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

Does instantaneous communication implies a preferrred Lorentz frame?

  1. Nov 12, 2009 #1

    Demystifier

    User Avatar
    Science Advisor

    Nonlocal entanglement of quantum mechanics suggests the existence of instantaneous communication between distant particles. Related to this, one often hears the following argument:

    Argument 1:
    "If the communication is instantaneous in one Lorentz frame, then it is not instantaneous in any other Lorentz frame. Therefore, there is a preferred Lorentz frame with respect to which the communication is instantaneous. Consequently, the principle of relativity is violated."

    However, such reasoning is completely incorrect. The simplest way to see this is to consider the following reasoning about classical relativistic mechanics where all massive particles move slower than light:

    Argument 2:
    "If a particle moves slower than light, then there is a Lorentz frame in which it is at rest. Therefore, there is a preferred Lorentz frame with respect to which the particle is at rest. Consequently, the principle of relativity is violated."

    Of course, Argument 2 is erroneous. But my point is that Argument 2 is completely analogous to Argument 1. In other words, Argument 1 is erroneous for the same trivial reason as Argument 2. In both cases, a "special" Lorentz frame does NOT need to refer to ALL particles, but at each time each particle may have another its own "special" Lorentz frame. In other words, the "special" Lorentz frame is not special at all.

    An explicit realization of instantaneous communication without a preferred Lorentz frame is provided by Lorentz-covariant Bohmian mechanics:
    http://xxx.lanl.gov/abs/0811.1905 [Int. J. Quantum Inf. 7 (2009) 595]
    Whatever you may think on Bohmian mechanics in general, this paper provides an explicit counterexample to the frequent claim that superluminal communication necessarily violates the principle of relativity. Perhaps it violates something else (like some versions of the causality principle), but not the principle of relativity.

    What do you think?
     
  2. jcsd
  3. Nov 12, 2009 #2

    dx

    User Avatar
    Homework Helper
    Gold Member

    The principle of relativity applies to the laws of motion, not the motions themselves. Obviously, the spacetime motion of an electron or an EM field is not lorentz invariant, i.e. it does not look the same in all frames. What does look the same is the law of motion, i.e. Maxwell's theory. Instantaneous action, by definition, has to refer to a special hyperplane of simultaneity, and therefore the laws of this action, whatever they are, cannot be lorentz invariant.

    Superluminal communication is not ruled out by relativity. Tachyons have Lorentz invariant dispersion relations, and therefore obey relativity. But comminication with tachyons has some peculiar features, such as recieving a reply before the message is sent, etc. So there are some difficulties with causality, but this is not strictly a requirement of relativity.
     
  4. Nov 12, 2009 #3

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The principle of relativity doesn't rule out the possibility that "messages" emitted by a particle are instantaneous in that particle's rest frame, or instantaneous in some other frame that can be calculated from information about all the particles involved in the interaction. It's ruled out by the paradoxes that it leads to.

    So if you want to allow this type of messages, you have to assume that the particles involved in this interaction have some pretty peculiar properties that eliminate the paradoxes. For example, the time it takes to emit or detect a single bit of information could grow rapidly with distance. You need something like that to make sure that it isn't possible to receive an answer before you ask the question.
     
  5. Nov 12, 2009 #4

    f95toli

    User Avatar
    Science Advisor
    Gold Member

    Not necessarily. One just as well say that entangled "particles" are not really separate entities, i.e. that the classical concept of a particle is never valid. Or one could just use a non-realist non-local interpretation of QM.

    Moreover, even IF there was some form of instantaneous interaction we would never be able to detect it, at least if SR is correct. This means as far as the physics of QM is concerned the idea is on the same footing as the flying spaghetti monster.
    Effects that can not be detected even in principle are the realm of philosophy and religion, not physics.
     
  6. Nov 13, 2009 #5

    zonde

    User Avatar
    Gold Member

    Reasoning is a bit short in argument 1 but is not incorrect.

    This is almost correct:
    "If the communication is instantaneous in one Lorentz frame, then it is not instantaneous in any other Lorentz frame."
    But I would add that there are Lorentz frames where this communication will have finite speed but it can be forward in time or backward in time.
    So in order to avoid causality loops you should abandon relativity and allow instantaneous communication only in one "preferred" frame.
     
