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Photon's frame of reference

  1. Jan 6, 2008 #1
    According to Einstein's relativity theories, each observer is afforded a frame of reference which can be considered central and stationary. What would the universe look like to the frame of reference associated with a photon observer. Would all things be moving past the photon at the velocity c? Would such things be moving in one direction or in all directions? What would other photons look like to the photon observer which considers itself at rest?
  2. jcsd
  3. Jan 6, 2008 #2


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    Actually, Einstein said -
    In this context 'inertial' means not accelerating and with relative velocity < c.

    There is no 'frame of reference' for the 'photon', so no answers can be given to your questions.

    Einstein reportedly had this thought and concluded it was logically impossible to 'catch up with light' many years before he formulated special relativity.
  4. Jan 6, 2008 #3
    You'll find plenty of discussion of this idea if you search through older threads; it comes up a lot.

    Mostly, I expect you'll find that people will respond by pointing out that no observer (in any sense of "observer" that we have) can be accelerated to the speed of light, so the question is meaningless. You can still work through the transformation equations of Special Relativity to see how various quantities behave as you go to the limit of v -> c, but you'll find that most of them go to infinity or to zero, with the result that you can't really speak in a meaningful way about what the universe would look like from such a frame.

    A more specific response might point out that time slows to zero as you go to v -> c, so in such a frame you can't talk about time passing. So what does anything look like from the point of view of a reference frame where there is no time? It means nothing.

    (Note: I don't mean this to be critical of the question - it's a perfectly reasonable one and is actually what allegedly led Einstein to come up with the theory of Relativity. It's not immediately obvious that it's a meaningless question.)
  5. Jan 6, 2008 #4


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    Hi belliot,
    I've no quarrel with what you've said, but

    This cannot happen, of course. Nothing ever graduates from from sub-light speed to light speed. I just want to emphasise this to the OP in case he misunderstands 'v -> c'.

  6. Jan 6, 2008 #5


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    He didn't say anything about an object "graduating" from sub-light to light speed. He was talking about the limit of the formulas in the mathematical sense.
  7. Jan 6, 2008 #6


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    yes, I understand that, as does belliot - but did the OP, who clearly is a neophyte to SR.
    Last edited: Jan 6, 2008
  8. Jan 6, 2008 #7
    Thanks for your response. I didn't take your response as critical. I'm grateful for the insight. Perhaps if Kaluza had been right and light were a phenomena of 4 spatial dimensions my question would make more sense. I'll think about it some more and see if I have any other questions. Thanks again.
  9. Jan 6, 2008 #8
    Wallin, you have had a lot of sensible formal answers so I hope you do not mind this this slightly light hearted point of view. :tongue:

    Imagine you were going at very close to the speed of light and some bizarre quantum fluctuation or uncertainty principle accidently boosts to you to exactly the speed of light. (It can't happen but bear with me ;) Assume you survive being squashed to a volume of zero width, which is a big problem in itself. Your rulers will be zero length and your clocks will have stopped so you won't be able to measure distances or relative velocities of anything else. The lenses of your telescopes (and the lenses of your eyes) will be completely flat so you wont be able to focus on anything because lenses have to be curved to work. So you won't see anything. All process including biological processes slow down as you aproach the speed of light, so your brain slow down too and at the speed of light your thought processes stop. So at the speed of light your eyes and brain don't work so you would not have a point of view. :surprised
  10. Jan 6, 2008 #9
    Thanks for the levity, Kev. I feel like that almost every morning!! Perhaps the condition you describe is more common than those who commented before you have let on! The reason I asked the question is because it seems to me that velocity we call c is something we take terribly for granted (kind of like the Dome of Fixed Stars in the millenium and a half before Copernicus made his correction to the geocentric model). Everyone held that the motion of the Dome of Fixed Stars was an unchanging constant. In some ways, it seems like the velocity c (which applies to all electromagnetics and the propogation of gravity) might be like the Dome of Fixed Stars. In other words, can we say with absolute certainty that the light and gravity are moving at c or is there another option? Could it be that we (the observable universe) are moving in relation to gravity and light in a way we have not yet detected? I better stop there because I can feel myself on the verge of breaking the rules and rushing into the realm of speculation. Apologies.
    Last edited: Jan 6, 2008
  11. Jan 6, 2008 #10
    I guess we could take something that is not constant like the velocity of an object falling in a gravitational field and declare that it is constant. With a bit of mathematical ingenuity we might be able build a model that is self consistant and yet has a non constant speed of light. I'm not sure because I haven't tried it, but it might be interesting and instructive to try. On the other hand, such a model might already exist or at least have been attempted. Anyone know if that is the case?
  12. Jan 6, 2008 #11


