Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question from the FAQ on Rest Frame of a Photon

  1. Aug 3, 2011 #1
    In a previous thread someone pointed me to DH's FAQ article on the Rest Frame of a Photon. In the article DH states, "Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero," and I've been thinking about the implications of that thought. The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?
     
  2. jcsd
  3. Aug 3, 2011 #2

    Dale

    Staff: Mentor

    Yes, it is all meaningless in the hypothetical rest frame of a photon. Therefore such a frame doesn't exist.
     
  4. Aug 3, 2011 #3
    Okay. But the light going at c isn't hypothetical. Must there be a "frame" for DH's principles to apply? If so, why?
     
  5. Aug 3, 2011 #4

    Dale

    Staff: Mentor

    Yes, light going at c is not hypothetical in any inertial frame.

    The mere fact that you can string together some words does not imply that they represent a self-consistent concept. In the case of the words "inertial rest frame of a photon" there is a self-contradiction since "inertial" implies v=c and "rest" implies v=0. That is why it is nonsense. There is nothing deeper to be learned, and the self-contradiction of a photon's frame does not imply anything amiss for self-consistent concepts.
     
  6. Aug 4, 2011 #5
    DaleSpam, I'm not sure what you are upset about. I did a search for your quoted phrase ("inertial rest frame of a photon") and could not find it used anywhere else in this thread. And the closest thing to it came from DH's article, not my thoughts. If you have a problem with it, I suppose you need to address it to the folks in charge here at PF that decided to use his stuff as the authority on the subject. But I don't think the phrase is mine, and I'm not sure how to field your complaint.
     
  7. Aug 4, 2011 #6

    Pengwuino

    User Avatar
    Gold Member

    Ok here's the deal.

    You CAN assume the rest frame of a photon exists. However, IF you do, everything ELSE stops making sense. We KNOW Maxwell's Equations work in all frames of reference. They've never been shown to fail, ever. IF you were to create this frame where a photon is at rest, in other words an inertial rest frame, you would be looking at a static electromagnetic field. Such a field is in contradiction to Maxwell's Equations, such a field is NOT a solution to the equations. That means Maxwell's Equations are wrong or the photon must not have a inertial rest frame one can observe from. What's more likely? The photon has no inertial rest frame, consistent with our experimental observations, or that maxwell's equations are wrong , completely contrary to over a century of millions if not billions of experiments showing it to be true?
     
  8. Aug 4, 2011 #7

    Dale

    Staff: Mentor

    I don't know what would make you think I am upset.

    I thought you understood that when we say "Bob's frame" it is short hand for the more correct but cumbersome "inertial rest frame of Bob" or "inertial frame where Bob is at rest".

    Similarly, when you say "photon's frame" it is short hand for "inertial rest frame of the photon". If you are interested in non-inertial frames then a whole different set of problems arise, and if you are interested in frames where the photon is not at rest then it isn't the photon's frame.

    The concept is self-contradictory nonsense.
     
  9. Aug 4, 2011 #8
    Pengwuino, I am not (knowingly) assuming anything about the photon rest frame. Relativity holds that a photon has speed (c) even without having having a rest frame, right? And DH's article states that as something approaches c certain characteristics in time and length/distance are noted. Therefore, I put the two together and get the ponderings of my original post -- with or without photon rest frame. So where have I screwed up?
     
  10. Aug 4, 2011 #9
    Okay, in the absence of voice inflection and the like it is easy to misinterpret the tone of written conversation. If that is the case here, my apologies.


    Yes, I understand the nomenclature. But what I don't understand is why you're addressing these issues with me -- they are not my words. The folks here at PF have set this article (with its specific terminology) as authoritative, not me; and if you believe the wording to be self-contradictory I don't think I'm the one that can do anything about it.
     
  11. Aug 4, 2011 #10

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    You don't understand this nomenclature which you quoted from the FAQ:
    Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero.​
    What that means is as a velocity approaches the speed of light, but never getting there, the time and length intervals approach zero, but they never get there. The whole purpose of the FAQ is to address the meaninglessness of the concept of a rest frame for a photon. But instead of leaving it at that, you say:
    The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?​
    Why do you persist in thinking about meaningless ideas and asking us to think about them too?
     
  12. Aug 4, 2011 #11
    I wouldn't say you've "screwed up".

    "And DH's article states that as something approaches c certain characteristics in time and length/distance are noted."

    I'm just someone who likes this stuff too.

    DH's comment about time and distance characteristics changing as something accelerates to c is critical in perceiving why a photon is not a frame for reference. Photons don't exist in 3D. They exist in the time dimension.

    The time dimension is not able to provide a reference for motion, like objects in the material dimensions can.

    As you accelerate to c, you are accelerating into the time dimension, leaving the 3D world behind so bye bye reference frames once you "enter" the time dimension entirely like a photon.
     
    Last edited: Aug 4, 2011
  13. Aug 4, 2011 #12

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    What was originally said by DH is equivalent to this:

    True statement: by going sufficiently fast, a traveller can travel an arbitrarily long distance (measured in the lab frame) in an arbitrarily short time (as measured by the traveler's watch).

