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Using General Relativity to analyze the twin paradox

  1. Mar 31, 2015 #1

    PeterDonis

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    In a previous thread, reference was made to an entertaining "defense" of relativity by Einstein, which can be found here:

    https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

    One of the arguments Einstein makes in this dialog is that the twin paradox can be analyzed using "pseudo gravitational fields"--the traveling twin can explain the fact that his clock shows less elapsed time when he and the stay-at-home twin meet again, by saying that, when he fires his rockets to turn around, a gravitational field exists, and since the stay-at-home twin is at a much higher altitude in the field, his clock runs much faster. The Usenet Physics FAQ article on the twin paradox includes an analysis based on this same idea:

    http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html

    In this post in a recent thread, harrylin mentions a criticism of this argument of Einstein's, by Builder. An actual quote from his criticism is given in that thread, but here I will try to paraphrase it to get at the essence of the argument: the problem with the traveling twin's explanation in terms of a "gravitational field" is that, when he fires his rocket to turn around, this "field" must instantly propagate to the stay-at-home twin, and when he completes his turnaround and turns off his rocket, the disappearance of the "field" must likewise propagate instantly to the stay-at-home twin. But this violates causality: no field can propagate faster than light. So this "field" explanation cannot be physically correct.

    The question I want to pose to the forum is: is this criticism correct? Does it invalidate the "GR analysis" of the twin paradox that is presented by Einstein and in the Usenet Physics FAQ? I have my own answer, but I'll put it in a separate post.
     
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  3. Mar 31, 2015 #2

    PeterDonis

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    My response to the question in the OP: no, Builder's criticism does not invalidate the GR analysis of the twin paradox. Here's why:

    (1) The "field" that appears in the analysis does not have to propagate, because it is a coordinate effect, not a physical effect. No physical influence has to travel from the traveling twin to the stay-at-home twin. This can be verified by the fact that, if you compute the curvature of spacetime, it does not change: spacetime is flat throughout the scenario. According to the Einstein Field Equation, changes in spacetime curvature are what must propagate, and those changes propagate at the speed of light. But if there is no change in spacetime curvature, as in this case, there is nothing to propagate.

    (A similar rebuttal can be made to the related claim that there is no source for the "gravitational field" that the traveling twin claims to exist, and a field cannot exist without a source. According to the EFE, spacetime curvature is what requires a source. There is no spacetime curvature in this scenario, so no source is required.)

    (2) The criticism makes an implicit assumption (which, to be fair, Einstein can also be argued to have made) that any entity which appears in any frame's account of events must be "real". So if the traveling twin explains observations by using a "gravitational field", that field must be "real"; if it turns out not to be "real", the explanation cannot be valid.

    However, this assumption is not correct. If it were, it would invalidate any explanation that invokes any frame-dependent quantity, since the argument for why the "gravitational field" invoked by the traveling twin is not "real" can be equally well applied to any frame-dependent quantity. So, for example, any explanation involving "time dilation" or "length contraction" would be invalid, since these are frame-dependent. The "gravitational field" that appears in the traveling twin's explanation is the same sort of frame-dependent quantity, and it is just as valid to use it in an explanation for that frame as it is to use time dilation or length contraction in an explanation.
     
    Last edited: Mar 31, 2015
  4. Mar 31, 2015 #3
    That implies regretfully a misunderstanding of the issue at hand. Not a modern GR calculation is at stake, nor your interpretation of GR, but Einstein's 1916 interpretation of GR. In a parallel thread I gave a quick impression of the view that he still tried to defend in 1918 (but apparently not in any later publication):

    https://www.physicsforums.com/threads/how-does-light-slide-sideways.804112/page-2#post-5058667

