4 ms·

> Yes, QM is incomplete, but it places hard constraints on the space of possible completions, and none of them allow FTL communications. This is entirely inco

> Yes, QM is incomplete, but it places hard constraints on the space of possible completions, and none of them allow FTL communications. This is entirely incorrect. All of the proofs about broken causality (all of which stem from special relativity, since non-relativistic QM doesn't put the speed of light on a pedestal) assume that the laws of special or general relativity hold everywhere. What you're saying is the equivalent of claiming in 1914 that the theory of special relativity proves that physics cannot operate without an intertial reference frame.

lisper 10y ago
Yes, I am assuming that relativity holds everywhere. There is zero evidence that this is not the case. This is what it means to be "as certain as anything in science."
- johncolanduoni 10y ago
  That's not a "hard constraint on the space of possible completions". That's assuming the answer to begin with.
  - lisper 10y ago
    No, it isn't. The mathematical formalism of QM is fully compatible with SR. QM can only be completed within the constraints of SR. To get FTL you would not have to complete QM, you would have to overthrow it and SR and GR and replace them all with something completely different. Not just that, but whatever you replaced it with would still have to account for the observed invariance in the speed of light in all reference frames. QM and entanglement are really red herrings with respect to the FTL question. The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. Entanglement only appears to violate this because people don't understand what entanglement actually is and how it works. When you make a measurement on one member of an EPR pair, nothing happens to the other particle. It's still in exactly the same state it was in before the measurement. So there is absolutely no reason whatsoever to believe that FTL could be possible, and a lot of reasons to believe that it's not. Granted, this is not "absolutely certain" (nothing in science ever is) but it's as certain as it gets.
    - johncolanduoni 10y ago
      > QM can only be completed within the constraints of SR. This is not true. QM theory in general (without talking about particular Hamiltonians, field or otherwise or not) has no reliance on special relativity; it doesn't even have a reliance on positions in three-dimensional space (physicists who work in quantum information routinely work with QM at this level)! So again, you're already specifying QFT (e.g. by something like the Wightman axioms), which includes special relativity as an assumption. > The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. This is what special and general relativity say. Not only are you begging the question again, but GR doesn't rule out violations of causality: there's a bunch of known models within GR that violate it[1]. Since GR is in fact more certain than SR (in that SR has strictly less explanatory power than GR since it's a special case of it), you must believe that it is as certain as anything else in science that causality violations are real! [1]: https://en.wikipedia.org/wiki/Closed_timelike_curve https://en.wikipedia.org/wiki/Closed_timelike_curve
      - lisper 10y ago
        You're right. Most people don't know the difference between QM and QFT, so I tend to use the terms interchangeably in casual exchanges. You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. So you can interpret this in two ways, as a prediction that FTL is possible, or a prediction that negative-mass matter does not exist. Nonetheless, since you clearly understand physics, I'll restate my claim more precisely: the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit. It would take something like the discovery of exotic matter with negative mass to move the needle on FTL.
        johncolanduoni 10y ago
        > You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. You only need matter with negative mass if you want to construct one by building a machine. Some models for closed timelike curves that could occur in nature (e.g. the Tipler cylinder with infinite length) do not need negative energy. > the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit Okay, that I agree with, but this unlikelihood is completely orthogonal to (pre-relativity) QM's compatibility with FTL travel/signaling. That was what I disagreed with.

lisper 10y ago

Yes, I am assuming that relativity holds everywhere. There is zero evidence that this is not the case. This is what it means to be "as certain as anything in science."

johncolanduoni 10y ago
That's not a "hard constraint on the space of possible completions". That's assuming the answer to begin with.
- lisper 10y ago
  No, it isn't. The mathematical formalism of QM is fully compatible with SR. QM can only be completed within the constraints of SR. To get FTL you would not have to complete QM, you would have to overthrow it and SR and GR and replace them all with something completely different. Not just that, but whatever you replaced it with would still have to account for the observed invariance in the speed of light in all reference frames. QM and entanglement are really red herrings with respect to the FTL question. The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. Entanglement only appears to violate this because people don't understand what entanglement actually is and how it works. When you make a measurement on one member of an EPR pair, nothing happens to the other particle. It's still in exactly the same state it was in before the measurement. So there is absolutely no reason whatsoever to believe that FTL could be possible, and a lot of reasons to believe that it's not. Granted, this is not "absolutely certain" (nothing in science ever is) but it's as certain as it gets.
  - johncolanduoni 10y ago
    > QM can only be completed within the constraints of SR. This is not true. QM theory in general (without talking about particular Hamiltonians, field or otherwise or not) has no reliance on special relativity; it doesn't even have a reliance on positions in three-dimensional space (physicists who work in quantum information routinely work with QM at this level)! So again, you're already specifying QFT (e.g. by something like the Wightman axioms), which includes special relativity as an assumption. > The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. This is what special and general relativity say. Not only are you begging the question again, but GR doesn't rule out violations of causality: there's a bunch of known models within GR that violate it[1]. Since GR is in fact more certain than SR (in that SR has strictly less explanatory power than GR since it's a special case of it), you must believe that it is as certain as anything else in science that causality violations are real! [1]: https://en.wikipedia.org/wiki/Closed_timelike_curve https://en.wikipedia.org/wiki/Closed_timelike_curve
    - lisper 10y ago
      You're right. Most people don't know the difference between QM and QFT, so I tend to use the terms interchangeably in casual exchanges. You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. So you can interpret this in two ways, as a prediction that FTL is possible, or a prediction that negative-mass matter does not exist. Nonetheless, since you clearly understand physics, I'll restate my claim more precisely: the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit. It would take something like the discovery of exotic matter with negative mass to move the needle on FTL.
      - johncolanduoni 10y ago
        > You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. You only need matter with negative mass if you want to construct one by building a machine. Some models for closed timelike curves that could occur in nature (e.g. the Tipler cylinder with infinite length) do not need negative energy. > the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit Okay, that I agree with, but this unlikelihood is completely orthogonal to (pre-relativity) QM's compatibility with FTL travel/signaling. That was what I disagreed with.