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Could there be an absolute 'state of reference?'

  1. Mar 18, 2014 #1
    Could there be an absolute 'state of reference' somewhere in the universe, perhaps at the location of the original center? If so, would it have to hold the same frame of reference in each dimension, including time? How would it do so?

    Looking at the Hafele-Keating Experiment in particular, the atomic clocks were affected in different ways due to different positions and velocities. The velocities/positions of course would be relative to the observer, however is it possible that there is a "correct" spot in which to observe the universe?

    I apologize if this is in the wrong location, a stupid question, confusing, or if this has been discussed before. This is my first post here at Physics Forums.
     
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  3. Mar 18, 2014 #2

    PeterDonis

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    Hi, mrnike992, and welcome to PF!

    No. There is no "original center" to begin with. But in any case, there isn't any absolute "state of reference" (by which I assume you mean a frame of reference from which to make observations); all frames of reference are on an equal footing in principle. Different frames of reference may be more convenient for making certain observations, but that's all.

    That doesn't mean there aren't any absolutes at all; see below.

    No, they're relative to *each other*. The difference in clock readings observed in the experiment is invariant; it will be the same regardless of which frame of reference you use to observe it. And that difference is caused by differences in the paths that the clocks take through spacetime; those differences in paths are also invariant.

    So even though there are no absolute frames of reference, there are still absolutes, in the sense of actual measured quantities, like the difference in clock readings in the H-K experiment, being invariant, independent of your frame of reference.

    I'm not sure what you mean by "correct". Some spots may be more convenient than others for making certain observations, as I said above. But that's a matter of practical convenience; there is no frame of reference that is privileged in principle.
     
  4. Mar 18, 2014 #3

    ghwellsjr

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    Even if it were possible that there exists an absolute "state of reference", there is no way to identify it. As a matter of fact, every inertial reference frame would look just like the absolute "state of reference", so how would you know which one it was?
     
  5. Mar 19, 2014 #4

    pervect

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    If you're asking in the context of general relativity, no. Non-GR theories might allow such a thing to exist, but I'm not aware of any with any experimental support. Non-GR theories would best be addressed in a different group, anyway.
     
  6. Mar 19, 2014 #5
    If the universe began at one point, and expands infinitely in every direction, would there not still be a 'starting point' at the center of the expanding universes?



    Was I mistaken in believing that clock A and clock B would not be the same if the experiment took place somewhere else? I thought I remembered reading that gravitational potential also affected the time of the clocks. This would mean that the two clocks could not be looked at solely as being relative to each other, if an outside system is affecting them differently.

    And I guess I don't understand this part here:

    How can the clocks be absolutes if the same clocks read different times (Even after starting synchronized)?


    Well, if there are discrepancies in the data recorded by the clocks representing time, then could there be a spot in which everything else is the same in relation to this spot?

    I'm sorry, I'm having trouble bringing words to what I'm thinking.
     
  7. Mar 19, 2014 #6

    ZapperZ

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    This is wrong.

    There was no "point" at the beginning of the universe. You are thinking of something that already existed in some space and time frame, and then expanding. This is the wrong view of the beginning of the universe. The beginning of the universe isn't just the beginning of some "thing". It is also the being of the expansion and formation of spacetime! A "center" or location just didn't exist when the universe began!

    Zz.
     
  8. Mar 19, 2014 #7

    PeterDonis

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    It didn't. As ZapperZ said, the universe did not start at a point and expand into a pre-existing space.

    You're right, it does; so I shouldn't have said the velocities are purely relative to each other. They're also relative to the gravitational field. But that's still not the same as them being relative to "the observer". The gravitational field itself can be described in a way that is independent of observers or frames or coordinates.

    That's why I was careful to define how I was using the term "absolute". The difference in the clock readings when the clocks come back together is the same regardless of which frame of reference you use; it's invariant. Invariant quantities are very important in relativity; in fact, Einstein once said that his theory was misnamed, and it should really be called the theory of invariants. Invariant quantities may not be "absolute" in the pre-relativistic sense, but that's because there really isn't anything that's "absolute" in the pre-relativistic sense.

    I'm not entirely sure what you mean by this, but I think the answer is "no".
     
  9. Mar 19, 2014 #8
    Alright, that really helps, thanks to everybody that contributed!
     
  10. Mar 21, 2014 #9
    Why are you saying it's "impossible?' Are you equating "impossible to detect" (CMBR notwithstanding) with "physically impossible?"
     