    In summary, he claimed that acceleration in GR is just as "relative" as velocity in SR, so that it is physically valid to regard an accelerating K' as not accelerating but truly in rest.
    He set out to defend that point of view (which was the starting point of his development of GR) by means of his 1918 paper.
    That's a nice way of saying that Einstein's field is fake ("fictitious"). Einstein argued that the field is "induced" by acceleration relative to the stars, and that it is just as "real" and "physical" as for example a magnetic field that can be made to disappear by a suitable choice of coordinate system. The way I look at it, you debunked Einstein's interpretation with your first analytical remark alone.
    First, that's putting the argument on its head; and second, I don't follow your argument. Any explanation involving "time dilation" would be invalid if it cannot pretend full physical reality. Relativity of "time dilation" (for inertial motion) means that other descriptions are also physically fully valid.
    Once more, I don't see how that can be right: if it is supposedly equally "physical" as for example time dilation and magnetism, then it cannot be treated as fictional according to the point of view that it truly exists. That looks like pure Doublethink to me! o0)
     
    Last edited: Mar 31, 2015
  5. Mar 31, 2015 #4

    PeterDonis

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    More precisely, he claimed that a non-inertial frame is just as valid for doing physics as an inertial frame, provided you are willing to allow a "gravitational field" to exist in the non-inertial frame that does not exist in the inertial frame. Note that this applies to flat spacetime, i.e., "in SR"; you don't need to have curved spacetime to use non-inertial frames. Rindler coordinates are a simple example of a non-inertial frame in flat spacetime in which a "gravitational field" exists in Einstein's sense.

    Whether this is equivalent to saying "acceleration is relative" is, to me, an unimportant matter of terminology, just like the question of what it means to say that "velocity is relative". It's unfortunate that we use the word "relative" to describe things that are, in fact, invariant: for example, we use the term "relative velocity" to describe something that, mathematically, is the inner product of two 4-velocity vectors, and is therefore a Lorentz scalar and is invariant. Similarly, as I remarked in a previous thread, we use the word "acceleration" to describe two things, one of which is frame-dependent and one of which is invariant. With our terminology in such a mess, it's no wonder that we get confused sometimes about what we are talking about.

    That's why I would rather focus on the physics and on the mathematical description of the physics, which is unambiguous. Mathematically, my restatement of Einstein above is just the observation that a non-inertial frame can have nonzero connection coefficients.

    Yes, and the justification for this is that, according to the Einstein Field Equation, the metric in a region of space is determined by the distribution of stress-energy in the past light cone of that region of space. When you fire your rockets and accelerate "relative to the stars", you are judging your acceleration by the light reaching you from those stars--which of course means you are looking at the sources in your past light cone. You are not judging your acceleration by the distribution of stars "right now". And of course the metric in your vicinity is indeed determined by the propagation (to the extent things even have to propagate--see below) of spacetime curvature from those sources in your past light cone, to your current spacetime location.

    It is easy to miss the above in many ordinary situations because many ordinary situations are, to a very good approximation, static, so that the distribution of sources in the past light cone is the same as the distribution of sources "right now". For example, the distribution of the "distant stars" around our solar system has been essentially static and spherically symmetric for billions of years, and therefore there has been plenty of time for the spacetime curvature induced by that stress-energy distribution to determine the metric in our vicinity. But when you fire your rocket and feel the acceleration induced, that acceleration is still being proximately caused by the metric in your vicinity, not by the distant stars "instantly"; it is only caused by the distant stars via the propagation of spacetime curvature in your past light cone. The fact that it appears that the inertial force is induced "instantly" by accelerating (or rotating) relative to the stars is only an appearance, due to the fact that the situation is static.

    The reason we know the above is the case is that we have measured the small corrections due to the fact that situations are not exactly static. The precession of the perihelion of the planets is an example of such a small correction (this measurement for Mercury was one of the "classic tests" of GR that Einstein made, but it has now been verified for at least all the inner planets, IIRC). This precession is due to the fact that the Newtonian "force" on a planet is not exactly pointed at the Sun "right now"; it only appears to be, to a good approximation, because the field of the Sun is, to a good approximation, static. But there are still small effects due to the non-staticity of the field, i.e., to the fact that changes in curvature can only propagate at the speed of light, and perihelion precession is one of them.