    Dr Don seem to be leaping from that to the following incorrect conclusion:

    False statement: a traveler can travel an arbitrarily large distance(as measured in lab frame) by the laboratory watch.

    The usual short version of this is "time is relative".

    Because it will probably come up later, I'll mention another false statement that doesn't follow:

    False statement: A traveller can travel an arbitrarily long distance (as measured in the traveller's frame) in an arbitrarily short time (as measured by the traveller's watch).

    Short version: distance is relative too.
     
  14. Aug 4, 2011 #13
    Because without asking, I can not understand what is meaningless and what is not. I am an admitted newbie in the field, and we infants have a way of putting everything into our mouth. Without a mentor to point the way I have no means of differentiating between the cheerios on the table, the match on the bar, and the dog droppings on the carpet. Many on this forum have been so kind as to hold my hand as I struggle with my early steps -- and I am thankful for that kindness. If such babysitting is not your cup of tea, please feel no obligation to read or respond to my baby drool.
     
  15. Aug 4, 2011 #14
    By Jove..., I think you're right (in identifying my problem), at least partly. :approve:

    Okay, so "light year" and "speed of light" do have meaning because they are measurements from my frame of ref, which is not traveling at c and thus not warping time or distance. I'm on board with that.

    But the part about the length of time that it takes for light to reach me from a distant star..., that question still isn't settled in my mind. Yes, from my ref. frame I can see how I would perceive it taking a super long time. However, (with or without its own frame of ref.) that light beam leaves the star traveling at c (a relativity given), and according to my understanding of DH's article, that makes both time and distance equal zero in its "travel". Thus, the light instantly covers the non-distance in zero-time for what from my ref. frame is measured to be a very vast distance and a vast time.

    My first reaction is to say that this doesn't match with experimental evidence which measures the speed of light. But as I think about it, those experiments are conducted on "our side" of the picture, and thus may not reflect what happens on the light's side.
     
  16. Aug 4, 2011 #15

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Let me repeat your statement from the first post:
    I've been thinking about the implications of that thought. The ramifications of this statement suggest (to me) that since light travels at c, when it leaves a distant star it is instantly visible to us on our planet (since the distance traveled at c is zero); the term "light year" becomes meaningless (since time at c is zero); and even the idea of "speed of light" has no meaning at all (since speed is a factor of both distance and time -- and both are zero at c). Any thoughts?​
    You don't have any trouble knowing that these are meaningless ideas. It's not an issue of your not knowing which ideas are meaningful and which are meaningless, you've identified them as meaningless. My question is: why do you want to continue to think about meaningless ideas after you have put them in your mouth and been told by your babysitters that they are meaningless and to spit them out and don't put them back in your mouth? Nobody's complaining about your asking questions, just why do you reject the answers?
     
  17. Aug 4, 2011 #16

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    DH's article does not say that the time and distance for a light beam are equal to zero. It says that's a meaningless idea. The light doesn't have a side. That's also a meaningless idea.
     
  18. Aug 4, 2011 #17
    Man, the interaction helps me know if the tentative hypotheses have merit. If they do, fine; if they don't those more knowledgeable can help me see the errors. While you have complained about my approach, Pervect has been gracious enough to walk me through what I needed to see (see post #14), and my search of understand has made a significant step forward.

    I do not reject the answers; however, my drive is to understand the answers, and that pushes for deeper questions as those initial answers are given to me. Just that simple.
     
  19. Aug 4, 2011 #18
    Very well said,
    Building an intuition for this stuff doesn't happen just because you are told something is "meaningless". You need to "see" for your self why.

    You've been told it's poop, but you have to "put it in your mouth" to be certain that it's not good.

    :rolleyes:
     
  20. Aug 4, 2011 #19

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    On the previous thread that you mentioned in your opening post, you asked this question:
    And I gave this answer:
    And you responded:
    You rejected my answer, made no attempt to understand my answer and did not even respond to my question.

    You are asking similar questions on this thread and you're not going to understand the answers until you quit being lazy and concentrate on the answers that you are given and then answer the questions that you are asked.
     
  21. Aug 4, 2011 #20
    I'm not sure if this has actually been pointed out to you [edit: now I see that it's been done], but those issues only come up if you use a (physically impossible) reference system in which the light ray is in rest. The purpose of that FAQ is probably to explain why you should not try to use such a nonsensical reference system.
    That has no bearing on the meaningful reference systems that we use, and all definitions relate to those systems.

    Hmm, perhaps it was not sufficiently elaborated?!
    There is no "photon frame". As a matter of fact, no frame "belongs" to anything, nor do we need a physical "frame". Many people simply use as a shorthand description "so-and-so's frame" for "reference system in which so-and-so is in rest. However, for a description of the photon trajectory(or any other trajectory) as function of time, you can freely choose any physical or imaginary (but physically possible!) reference system that is in uniform linear motion. As no clocks or rulers can move at the speed of light, and measurements with such a system would be meaningless (like 0/0), it doesn't make sense to choose such a system.

    Best,
    Harald
     
    Last edited: Aug 4, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Question from the FAQ on Rest Frame of a Photon
  1. Rest frames (Replies: 2)

  2. Rest frame of the photon (Replies: 16)

Loading...