  11. Mar 21, 2014 #10

    PAllen

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    CMBR allows you to identify a locally inertial frame with the property that the CMBR is isotropic. This is not different from the air on earth allowing you to identify a state of motion in which there is locally no wind. Neither is fundamental. The key word is local. There is no globally inertial frame at all that is possible over cosmological distances because local frames each 'at rest' per CMBR isotropy are separating from each other, so you clearly do not have a single inertial frame.
     
    Last edited: Mar 21, 2014
  12. Mar 21, 2014 #11

    ghwellsjr

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    No (to your second question), I thought my post made that clear.

    If you understand Special Relativity, you will know that attempting to promote an absolute "state of reference" is a pointless pursuit.

    The question is: do you want to understand Special Relativity? If not, then you shouldn't be posting on this forum as that is that stated purpose of this forum and continually violating that purpose will get you banned.
     
  13. Mar 21, 2014 #12

    Nugatory

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    Impossible, because it would require detecting a difference that cannot exist.

    The laws of physics are the same in all inertial frames, meaning that the speed of the inertial frame doesn't appear anywhere in these laws (it does appear in the coordinate transforms between inertial frames, but that is by definition relative motion not absolute). Therefore the result of any experiment carried out in any inertial frame will be the same no matter what speed you choose to assign to the frame.

    This is why ghwellsjr says that it is impossible to identify the hypothetical absolute frame; it cannot be distinguishable in any way from any other inertial frame.
     
  14. Mar 21, 2014 #13

    Dale

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    Hmm, I think that I understand what you are trying to say, but I am not sure that I fully agree. I think that what you are stating is a logical impossibility based on a specific definition of "inertial frames" that you have in mind. Under that definition for a frame to qualify as "inertial" all of the laws of physics must be identical, so if a difference between two frames is detected then the logically the frame cannot be "inertial".

    I think that even with such a definition there remains an experimental question, which is about how many inertial frames there are and what are the admissible transformations between inertial frames. We know from experiments that there are an infinite number of inertial frames, and that the admissible transformations are the Poincare group.

    If the laws of physics were different than they are, then it could be that there were only 1 inertial frame, or that inertial frames were related by the Gailiean transform, or that the admissible transformations did not contain boosts, and so forth. Such transforms are contrary to available evidence, and therefore physically impossible, but not logically impossible.

    Layman, please don't mistake this as a vindication of your position. Nugatory and I agree on the facts, this is just a minor comment on terminology.
     
  15. Mar 21, 2014 #14
    DaleSpam, may I ask what you presume "my position" to be? Have you made your inferences about "my position" from the question I asked in this thread (and it was merely a question, not a statement of position) or something I said elsewhere?

    I happen to agree completely with what you just said about there being a distinction between logical and practical impossibility. I actually thought that was the question I was asking of ghwells: "Are you equating "impossible to detect" (CMBR notwithstanding) with "physically impossible?"

    He said he was not doing that, as I understood him, so I think we all agree. It's just that he conjoined the ontological and epistemological aspects in a way that made me unsure of what he was saying.
     
  16. Mar 21, 2014 #15
    I started a thread in this forum that was first moved to the general physics forum, then closed. Now I am being threatened with banishment. As I recall, in the guidelines there was mention of "civil debate." Was that misleading? Is debate prohibited? If I don't understand, or agree with, a statement about SRT made by another poster in this forum, am I supposed to refrain from expressing disagreement?

    I guess I really don't understand the "forum rules."

    What do you mean by "attempting to promote?" Making a reference to test theory studies, like those conducted by Sexl and Mansur, is that it?

    Maybe it's just me, but I see those studies as being absolutely crucial to any in-depth understanding of the theory of special relativity. Understanding the theory, qua theory, is different than merely understanding the mathematical rules you are directed to follow by SRT, isn't it?
     
    Last edited: Mar 21, 2014
  17. Mar 21, 2014 #16

    Dale

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    Based on your comments and questions both here and in other threads you appear to be a LET proponent. If that is not the case then you should be aware that you give that impression rather strongly.
     
  18. Mar 21, 2014 #17

    PeterDonis

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    It depends on why you disagree, and how you express the disagreement.

    For example, if someone were to say on these forums that "special relativity uses Galilean transformations to go from one inertial frame to another", you would be quite justified in pointing out that that's false; SR uses Lorentz transformations, not Galilean ones.

    But if someone says that, in SR, the concept of an "absolute frame of reference" is not valid, and you disagreed with that, you would have to be careful how you expressed that disagreement, because your disagreement would be wrong: SR *does* say that the concept of an "absolute frame of reference" is not valid (more precisely, it's not valid within the context of SR as a theory; SR as a theory has no room for it, doesn't use it, and doesn't assign any meaning or weight to it). All inertial frames are equivalent in SR, in the sense that they are all equally valid for expressing physical laws. Disagreeing with that statement, or even appearing to disagree with it, just leads to pointless threads that go on forever; those of us who have been here on PF for a while have seen it happen all too many times, which is why we try pretty hard to nip it in the bud if it looks like it's happening again.