    Einstein did choose an unfortunate comparison here, because a magnetic field is not a coordinate artifact the way connection coefficients are. For some electromagnetic fields, it is possible to find a frame in which the magnetic field is zero; we call these "electrostatic" fields, and they correspond to a very special physical situation. But for any other EM field, it is not possible to find a frame in which the magnetic field vanishes completely. However, it is always possible to find a frame in which the connection coefficients vanish.

    As for the terms "real" and "physical", I personally would be just as happy to restrict those terms to direct observables: for example, I would be just as happy to say that length contraction and time dilation are "fictitious" in the same sense that you are saying the "pseudo gravitational field" in Einstein's analysis is "fictitious". Then we could focus on frame-invariant observables in place of those "fictitious" things, such as the observed Doppler shift in place of "time dilation". But if we are going to allow some of those frame-dependent things to be "real and physical", then I don't see any way to pick and choose and say that time dilation, for example, is "real" but the gravitational field in Einstein's example is not.

    Why the restriction to inertial motion? Are you saying that, if I am moving non-inertially, "time dilation" somehow becomes unreal, but it's real if I'm moving inertially? On what do you base that distinction? Without that distinction, it seems to me that the same argument you are making for time dilation could be made for the gravitational field in Einstein's example.

    I don't understand what you're trying to say here.
     
  6. Mar 31, 2015 #5

    PeterDonis

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    But what is "full physical reality" supposed to mean? How do I tell that time dilation has "full physical reality", even though it's frame-dependent, whereas Einstein's "gravitational field" does not?

    The only distinction I see being made in Builder's criticism is propagation: something "real" has to propagate no faster than light. But time dilation is not "real" in this sense any more than the "gravitational field" is: time dilation does not propagate. You can change it "instantly" by changing coordinates. That is basically the point of my rebuttal: that without some principled way to distinguish frame-dependent things that are "real" from frame-dependent things that are not, Builder's criticism of Einstein cannot stand.
     
  7. Mar 31, 2015 #6

    Dale

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    There is nothing wrong with an explanation in terms of a non causal field. For instance, the Coulomb gauge is a perfectly valid gauge in which to describe EM.

    The object that Einstein refered to as "the gravitational field" in his piece was the Christoffel symbols. They are a legitimate part of the theory, just like the potentials are a legitimate part of EM, regardless of their "propagation speed".

    That said, this presentation by Einstein is a good example of why pop-sci sources are not considered valid references here. Even Einstein couldn't do a pop-sci piece that is also rigorous.
     
    Last edited: Mar 31, 2015
  8. Apr 1, 2015 #7

    Orodruin

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    Another issue is if to really call this a GR analysis or not. Space time is still Minkowski space and the only difference is that we have tried to find a set of coordinates which are locally equivalent to the standard coordinates for the non-accelerated parts of the traveling twin. It is still just SR, but in some set of curvilinear coordinates.

    I have to agree with Dale, this is an example of the impossiblility to write accurate pop-sci.
     
  9. Apr 1, 2015 #8
    From those remarks it appears that Einstein's 1916 interpretation of GR as well as the objection to it are not sufficiently rehashed in this thread. First of all, your remark is the essence of the early criticism on Einstein's views. Moreover, you are sharing Builder's criticism here. And also Baez claims that this is not really a GR analysis.

    Next, Einstein's claims as well as the objections against them have nothing to do with "pop-sci". As referenced (admittedly not one, but two clicks away), he made those claims in his 1916 overview paper on GR. The objection was that -contrary to Einstein's claims- a rigorous and self consistent analysis in which a frame that is in arbitrary motion can be pretended to be truly in rest all the time, is not possible.

