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Distances for classical objects vs. photons

  1. May 26, 2015 #1
    Good afternoon all,

    A few days ago, I had been reading a book on general relativity and cosmology by Dr. Brian Greene, in which something was written that I found to be very profound. (At least, from the standpoint of my own ignorance on the subject.) I was wondering if any professionals could point me in the right direction to learning more about this quandary.

    In the book, Dr. Greene went on to describe how classical objects, like us, experience the vast enormity of spatial extent in the heavens, yet photons do not. More specifically, it was mentioned that the distance between objects from a photons perspective is zero.

    This utterly blows my mind. How is this even possible? Is there some kind of underlying prevailing wisdom I'm unaware of wherein distances are just an artifact of spacetime experienced by classical objects?

    Forgive my speculation, but I'm completely unaware of how to reconcile this statement.
     
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  3. May 26, 2015 #2

    PeterDonis

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    While Brian Greene is a well-known physicist, his pop science books are not good sources if you actually want to learn about physics. In this particular case, his description depends on an implicit assumption that a photon has a "rest frame" in which the concepts of "distance" and "elapsed time" make sense. That assumption is false: see the PF FAQ entry on this topic, and also the Usenet Physics FAQ entry on a similar topic.
     
  4. May 26, 2015 #3
    Indeed; thanks Peter! Appreciate your help.
     
  5. May 26, 2015 #4

    Chronos

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    Yes, under relativity the passage of time is relative to the observer's inertial reference frame. Since a photon does not have an inertial reference frame, it does not perceive the passage of time - which renders the concept of distance meaningless under the familiar equation d = vt.
     
  6. May 26, 2015 #5

    PeterDonis

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    It's probably better to say that the concept of "perceived passage of time" does not even apply to a photon, to avoid any possible misunderstanding.
     
  7. May 26, 2015 #6
    Incredible. I'm reminded daily that I picked the right career to get into.

    Let me push this a little further, and please reign me back into the correct ideas if I'm getting too far into left field.

    Could this effect *ever* apply to non-photon objects if acceleration to near speed of light was a technological possibility? Or do these explanations apply explicitly only to photons?

    Again, thank you both.
     
  8. May 26, 2015 #7

    PeterDonis

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    What "effect" do you mean?
     
  9. May 26, 2015 #8
    Excuse me, sorry; the effect analogous to a photon experiencing timelessness and a distance of zero.

    It seems strange that this is something only experienced by a photon in its own perspective. The whole scenario makes me speculate whether this is some underlying fundamental characteristic of the universe itself, yet we experience things completely differently and are completely unaware of it in our own inertial frames of reference. (i.e. - is the universe really of "zero" size?)

    However, I feel like I'm getting far too into the lands of philosophy and armchair rationalization.
     
  10. May 26, 2015 #9

    PeterDonis

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    Ok, but the whole point is that the photon does not "experience" these things; the concepts of "experienced time" and "distance" are meaningless for a photon.

    A photon does not have a "perspective" in this sense; as the links I gave say, there is no such thing as an inertial reference frame in which a photon is at rest. This is a fundamental difference between lightlike objects (like photons), which travel on null worldlines, and other objects (like us) which travel on timelike worldlines. The concepts of "experienced time" and "distance" only make sense for objects traveling on timelike worldlines.

    In terms of SR and GR, the difference between null and timelike worldlines (and objects that travel on them) is a geometric property of spacetime. So in that sense it is an "underlying fundamental characteristic".
     
  11. May 26, 2015 #10
    Okay. I think it finally clicked now.

    I apologize for making you repeat yourself, Peter - my understanding in physics is purely Newtonian at this point (classical undergraduate), so shaking the incorrect, preconceived notions I've carried all my life for the prevailing wisdom in SR and GR is still very alien and counterintuitive.
     
  12. May 26, 2015 #11

    Chronos

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