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    I think a theory that violated SR would predict contradictory things, like two observers seeing different outcomes to the same experiment. Special relativity ( and logic) demands that this is not so, and the invariance of local velocity of light is a necessary condition. If locally measured light velocities vary, it means local values of epsilon and mu and that causes problems of another kind.
    There are cosmological theories where c changes over time, but SR applies at any given time.
  13. Jan 6, 2008 #12
    Just for fun, or actually serious interest of a non-expert, I'd like to pose a few challenges:

    1. Doesn't a photon (or anything moving at c) deserve a frame of reference, like any element of reality?

    2. Even though this is not a gradual process, the energy that constitutes a photon was at sub-c before the photon was emitted. So it is possible for energy to 'jump' from sub-c to c.

    3. Time stands still for a photon ... but isn't that only from a sub-c frame of Reference? Any other particle would not notice a slow down of time in its own frame of reference, when accelerated to a speed close to c.

    4. Isn't the fact that a photon can be seen as a particle and/or an element of reality, yet that the formula of SR don't apply to photons, a paradox not resolvable within SR (nor GR), and points to SR being incomplete? And there is no other theory completing that picture yet?
  14. Jan 6, 2008 #13
    Completely wrong.
    Approaching light's speed at an acceptable acceleration, nothing special happens to you and to your starship (excepting for the effects of acceleration).

    In your ref. frame, what you measure as lenght and time contracted are objects moving at near c with respect to you.
    Last edited: Jan 6, 2008
  15. Jan 6, 2008 #14


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    What for ? No physicist that I know of has ever found it necessary.

    You're making assumptions about the emission process that might be wrong , but obviously energy from the emitter takes the form of light, and therefore is moving at c.
    What's the challenge ?

    No, this doesn't have meaning because the photon does not have a frame of reference. The time dilation effects apply between inertial frames. You're mis-using the formulae.
    No. How does the existence of a light quantum violate SR ?
    Last edited: Jan 6, 2008
  16. Jan 6, 2008 #15
    It seems that this means that in relation to a photon, even though it exists in physical reality, the laws of physics do not take the same form as in relation to other elements of reality.

    Also, it seems to me that for Einstein it was important that physical statements are about reality, rather than just of computational usefulness for the physicist doing his job.

    I couldn't see which assumption is wrong. Are you saying it is (or might be) a gradual process, and that the energy is gradually accelerating to c, in so far as one can say that energy accelerates?

    Either way it seems that a photon, as a mass-less particle, is an exception to otherwise general "rules", such as that acceleration to c is not possible. Which means that these rules are not universal.

    I don't really understand that statement. To me it would seem that one could try, as a theoretical exercise, to describe events and establish a coordinate system, so to speak, from the perspective of a particle moving at c. It maybe that SR can't do that, but why would that be a-priori not a valid attempt?

    I don't think it was me misusing the formula, on the contrary, you seem to be saying the same thing as I did.
  17. Jan 6, 2008 #16


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    The existence of photons as free 'particles' is not established. You are mistaken in giving to them particle properties. Any inference based on this assumption is wrong. Please read some books about this, you are just wrong.

    All and any. There is no model for the emission process in current physics. How do think emission proceeds ?

    There's no such thing as a particle moving at c !

    You're free to waste your time as you please.

    I absolutely deny that. How do you work that out ?

    Answer my question, please.

    [sorry about all the edits - lots of typos]
    Last edited: Jan 6, 2008
  18. Jan 6, 2008 #17
    I should have responded to this separately since it was the reply to #4. However my answer is implicit in my other responses. I did not say that SR is violated, but that it is incomplete, based on that photons (or anything moving at c) is an exception to many rules and formula, which are therefore not universal. And this even though energy that wasn't moving at c can change to doing so, and vice versa (when a photon is emitted or absorbed). That means that the same energy becomes subject to the SR rules, then not, then again, apparently without a reflection of this "mutation" within the theory itself.

    [Edit: I just wrote this response before reading your latest reply above. I hope it still applies... I'll be reading it now.
  19. Jan 6, 2008 #18
    My argument doesn't really depend on photons being particles. Let's say they are wavicles. Or do you deny quantum physics?