    Once more:

    - Einstein main goal was not to predict the effects of gravitation, although that very important achievement resulted from his work. He held that SR's Minkowski space (which is still "Galilean space" for rotation) should be rejected and replaced by a Machian space ("The Galiliean space, which is here introduced is however only a purely imaginary cause, not an observable thing"). While that also has been the subject of discussions, it's still not the issue that became the topic of this thread. He went even one step further: just as it is the case that in SR, the laws of physics are valid in any Galilean frame so that one may treat any such frame as a true rest frame, "The laws of physics must be so constituted that they should remain valid for any system of co-ordinates moving in any manner."
    He claimed that GR achieves this, so that an observer at rest in a frame in arbitrary motion can always consider that frame as a true rest frame for the description of phenomena according to the established laws of physics. Einstein tried to kill three flies with one blow.

    - The main objections focused on this last mentioned claim, which was in direct contradiction with Langevin's claim about his example of the traveling astronauts. The accelerating astronaut can not just as well claim to be not accelerating but all the time truly in rest; that is not a valid point of view for describing physical phenomena if one wants the laws of nature to hold with respect to that frame. But Einstein now claimed exactly the contrary. That created the "twin paradox", and most modern authors don't know or don't acknowledge the existence of Einstein's proposed solution (however, Moller provides the calculation that Einstein did not include in his 1918 paper).

    Perhaps I should have clarified in my first reply that it is simply not correct that "One of the arguments Einstein makes in this dialog is that the twin paradox can be analyzed using "pseudo gravitational fields" " (emphasis mine), just as in SR a capacitor is not argued to have a "pseudo magnetic field" according to a frame in which the capacitor is in motion.

    I'll get back to peterdonis' comments later.
     
    Last edited: Apr 1, 2015
  10. Apr 1, 2015 #9

    Ben Niehoff

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    I've only read the OP, so this may have been said. But...

    I think the best criticism of this argument is that it assumes that one can define a global notion of simultaneity for observers in arbitrary motion. For observers in inertial motion in Minkowski space, it is obvious that the spacelike plane perpendicular to their four-velocity vector is the appropriate surface of simultaneity. But for observers in accelerated motion, there is no natural way to extend local notions of simultaneity to global ones. We can certainly agree that a "surface of simultaneity", if one should exist, should be perpendicular to the observer's worldline; however, this does not give us enough information to define a unique surface.

    The implication of this is when either observer is undergoing accelerated motion, there is in fact no unambiguous way to compare the rates at which their clocks tick. So the question of considering "Twin A in the pseudo-gravitational field experienced by Twin B" is nonsensical to begin with. The time difference when the twins meet again does not come from comparing local rates of time along their worldlines; it comes from integrating the proper time along the worldlines and concluding that their paths through spacetime have different lengths.

    Secondly, I think arguments about the speed of field propagation are subtle, and may not be correct here. For example, in standard (Lorentz-invariant!) electrodynamics, the monopole part of the electric field always manages to point directly toward the current location of the source charge (in arbitrary motion), due to a mathematical conspiracy with the magnetic field.

    Finally, as probably others have mentioned, in Minkowski space there is of course no gravitational field. That one observer is undergoing acceleration does not allow you to use the equivalence principle to say there is a gravitational field, let alone one that applies to the inertial observer. The equivalence principle does not actually equate gravitational fields and acceleration; it equates acceleration with uniform fields. But the gravitational field is precisely a non-uniform acceleration field, because the Riemann tensor specifically measures the non-uniformity.
     
  11. Apr 1, 2015 #10

    stevendaryl

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    Just to add to what you've said: In GR, gravitational fields have exactly the same ontological status as fictitious forces such as "centrifugal force". Centrifugal forces seems completely real in the sense that you can reason about them using force diagrams: Anything that doesn't have a centrally directed force (such as the tension force of a string) will be flung away by centrifugal force. This includes buildings and planets infinitely far away; they are all affected equally by centrifugal force. But centrifugal force ISN'T real, in the sense that it has no source, and it propagates instantly; it has very different behavior from real fields that have sources and propagate at the speed of light.
     
