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Why is the speed of light erm the speed of light?

  1. Jul 15, 2009 #1
    I'm not entirely sure if this is the right place to ask this question, but here goes:

    I have a question that's been plaguing me for a while. I understand why it is pointless to think in terms of a photon's rest frame, as it is either impossible or meaningless to define said frame. I also understand why the speed of light should be finite and constant, given the permittivity and permeability of free space, and the aspects of relativity that state it should be constant in every frame of reference.

    However, I cannot think intuitively to reconcile these bits of knowledge. How is it that to us, a photon takes some finite amount of time to travel any given distance, but the photon does not experience time, and is emitted and absorbed at the same instant?
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  3. Jul 15, 2009 #2


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    Does a rock experience time ? Or anything ? Photons are inanimate ( if they exist at all ) so I don't see the point of your question. When a photon is created, travels somewhere and is absorbed - time has passed between those events.

    What's to reconcile ? You're just confusing yourself with naive literalism which you mistake for insight.

    The title of this post is misleading because your question has nothing to do with the speed of light.
  4. Jul 15, 2009 #3
    Well of course I do not mean 'experience' as in the photon somehow is conscious and can take in information about its surroundings. I use it in a figurative sense. I thought that much would be apparent.

    Time does not pass for the photon, yet we observe time passing between the emission and absorption of said photon. Am I missing something really simple here?

    I do not appreciate the tone you take for me asking about something that puzzles me. I get that I may not accurately have expressed what it is I wish to. That does not warrant mocking me.

    Do you have any suggestions for what I should change the title to?
  5. Jul 15, 2009 #4


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    There's no better place. :smile:

    That sounds good, but then you say this:

    Yes! The third quote above only makes sense if the photon has a co-moving inertial frame (i.e. a "rest frame"...which you claimed to understand that it doesn't have).
    Last edited: Jul 15, 2009
  6. Jul 15, 2009 #5


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    What you're missing is that the above is meaningless. You are mistaking the statement "light travels on null geodesics" for a statement about experience.

    It is a postulate of SR that for time-like ( by which I mean sub-luminal ) worldlines the proper time may be understood to mean the elapsed time on clocks on the worldline. This postulate does not include null geodesics. So SR does not say that the time experienced by a photon is zero, even if you admit to the concept of 'photon experiencing time' which I do not.

    I'm sorry about the tone. Forget the title. Maybe the above will help.
  7. Jul 15, 2009 #6
    I see now.

    Thanks for the answers. That really was something simple I was overlooking.
  8. Jul 15, 2009 #7


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    Last edited by a moderator: Apr 24, 2017
  9. Jul 16, 2009 #8


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    Yeah, I was hoping for a good discussion on dimensionful and dimensionless universal constants (or at least on the numerical value of c).

    It seems like we haven't had one of those in a while.
  10. Jul 16, 2009 #9
    Dave, what are the implications of this interpretation for causality? That is, wouldn't an event somewhere along the photon's path affect its entire 'lifespan', if the entire lifetime of a photon - from emission to absorption - is one event?

    I was thinking about this at age 15 or so, certain that I had just stumbled on the explanation for 'spooky actions at a distance' - that two entangled photons shared behavior without delay because the information was being transferred 'backward' up one photon-path and then down the other photon-path, all in the same 'time-slice'. It broke my heart when I learned that entanglement occurred with sublight particles, too, as I was looking forward to that Nobel prize :)
    Last edited by a moderator: Apr 24, 2017
  11. Jul 17, 2009 #10
    As I see it all the problem is of philosophical nature in particular epistemological nature.

    Physics relate the world of physical phenomena with a certain world of discourse that in this case is a mathematical discourse that have the task to make predictions. Unfortunately there is no guarantee that there is a 1-1 mapping of the physical phenomena to the discourse or even that all the predictions made by the discourse have a physical phenomenon related. In fact the leap from the physical world to the physical discourse (the converse is no so controversial) have no recipes (although some people think that actually there is a SCIENTIFIC METHOD).

    So if you think that the time for a photon "does't pass" you think that only because Relativity (the Discourse) says (through the mathematical machinery) that arc length over a null geodesic with the Minkowski metric is always zero.

    But that's only a mathematical result, a prediction of the Discourse. There's no Physics on it (no more than the physics that is implied by the axioms of relativity), then you have to pass from the Discourse to the Physical World.
  12. Jul 17, 2009 #11


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    The point is, there is no causality; space-time is static. The event that intersected the photon is also static.
  13. Jul 18, 2009 #12

    Causality doesn't exist? Spacetime is static? That's going a bit far I think.

    What makes you say this?
  14. Jul 18, 2009 #13


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    The "static" view of spacetime in no way negates the causal structure of spacetime. For any event there is a past light cone and a future light cone. Any causes must lie within the past light cone, any effects must lie within the future light cone, and any events outside both light cones cannot be causally related. This causal structure remains in the "static" view of spacetime.
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