  7. Nov 13, 2009 #6
    Well, 'abandon relativity' is a bit strong. You mean 'abandon the Einstein-Minkowski interpretation of relativity'. You just want the neo-Lorentzian interpretation of relativity - which is perfectly in agreement with experiment.. As Bell said:

    I think it's a deep dilemma, and the resolution of it will not be trivial; it will require a substantial change in the way we look at things. But I would say that the cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincare thought that there was an aether - a preferred frame of reference - but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether. ... that is certainly the cheapest solution. Behind the apparent Lorentz invariance of the phenomena, there is a deeper level which is not Lorentz invariant. ... what is not sufficiently emphasized in textbooks, in my opinion, is that the pre-Einstein position of Lorentz and Poincare, Larmor and Fitzgerald was perfectly coherent, and is not inconsistent with relativity theory. The idea that there is an aether, and these Fitzgerald contractions and Larmor dilations occur, and that as a result the instruments do not detect motion through the aether - that is a perfectly coherent point of view. ... The reason I want to go back to the idea of an aether here is because in these EPR experiments there is the suggestion that behind the scenes something is going faster than light. Now if all Lorentz frames are equivalent, that also means that things can go backwards in time. ... [this] introduces great problems, paradoxes of causality, and so on. And so it is precisely to avoid these that I want to say there is a real causal sequence which is defined in the aether.

    So as Zonde says, superluminal signalling violates causality only if one assumes a locally Minkowski structure for spacetime, and not with Lorentz. Historically, Minkowski structure was developed for a local physics. If Nature turns out to be nonlocal, then we might consider revising that structure.
     
    Last edited: Nov 13, 2009
  8. Nov 13, 2009 #7

    Demystifier

    User Avatar
    Science Advisor

    I fully agree.

    No, instantaneous action does not need to refer to a SPECIAL hyperplane of simultaneity. The law of motion with instantaneous influences may look the same in all frames. The explicit example is provided in the paper mentioned in the first post above.

    With that I agree again.
     
    Last edited: Nov 13, 2009
  9. Nov 13, 2009 #8

    Demystifier

    User Avatar
    Science Advisor

    I don't think that it leads to paradoxes. At least not if "free will" does not exist, except as an illusion.

    I don't think there is any problem with receiving an answer before asking the question, provided that "free will" does not exist, except as an illusion.
     
  10. Nov 13, 2009 #9

    Demystifier

    User Avatar
    Science Advisor

    What I push forward is the idea that it is causality (i.e., the principle that events can be ordered into "causes" and "consequences") that should be abandoned. If so, then both nonlocality and Minkowski structure can be retained.
     
  11. Nov 13, 2009 #10

    Demystifier

    User Avatar
    Science Advisor

    That is true, but one of my points is that there is no need to avoid causality loops.
     
  12. Nov 13, 2009 #11

    Demystifier

    User Avatar
    Science Advisor

    Re: Does instantaneous communication imply a preferred Lorentz frame?

    Let me organize my line of thoughts in the following way:

    Step 1: Take the laws of physics seriously!
    It means that the laws of physics (whatever they are) describe EVERYTHING. Clearly, free will is not described by the laws of physics (as we currently understand them), which implies that I don't take the existence of free will seriously (except as an illusion).
    Without free will, grandfather-like paradoxes are no longer paradoxes.

    Step 2: Take Minkowski spacetime seriously!
    It means there is no big difference between time and space (except in the sign of the metric). Therefore, there is no flow of time (because there is no flow of space), except as an illusion (due to the fact that we happen to live in a universe in which entropy increases in some direction).
    Without a flow of time, the spacetime is best viewed as a static 4-dimensional structure (block universe). Consequently, there is no causality (i.e., no "causes" and "consequences"), except as an illusion.
     
  13. Nov 13, 2009 #12

    Demystifier

    User Avatar
    Science Advisor

    Can you explain WHY do you think so?
     
  14. Nov 13, 2009 #13

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the reply you get is "the message you sent hit your daughter in the head and killed her", and you're still unable to stop yourself from sending the message, you don't even have the illusion of free will.
     
  15. Nov 13, 2009 #14

    Demystifier

    User Avatar
    Science Advisor

    Sure, but I don't see any paradox or inconsistency with it. The paradox/inconsistency would appear, e.g., if my daughter was both alive and dead at the same time, but I see nothing like this in the scenario above.

    Additional comment: If I receive such a message in real life, I will not interpret it as a true reply. Instead, I will interpret it as someones joke. If I don't have any other evidence that my daughter is already dead, I will send the message to her anyway, and I will feel that I am doing it freely (even though the free will is only an illusion).
     
    Last edited: Nov 13, 2009
  16. Nov 13, 2009 #15

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Edit: I didn't like my first reply to this, so I'm replacing it. See my next post for a paradox of that sort. Let's recap the discussion so far:

    F: Superluminal communication leads to paradoxes (or long emission/detection times).
    H: Not if free will is only an illusion.
    F: If you can't stop yourself from sending a message (which is necessary to avoid the paradox), you don't even have the illusion of free will.
    H: OK, but I still don't see a paradox.