  12. Apr 1, 2015 #11

    bcrowell

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    Einstein originally wanted to interpret GR as a generalization of SR in which all frames of reference, including accelerated ones, were equally valid. It has probably been 70 to 90 years since this was considered a viable interpretation of GR. So to me the question posed here seems analogous to something like this:

    In the Ptolemaic cosmology, the planets' cycles and epicycles are organized around the position of the earth. Is this consistent with special relativity, under which we would expect the earth's influence to propagate at a velocity no greater than c?
     
  13. Apr 1, 2015 #12

    PeterDonis

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    I don't understand. Doesn't GR say that the laws of physics must look the same in all valid coordinate charts, inertial or not?
     
  14. Apr 1, 2015 #13

    PeterDonis

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    This is not correct. The Einstein Field Equation, along with all other laws of nature in proper tensor form, holds in all valid coordinate charts. So an accelerating astronaut can use a chart in which he is always at rest and call that his "rest frame". He may find that the calculations he has to do are more complex, but that doesn't make his claim invalid.
     
  15. Apr 1, 2015 #14

    PeterDonis

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    There doesn't need to be a unique way to do this for the analysis to be valid; there only needs to be some valid coordinate chart that covers the required region of spacetime and has nonzero connection coefficients (which are the "gravitational field"). This is certainly true for the twin paradox scenario under discussion.
     
  16. Apr 1, 2015 #15

    Dale

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    harrylin, this thread (as posed by Peter Donis in the OP) is about a specific objection (superluminal propagation of the gravitational field) to a specific paper by Einstein ("Dialog about Objections ..." 1918). There is no need to rehash everything on the subject, and I don't know why you are bring in 1916.

    The specific paper by Einstein which we are discussing is "pop-sci". The fact that it is "pop-sci" is not the specific objection that this thread is discussing, but a different objection that I personally have to the paper under discussion. It is not really relevant to the topic at hand, but I didn't want to leave the impression that I agreed with the paper simply because I disagreed with the specific objection under discussion.

    Such an analysis is certainly possible, although that is not the topic of this thread and was certainly not something that was done in the pop-sci work in question.
     
  17. Apr 1, 2015 #16

    PeterDonis

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    If it's just the "pseudo" that you object to, feel free to ignore it. I was simply referring to the fact that Einstein himself, in places, points out the difference between this field, in flat spacetime, and the field that is present in a curved spacetime.
     
  18. Apr 1, 2015 #17

    PeterDonis

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    Yes, in the modern view, "GR" means "curved spacetime", rather than "curvilinear coordinates" or "gravitational field" (nonzero connection coefficients), which was how Einstein was (sort of) thinking of it. So in the modern view, this analysis is not a "GR" analysis, it's just "SR in curvilinear coordinates".

    However, calling it a "GR analysis" also draws attention to the essential unity between the flat and curved spacetime cases, from the standpoint of GR: flat spacetime is just one particular solution to the EFE, not some separate domain of physics.
     
  19. Apr 2, 2015 #18

    bcrowell

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    There are many problems with Einstein's early interpretation. The biggest problem with it is that SR has absolutely no problem dealing with accelerated frames of reference (or with arbitrary coordinate charts). The modern way of defining the distinction between SR and GR is that SR assumes zero curvature.
     
  20. Apr 2, 2015 #19

    Orodruin

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    This is something I never really thought about: The flatness requirement would not exclude some strange geometries (such as a torus world) which are flat, but clearly distinct from Minkowski space. Do we consider this SR? It could include things such as closed timelike curves etc.
     
  21. Apr 2, 2015 #20

    bcrowell

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    I don't think there is any widespread agreement on whether topologically nontrivial SR is still referred to as SR. It does introduce some issues that are not present in ordinary SR. As you point out, there can be CTCs. Other examples: (1) there can be a preferred frame, e.g., in 1+1 dimensions where the spacelike dimension wraps around, there is a frame in which the circumference is maximized; (2) you may be unable to cover all of spacetime with a single coordinate chart.
     
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