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What is the relationship between light, time and space?

  1. Jan 26, 2013 #1
    I looked for this topic in the FAQs but to my surprise could not find it.

    The question as to whether or not one can say that light experiences time and distance seems common on the internet, as are answers. The trouble, from the point of view of someone like myself (a hitch-hiker on the scientific journey of discovery) is that conflicting answers can be found.

    One of the most troubling responses is along the lines that, because one cannot define a frame of reference in which a photon is stationary relative to anything else in the Universe, the question is meaningless and should not be asked. This strikes me as being akin to the following:

    Where did the Universe come from?

    God created it.

    beyond this there is no discussion, which is more than a little restricting where scientific inquiry is concerned.

    I have formulated some ideas of my own, but as a non-scientist I could be very wide of the mark, so I would appreciate thoughts from the world of science before I go any further.
     
  2. jcsd
  3. Jan 26, 2013 #2

    Drakkith

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    Relativity, which is what describes spacetime, simply doesn't allow for a frame of reference to be assigned to a photon. If another theory is developed that does, then we can discuss how time passes/doesn't pass for a photon according to that theory. We can discuss what happens to a photon now, but not in a meaningful manner since any such discussion is pure speculation.
     
  4. Jan 26, 2013 #3

    Dale

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    Conflicting answers to the question "what is 2+2" can also be found. The fact that a conflicting answer exists says more about the nature of humans than about the quality of the answer.

    That seems like an extremely poor analogy. In fact, I cannot see the least bit of resemblance. The reason that there is no inertial frame where a photon is at rest has no religious overtones whatsoever. It is purely a matter of logic, such a frame is a logical self-contradiction.

    Just because a question can be asked doesn't mean that it is a valid question. And just because someone points out that a question is invalid doesn't make them a religious zealot.
     
    Last edited: Jan 26, 2013
  5. Jan 26, 2013 #4
    There is a mysterious relationship between these things. But, to understand those things you have to clear the ideas of space and time. First of all you have to be familiar with Einstein's theory of relativity.
     
  6. Jan 27, 2013 #5

    pervect

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    One poster suggested (and it's a good idea, I think) that instead of saying "X's frame of reference" that one should say "a frame of reference in which X is at rest".

    Then "the rest frame of a photon" becomes "a frame of reference in which a photon is at rest".

    It then becomes reasonably obvious I think, as to why no such thing exists in relativity.

    A bit of historical perspective may help as well. People originally expected to find that Maxwell's equations applied only to one special frame of reference. They therefore expected to be able to detect an "ether wind" in any other frame of reference. When experiments were performed, no such "ether wind" was observed. Looking for logical explanations for this, Einstein came up with relativity, which basically says that Maxwell's equations apply in ALL frames of reference.

    Since Maxwell's equations do not allow for a stationary photon, the "frame of reference in which a photon is at rest" aka "the frame of reference of a photon" doesn't exist (within the theory) either.

    So - to recap. Relativity wasnt the first idea humans had, it's just the one that best matches experiment. And it does not allow for "frames of reference" in which a photon is stationary - this is integral to the theory. Insiting that it does or should permit "frames of reference of a photon" will prevent one from learning this experimentally well-tested theory.
     
    Last edited: Jan 27, 2013
  7. Jan 27, 2013 #6

    bcrowell

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    PF has a FAQ on this:https://www.physicsforums.com/showthread.php?t=511170

    Here is a longer one that I wrote:

    FAQ: What does the world look like in a frame of reference moving at the speed of light?

    This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

    The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

    Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

    What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.
     
  8. Jan 27, 2013 #7
    Thanks folks for the responses. A particular thanks to Ben for your patient explanation.

    I still have several questions, but perhaps I can dispense with a few of them by clarifying one point from Ben's explanation:

    "(This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.)"

    Is this not the same as saying that a particle moving at c would experience no time or distance?
     
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