as usual, but then, bam, if it were to hit c, suddenly it stops experiencing time?
     
    Last edited: May 15, 2008
  19. May 15, 2008 #18
    Ok, so following this, how would you answer the OP's question about what this "unchanging line" would see? Would it see:
    a) all point in time simultaneously - so the observer's eyeball, the hippie from the 60's who was there before him, the martian from the future who will be there after him, etc?
    b) only the observer's eyeball and nothing that came before or after him?
    c) only what was in the point in space that the observer's eyeball was located when the light was emitted (so, probably some hippie from the '60s)
    d) none of the above [I'm guessing you're going to choose this - I'd appreciate an explanation rather than the usual cop out of it being a bad analogy, etc...after all, this is your analogy].
     
  20. May 15, 2008 #19

    You are absolutly right, but missing one thing.. length contraction. When you are traveling a finite distance then as you aproach speeds closer and closer to c the universe contracts more and more. When you finally reach C the universe is completly contracted and your start and finish have no distance, hence time would continue as normal if there was any time to travel. So its not so much a Bam everything is changes but a gradual shrinking of distance traveled.
     
  21. May 15, 2008 #20
    That's right. If you used all the energy in the universe to continue accelerating the particle moving at 99.99999999999999999999999999999% of the speed of light and continued accelerating the particle for the rest of eternity, it still wouldn't be any closer to the speed of light because the speed of light relative to the particle in the particles own reference frame is still c.
     
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