    Again my argument doesn't depend on any specific understanding of the emission process.

    (Just on the side, though, I've recently heard that they are emitted for example when electrons change their energy level, but I've also heard there are other ways a photon can be created.)

    I simply haven't said what you are objecting to. Please re-read my original statement.

    I've meanwhile done so, above, before reading your immediately preceding reply.
  20. Jan 6, 2008 #19


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    Now you are being insulting. So it's my turn to point out to you -

    Congratulations, considering that's been known since about 1913. I learnt that at high school many years ago, and have learnt a few more things since then.

    Keep learning and you'll be OK, but I'm tired of arguing this with you now, so, good night.
  21. Jan 6, 2008 #20
    That's not an insult, but a question that seemed adequate given your previous response: I've had many (interesting) discussions with people very knowledgeable in physics, yet actually denying quantum physics.

    Oh, I thought "There is no model for the emission process in current physics". Perhaps you meant: not a detailed model, however I don't see how that would be relevant to our discussion. BTW, I guess at some time long ago, I learned that as well... :)

    Ok, good night.
  22. Jan 7, 2008 #21
    An interesting thought. I don't know whether this means SR is incomplete other than SR's equations does not apply to object of speed c. And if photon can't have a (inertial) reference frame, what other sensible (whatever it maybe) reference frame it may have? It does seem unsatisfactory that SR has no answer about this.

    I am not sure exactly what you are asking or what it means by "the speed of energy" or "speed change of energy".

    http://groups.google.com.tj/group/alt.philosophy.debate/browse_thread/thread/7cfefc9c6bfb0f96 suggests the following.

    The force, according to the Newton’s Second Law,
    is equal to : F= ma.
    This force is possible to consider as absolute independent
    quantity - impulse. When in case with light quanta
    the impulse is equal to: mc.
    He continued.
    Let us now imagine that light quanta falls on a black body,
    and it absolutely absorbs this light quanta
    ( it means light quanta stops).
    Then, according to the Lebedev,s law, light quanta
    renders pressure on the black body: E/c.
    Therefore it is possible to write: mc=E/c.
    It means that light quanta has not mass of rest equal to zero,
    but it has potential energy/mass: M=E/c^2. (E=Mc^2).

    Since SR does not apply to v=c, I am not sure whether your premise of stand-still time is sound.

    good question.
  23. Jan 7, 2008 #22


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    Clearly there are some distinctions between massive and massless particles in relativity, but I don't see any overriding reason to exclude the latter from particlehood. That of course that entirely depends on your definition of particle, though the very existance of the term 'massless particle' should indicate that massless particles are indeed widely held to be particles.

    Clearly the formulae for massive particles don't apply to photons as they are massless, but SR as a whole applies to photons just as much as it applies to massive particles. Photons cannot be acclerated to c as they can only travel at c (that this is true in SR is undeniable as it's a postulate of SR)
  24. Jan 7, 2008 #23
    "Time stands still for a photon" is a reference to previous posts. My point is that this can't be said from the photons own frame of reference. That is, I'm objecting to that statement in the form in which it was made in other posts.

    For other particles, as they are accelerated closer and closer to c, if they had a clock "on board", this clock would be perceived as going slower and slower from a stationary observer's point of view. So one could argue that for a photon, the clock would stand still. But that would be only from stationary observer's point of view. This is what I am pointing out above. From "aboard" any object, the clock will still appear to tick as it ticks, no matter how close to c the speed becomes.

    I see this as a challenge, since when trying to approach this question by letting v come close to c, the external observer's point of view appears difficult to reconcile with the "local" (to the photon) point of view. The external observer could assume to perceive the clock of the photon to be at a stand-still, but for the photon itself the clock would have to continue to tick. Will it tick once more, or not?

    Thank you! :)
  25. Jan 7, 2008 #24

    I don't think SC says this though. The best I can get from SC is it's unknown whether it ticks or not.
  26. Jan 7, 2008 #25


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    Special relativity says that photons do not have a reference frame. When constructing a reference frame for an object in relativity you start by choosing a basis vector for the time coorindate which us the unit vector (whose 'length' of |1|) that is tangent to it's worldine. However for a photon (or an object travelling at c) all vectors tangent to it's worldlien are null vectors (whose 'length' is 0) so you cannot construct a reference frame.

    This is not a flaw in theory it is a feature of the theory as in SR there is no reason that a photon shoudl have a reference frame.
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