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

Conundrum Regarding the Lack of an Absolute Frame of Reference

  1. Jun 13, 2004 #1

    loseyourname

    User Avatar
    Staff Emeritus
    Gold Member

    This just popped into my head earlier tonight, for whatever reason. Imagine a photon, a lone photon, that was the only thing in existence. According to relativity theory, am I correct to say that without any other object by which it may be said to be in motion relative to, we cannot say that the photon is in motion? However, due to relativity, the photon would experience no passage of time, a phenomenon that is attributed to its velocity of c. It seems paradoxical to say that an object that is not in motion has a velocity, yet this seems to be what is happening here. Can anyone clarify exactly what is going on here for me?
     
  2. jcsd
  3. Jun 13, 2004 #2

    Janitor

    User Avatar
    Science Advisor

    A very interesting question, that. Special relativity physics is in some sense embedded in a taken-for-granted background of spacetime that itself may only make sense if the universe has a whole bunch of stuff in it. Are there any threads on Mach's principle in the Physics Forum? I myself used to wonder what a universe consisting of one material particle would be like, but I never thought of asking what if a universe just had one massless particle in it and nothing else.
     
  4. Jun 13, 2004 #3
    I posted a similar contemplation a little while ago in which an empty universe was populated only by a spacecraft with an emission-free propulsion system (a kind of theoretical "impulse drive" I saw on the web some time ago which works thorough a revolving coil which is briefly energized at exactly the same point of each rotation - thus producing a pseudo-mass which accelerates the apparatus in a particular direction).

    Given that such an engine could actually work, the question I asked was, "How can an object experience acceleration, but no change in velocity?" Acceleration is experienced directly, while velocity is only experienced relatively.

    Several people picked my thought-experiment apart on practical grounds, but I can't remember anyone trying to answer the basic riddle.
     
  5. Jun 13, 2004 #4
    loseyourname,

    How can there be a photon without any particles? You can't expect physics to explain a universe that physics says can't exist.
     
  6. Jun 13, 2004 #5

    loseyourname

    User Avatar
    Staff Emeritus
    Gold Member

    Whether or not it's possible for a photon to exist in isolation isn't the point. I'm just concerned that, without introducing any exotic elements, this seems to reveal an inconsistency in special relativity. The theory states that all velocity is relative, does it not? Yet it is at least possible to envision a scenario in which there could be velocity without relation. I'm just trying to think of a way to reconcile this. I've considered self-reference, but while that seems possible with temporal movement, it doesn't seem like it can explain spatial movement, at least not at a constant velocity. I'm far from being an expert on this subject. The puzzle just popped into my head a couple nights ago and I assumed that I'm just overlooking something, given that I really have very little knowledge of the details of relativity theory. I hoped someone here could fill in the gaps.

    It seems a pointless besides to ask if this universe could actually be. Physics has yet to explain the existence of our own universe, and it probably never will. If you don't like the idea of a single photon because it is associated with a change in the energy state of another particle, just substitute it with any other massless particle.
     
  7. Jun 13, 2004 #6

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    TMK, The problem of measurement is a very sticky one in modern physics. Measurement is extraordinarily difficult in General Relativity; for a time it was thought that it predicted measurement was impossible! In Quantum Mechanics the problem is real, and I don't mean the uncertainty principle; as I recall, there is a theorem that any clock has a nonzero probability of occasionally running backwards!
     
  8. Jun 13, 2004 #7

    loseyourname

    User Avatar
    Staff Emeritus
    Gold Member

    Sure. There's even a non-zero probability that the entropy of a closed system could decrease. Is no law sacred anymore?
     
  9. Jun 13, 2004 #8
    loseyourname said, "Whether or not it's possible for a photon to exist in isolation isn't the point."

    You're mistaken; it's exactly the point. SR says that when the speed of light is MEASURED it will be c with respect to whoever measures it. It says absolutely nothing about what light does or doesn't do when a measurement isn't being made. It may seem like an irrelevant point, but the fact that instruments used to measure light speed are made out of particles with mass is absolutely essential to the theory. If there were a way to measure light speed without using any mass, SR would colapse!
     
  10. Jun 14, 2004 #9

    loseyourname

    User Avatar
    Staff Emeritus
    Gold Member

    You obviously ignored my point about how the velocity was determined. The fact that the photon experiences no passage of time is a relativistic effect meaning that it must be travelling at c. Obviously, unless the photon was conscious, there wouldn't be anyone in this universe to realize this, but it would still be happening. Nothing about SR says that an event must be observed in order for it to happen.
     
  11. Jun 14, 2004 #10
    loseyourname,

    SR says:

    1) All massless particles travel at c with respect to particles that have mass.

    2) All particles that have mass travel at speeds less than c with respect to other particles that have mass.

    In a universe with only one particle (of either kind) both those statements are meaningless. So in a universe with only one particle (of either kind), SR would be meaningless. SR would have nothing to say about that universe. It wouldn't apply to that universe.

    But the job of physics is to explain OUR universe. Now sometimes physicists will imagine an idealized version of the universe in order to learn something about the real universe. But there are rules in physics about idealizations that can be used and ones that can't. And one absolute rule is that you can't disprove a theory by imagining a situation where it would be disproven.
     
  12. Jun 14, 2004 #11

    russ_watters

    User Avatar

    Staff: Mentor

    Right, all it says is that the event must be observed for Relativity to be relevant.
     
