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> 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 otherw

> 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
    OK, fair enough. But just for the record: > You only need matter with negative mass if you want to construct one by building a machine. As I'm sure you're aware, things do move faster than light and/or backwards in time on the microscopic level. But that's not what most people care about when they talk about this stuff. They want to know if you can build a time machine. And the answer to that (on current knowledge) is a definitive "no".

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
  OK, fair enough. But just for the record: > You only need matter with negative mass if you want to construct one by building a machine. As I'm sure you're aware, things do move faster than light and/or backwards in time on the microscopic level. But that's not what most people care about when they talk about this stuff. They want to know if you can build a time machine. And the answer to that (on current knowledge) is a definitive "no".