    In your case, you implicitly equated measuring the CMBR with measuring an "absolute frame of reference". What you should have done, IMO, was to ask a question something like this: "I can measure the CMBR, and that measurement picks out a particular frame of reference: the one in which the CMBR is isotropic. How does relativity reconcile that with the principle that all inertial frames are equivalent?" That would have made it clearer that you were not trying to claim that SR was wrong; you were just trying to understand how SR reconciles two things that look, on the surface, like they don't fit together. (There are a *lot* of examples in relativity where things look, on the surface, like they don't fit together, but can be reconciled perfectly well; so when you see things in relativity that look, on the surface, like they don't fit together, it's always a good idea to assume, or at least behave as if you assume, that there *is* a reconciliation at a deeper level and ask what it is.)

    PAllen went ahead and gave a partial response to that question, even though you didn't ask it. A further response would be to say that inertial frames are equivalent for expressing physical laws, but obviously that doesn't mean that all inertial frames look the same in every respect. The physical laws say things like: the frequency you will measure for a particular photon (such as one in the CMBR) depends on your 4-velocity and the photon's 4-momentum. You can assess that in any inertial frame; the calculation may be simpler to do in the frame in which the CMBR is isotropic, but the same physical law can be expressed in any frame, and is equally valid in all of them. The experimental fact that the CMBR exists shows that the universe is filled with photons that have particular properties; but that in no way privileges the frame in which those photons look isotropic, from the standpoint of physical laws.

    In the other thread you refer to, while the test theory studies you mention may have established that assuming that there is absolute motion can be reconciled with experiment (I'm not familiar enough with the studies to know whether they actually did that), you appear to me to have been making a stronger claim: that *Special Relativity*, that specific theory (as opposed to some generalized test theory of which SR is one special case), allows one to assume absolute motion. That's not correct; SR, the specific theory, has no room for a concept of absolute motion, any more than for a concept of an absolute reference frame. You also appeared to continue to insist that SR requires everyone to assume that they are "not moving", i.e., to always adopt an inertial frame in which they are at rest, even after at least two posters in that thread pointed out to you that SR doesn't require that.

    Once again, I understand that these are things that look, on the surface, like they are difficult to reconcile; but your response should be to ask what the reconciliation is, not to repeat claims that you have already been told are wrong.
     
  19. Mar 21, 2014 #18

    WannabeNewton

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    A statement of pithy indeed-if only this could be written across the cover of every modern physics textbook ever written...:wink:
     
  20. Mar 21, 2014 #19

    ghwellsjr

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    Getting your first thread closed within 4 hours of opening should be a warning to you.

    I have no ability to banish you, I'm just warning you from past experience on this forum.

    I am not aware of anything uncivil going on here. There's lot a debate happening on this forum but it should all be directed at increasing understanding.

    Here are two of the rules that I was referring to:

    •Challenges to mainstream theories (relativity, the Big Bang, etc.) that go beyond current professional discussion
    •Attempts to promote or resuscitate theories that have been discredited or superseded (e.g. Lorentz ether theory)

    This forum is to help people learn and understand relativity. Do you consider yourself to be at the level of a teacher or are you trying to learn?
     
  21. Mar 21, 2014 #20
    Dale, I wouldn't say I'm an LET "proponent." I do think it has been well established that theories which posit absolute simultaneity are consistent with all experimental evidence. In the other thread, I took the poster I was responding to to be claiming otherwise (i.e., that ONLY SRT was consistent with the empirical data), and I disagreed with that.

    The other thread made specific reference to an example Einstein used to "promote" the "relativity of simultaneity." I personally found that explanation to be questionable, since it requires, as does the theory as a whole, all observers to assume that they can't be moving, and that only the "other guy" can be moving.

    Let me give an example of what puzzles me. Maybe someone can explain why it is more reasonable than it appears to me to be:

    Let's say I'm an astronaut who has prepared to years to take an inter-stellar space flight. I get on board my rocket. I blast off. Thereafter, for days, weeks, and months I feel a constant acceleration, and then, one day, the acceleration stops. I am now in an inertial state of, let's say, .9c (relative to earth).

    Would I, as an astronaut with all that knowledge, actually assume that I am motionless? Would I assume that the earth is moving away from me, and not vice versa? I don't think so. And yet, as I understand it, that's what SRT would require me to assume, if everything is to work out "right' (per SR) mathematically.

    This seems to contradict reason, to me.
     
    Last edited: Mar 21, 2014
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