  13. Jun 14, 2004 #12

    loseyourname

    User Avatar
    Staff Emeritus
    Gold Member

    I hope you guys don't think I'm one of those people trying to disprove relativity. It is well beyond my capacities to even think of such a thing. I would need to fully understand the theory first and besides, it makes perfect sense to me and as far as I know, it's been confirmed in every experiment capable of confirming it.

    Addressing what Russ brought up, I was under the impression that relativity is relevant regardless. For instance, take the synchronization of clocks on the satellites we have in orbit. They use relativistic equations to remain in synch, do they not, and this is all automated, is it not? If all of the conscious lifeforms on this planet died off and this could no longer be observed, it would continue to take place and the equations would continue to work.

    I suppose in the truest sense, nothing is relevant unless it is given relevance by a conscious observer, but that isn't to say that relativity doesn't apply unless the events it is describing are observed. Am I incorrect?
     
  14. Jun 14, 2004 #13

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    Oneye re acceleration:

    Think about it this way in SR spacetime can be thought of as 'absolute' (that is all observers agree on certain things that are happening in spacetime), objects all 'travel' with the same magnitude of 4-velocity in spacetime, but the 'direction' of this 4-velocity is depednent on both the magnitude and direction of their 3-velcoity (in some reference frame). Therefore we can recognize when an object is accelerating as it 'rotates' in spacetime and everyone agrees thta it rotates.

    Looseyourname: It doesn't make much sense to talk about things from a photons point of view in relativity.

    Hurkyl re the clock going backwards: that sounds like decoherence to me.
     
  15. Jun 14, 2004 #14

    hellfire

    User Avatar
    Science Advisor

    I think the formulation of the postulates of special relativity given by jdavel is misleading. According to SR, speeds are defined to be relative to reference frames (the term "observer" is usually used to mean "reference frame"). If there is spacetime (a background globally flat spacetime), then there exist reference frames (infinite ones) and thus it exists the posibility to define motion. Whether only one physical body, or more than one exist on spacetime seams to be an irrelevant point to me.

    Regards.
     
    Last edited: Jun 14, 2004
  16. Jun 14, 2004 #15
    it seems to me like only jcsd has actually tried to address the fundamental answer to this particular riddle...

    you are taking away all frames of reference and leaving just a photon and asking if it is in motion in the absence of reference frames...whether or not this is physically a valid idea, the essence of the riddle is whether motion is independent of frames of reference, is it not?

    in relativity, isn't continuous motion only motion relative to other reference frames? and isn't only accelerated motion measured against absolute spacetime?...so the question is maybe whether light waves count as accelerated motion...they definitely can change direction - but not without other particles to warp space-time!
     
  17. Jun 14, 2004 #16

    russ_watters

    User Avatar

    Staff: Mentor

    I don't recall anyone ever saying you need a human observer. A computer checking different clocks in different frames satisfies the observer requirement because it includes different clocks in different frames.

    The importance of the observer isn't its/his/her nature, its its/his/her existence, takin a measurement that complies with the theory.

    A photon with nothing to measure it fits some of the critereon for applicability of SR (its a photon, so it should be traveling at C), but not all the criterion for applicability of SR (without any reference points [the "observer"], it can't be said to have any velocity, not even C).

    What is speed? Distance divided by time. For a photon with no observer, what is the distance and what is the time?
     
    Last edited: Jun 14, 2004
  18. Jun 14, 2004 #17
    I would agree with hellfire - photons exist in largely empty space and they travel very nicely from place to place (whether or not there are masses or measuring fames) in accordance with Maxwell's equations. The fact that the photon is always measured to have the same velocity relative to an isotropic light frame, does not impact the physics that are responsible for the photon's trajectory.
     
  19. Jun 15, 2004 #18

    Janitor

    User Avatar
    Science Advisor

    Does it do violence to modern-day physics ideas to imagine a universe with no real particles in it at all, just virtual particles? Is there any sense in which such a universe could be said, for instance, to be four-dimensional, or if you prefer, eleven-dimensional or whatever it is supposed to be in string theory?
     
  20. Jun 15, 2004 #19

    hellfire

    User Avatar
    Science Advisor

    Yes, may be you are right and I might not have fully understood the problem.

    In my previous comment I was addressing the question of internal consistency of special relativity. It seams to me that the existence of a globally flat background spacetime is a sufficient condition to define the motion of a single paticle.

    But I agree that one could also, in principle, remove the background spacetime. I don’t know how to proceed then...

    Regards.
     
  21. Jun 15, 2004 #20
    The thing that I find interesting, in my little Gedanken, is that one can accelerate without a change in velocity. This may be the natural consequence of GR (wherein such acceleration is regarded as a gravity field, if I am not mistaken), but then this means that it is theoretically possible in GR to have a sourceless gravity field!

    I understand that such thought experiments must be restrained to sensible grounds. But I also feel that this particular contemplation implies an important conclusion:

    Relativity - SR,at least, - is really only about observation using light - the natural consequence of using a non-inertial finite-velocity measuring tool. The math works, and it squares with observation, but SR can really only comment on what we can observe. The limits of v<c would then (as illustrated by my thought experiment) talk about what can be observed by someone who did not share my reference frame, not about what actually happens to me - precisely because we are using light as our yardstick.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Conundrum Regarding the Lack of an Absolute Frame of Reference
  1. Absolute reference frame (Replies: 22)

Loading...