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Crazy one - photons and time

  1. Apr 24, 2008 #1
    As a photon travels at the speed of light, it does not experience time. For a photon the beginning and end of time are simultaneous.

    However then does anything ever happen to a photon, as events must happen in time?
  2. jcsd
  3. Apr 24, 2008 #2
    I am sitting in front of my desk, and photons are created on my screen and allow me to write those lines as I collect them with my eyes. Philosophical arguments about what they feel about their fate is not much part of a scientific discussion. Indeed, in the mathematical limit of a referential at light speed with respect to a given observer, proper time freezes. But no actual device can be brought to this limit physically.
  4. Apr 24, 2008 #3


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    I made https://www.physicsforums.com/showpost.php?p=1684890&postcount=20" to explain this just last week.
    Last edited by a moderator: Apr 23, 2017
  5. Apr 24, 2008 #4
    You could argue, quite successfully I might add, that the photon is created here, destroyed there, and nothing happens to it in between.

    Alternatively, you could parametrize the photon's geodesic by an affine parameter that is not proper time. This approach is often used in general relativity for null geodesics. This parameter encodes how the photon's phase evolves as it moves through space, but of course no real observer could attach a clock to that phase.
  6. Apr 25, 2008 #5


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    This argument falls short when you consider things that can happen to photons in between, such as refraction.
  7. Apr 25, 2008 #6
    Well, the time between interactions just becomes very short. Refraction in a material happens due to the absorbing and re-emitting of photons. You can visualize it as a born series of sort. In vacuum, the only thing you'd have to worry about is gravity, and in which case I will refer to my second point =)

    The question isn't all that meaningful anyway, but seems to be popular.
  8. Apr 26, 2008 #7
    PF mentors - please place a link to the main thread regarding "photon perspective" if you lock this one out. I couldn't find it.

    I think someone could pose an argument that the photon frame doesn't "experience" a change in time. I've always argued for that perspective in the past. But now I also believe the better argument would be that, since it is an entity with energy and information, that it should "experience" a minimum of the plank length of time, and that the extent of the photon energy / information transfer is simply time dilated to it's distance in a given observer's frame of reference.

    So I believe that starlight from a star 500 light years away - cannot be contracted to less than the Planck length from any frame. Then the question comes up - is the photon really a light speed entity, or is it really the speed of light less the fraction 1 Planck length / 500 light years? Only photons on the scale of the Planck length, if this were the case, would be discernable from the full speed of light... The entity could be a sublight entity time dilated to appear to be the spead of light - or such a small fraction less than c, that it is practically c in all cases (even sub-atomic distances).

    However, regarding the interactions along the way - such as refraction, etc. These could also be considered as part of the singular entity as a whole - i.e. there would be different potentials along the entities surface area (as if it were a static effect) - because the final interaction for each photon is always singular... it's always behaving in a quantum way at the endpoints regardless of the potential mess that effects the path between the points.
  9. Apr 26, 2008 #8


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    There is no "main thread" as such. This subject has come up repeatedly over the years. The best way to search PF is via Google. You can restrict Google to search only PF, by including "site:physicsforums.com" in the list of words that you're searching for. For example:

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    Has this idea been published in the usual scientific literature and/or is part of current discussion among physicists, or is it purely a personal speculation? If it's the latter, you should be aware that we don't go in for that sort of thing here, as per the Physics Forums Global Guidelines, in particular the section on Overly Speculative Posts.
    Last edited: Apr 26, 2008
  10. Apr 26, 2008 #9
    Here you are saying both that a photon experiences time and that it does not experience time. You say that As a photon travels at the speed of light, it does not experience time and then go on to say For a photon the beginning and end of time are simultaneous which implies one is comparing times and therefore time has a meaning for a photon. It is meaningless to say either than a photon does not experience time or that it does. No experiment can determine this since no clock can be at rest in a photon's frame of reference. The statement that the proper time between two lightlike events being zero is merely an extrapolation since a proper time interval between two events is the time as measured by a clock which passes through these two events. This is impossible when the events have a lightlike spacetime seperation as are all the events on the worldline of a beam of light (or worldline of a photon).

  11. Apr 30, 2008 #10
    The answer is simple. Imagine someone knocked unconcious. Then people around this person partying and finally one of them get drunk and take a knife to kill that person. That person knows nothing but things still happens around him. That is similar to what happen to a photon. Things do happen ( including the creation and destruction of that photon ) but that photon cannot percieve even with the photon having a conscious mind because everything happened to it within an infinsimal amount of time.
  12. Apr 30, 2008 #11
    A related question that I've wondered about is this: I read somewhere that spin is a "relativistic effect", i.e. it arises when one applies relativistic principles to quantum mechanics. Does this mean that the photon doesn't "experience" its own spin?

    Regarding locking threads that mention the perspective/frame of reference of photons/light, even though photon perspective is a fiction, can't we at least keep such discussions active for their assistance in illustrating principles? In other words, isn't it useful to just accept "perspective of a photon" as an academic crutch - like the idea of an asymptote, something that is never really reached but that can help one understand the thing that approaches the asymptote. I mean, really, all of calculus is an example of the use of such an academic crutch, as there is no such thing as an "infinitesimal" interval of a dimension along which any real world object extends.
  13. Apr 30, 2008 #12


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    Because it's a bad analogy. Which is why it leads to trouble. This thread serves as an illustration of the principle that starting with a bad analogy leads to erroneous ideas.
  14. Apr 30, 2008 #13
    I agree with Dave here, the path of a null curve is categorically different from the path of a timelike curve. Also notice that a timelike curve is not a null curve in the limit.
  15. Apr 30, 2008 #14
    DaveC, you said:

    But earlier you said:

    I liked your elevator analogy - but are you saying now that this elevator analogy isn't any good?

    Also, let's not forget that Einstein himself spoke of how things would look from the perspective of light - this was the line of thought that eventually resulted in his theory of relativity. So, clearly, considering the perspective of light can lead to useful insights.
    Last edited by a moderator: Apr 23, 2017
  16. Apr 30, 2008 #15


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    No, the elevator analogy is a good analogy. It demonstrates how we observe photons as we move through time, while simultaneously demonstrating how the photons themselves do not experience time. QED.
  17. May 1, 2008 #16
    Right; so what's wrong with saying that "photons don't experience time"? I don't think there's anything wrong with that statement, but previous posts suggest that there is.
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