    I find that last reply (the one I quoted here) pretty confusing. Here's what I think you meant: You're saying "sure" to my observation that it's not enough to say that free will is an illusion (if we want to eliminate the paradoxes), and then you're saying that there's nothing internally inconsistent about a special relativistic universe with tachyons that can be detected in zero time, if it doesn't even have the illusion of free will. I agree that there are no internal inconsistencies there. The problem is that we do have the illusion of free will in our universe, so I don't see how your argument is relevant. If you think we don't have the illusion of free will, I have to ask you to define what you mean by "illusion of free will".

    There are ways to make sure that the reply can be trusted, at least to such a degree that you would feel that sending the message would be to gamble with your daughters life with nothing substantial to gain. You can e.g. use encryption and digital signatures, and put someone you trust at the other end. That someone doesn't even have to be a person. It could be a computer that you programmed yourself, and rigged to explode if tampered with.
     
    Last edited: Nov 13, 2009
  17. Nov 13, 2009 #16
    I would agree. I think this issue demonstrates how easy it is to get confused between two separate systems and one connected one. You can't think of this as two separate systems communicating, but one system just changing state. As long as the fields are "entangled" they have to be treated as a single system.
     
  18. Nov 13, 2009 #17

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Consider the following scenario: (You probably have the spacetime diagram in your head already, so I won't describe everything in detail).

    A computer that's hooked up to a tachyon transmitter is running a program that tells it to send a 1-bit message at t=1 that contains the NOT of the 1-bit message it received at t=0. (If we receive 1, send 0. If we receive 0, send 1). The computer at the other end is set up the same way, except that it's running a program that tells it to send the same 1-bit message that it has received, immediately after it has received it. This scenario contains a paradox:

    Receive 1 → send 0 → receive 0
    Receive 0 → send 1 → receive 1

    You're suggesting that a way out of this is to say that free will exists only as an illusion. How does that help? Do you mean that computers running these two programs can't be built? Do you think they can't be hooked up to tachyon emitters or tachyon detectors? What do you think happens if I try? Do I get hit by a meteor? What if we all try? More meteors?

    Do you think we would be unable to choose to try? I have two problems with that option. First of all, that doesn't just eliminate free will, but also the illusion of free will. How can we resolve a paradox by assuming that something we know is true (that we have the illusion of free will) is false? Second, if our brains are just physical systems that gather and utilize information, then we're not very different from the computers mentioned above. So why would it be more likely that the event that prevents the paradox happens in my brain than in the computer?
     
  19. Nov 14, 2009 #18

    RUTA

    User Avatar
    Science Advisor

    I'm trying to understand this spacetime pic. There are two computers at rest wrt each other. The computer on the left, say, is programmed to emit the opposite signal it receives while the one on the right is programmed to emit the same signal it receives. The signals' worldlines are superluminal. At t = 0 (in computer rest frame), L receives 0 from R (event 2), emitted at t < 0 (event 1). At t =1, L emits 1 (event 3), received at t > 1 by R (event 4). In the computer frame, the order of events is 1, 2, 3, 4. In some other frame, the order of events becomes 2, 1, 4, 3. In this second frame, it appears that R receives 0 and emits 1, in contradiction to its program. Do I have this correct or have I missed the point?
     
  20. Nov 14, 2009 #19

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Fredrik, I didn't follow your example of the contradiction.

    I assumed that if there were instanteous transmissions, everyone could agree (synchronize their clock) on what T=0 is. Once that happens, the example above doesn't seem valid because you don't send messages to the past, just to the same absolute time frame. I get that each observer's frame may evolve differently (so my T=1 clock may be different than someone else's T=1 clock if we are in different inertial frames) but that could be fixed simply by refreshing our agreed synchronization (say from an agreed upon 3rd party).

    Obviously I have trouble understanding WHY and HOW instantaneous transmission (or preferred frames) lead to causal loops. Can you or Demystifier explain in more detail for me?
     
    Last edited: Nov 14, 2009
  21. Nov 14, 2009 #20

    zonde

    User Avatar
    Gold Member

    Not at rest but in inertial motion toward each other.
    Because they are in different reference frames they have different simultaneity and what is instantaneous (simultaneous) in one reference frame is backward in time in other frame.

    EDIT: Sorry, not toward each other but away from each other.
     
    Last edited: Nov 14, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does instantaneous communication implies a preferrred Lorentz frame?
Loading...