6 ms·

Thermodynamics constrains interpretations of quantum mechanics

35 points by evanb 10y ago

squozzer 10y ago
> Cabello and colleagues show, however, that if an experimenter is free to make spin measurements anywhere in the xz-plane, the heat generated per measurement is unbounded—obviously, an unphysical result. First thing that popped into my head was "ultraviolet catastrophe" -- and saw that one of the scitation.org commentors had beaten me to punch.
hprotagonist 10y ago
Leibniz would be thrilled.
lisper 10y ago
There is a much simpler argument to be made here: send a stream of entangled particles through a polarizing beam splitter. At each branch, put another polarizing beam splitter oriented at 45 degrees to the previous one. You can continue this indefinitely to split the beam an arbitrary number of times. QM predicts that entangled particles must remain correlated throughout the splitting no matter how many splits there are. If there were only a finite number of hidden variables the correlation would have to stop after a finite number of splits.
- platz 10y ago
  unless the hidden-variables are non-local/global, though
  - lisper 10y ago
    What would that make a difference? You still need a potentially unbounded amount of information to specify which path the particles take.
- SomeStupidPoint 10y ago
  Why can't splitting just be creating a new non-local hidden variable by the very action of splitting, carrying the information for the split pair? (And really, several consecutive splits is just defining the shape/topology of the single, original split non-local variable.)
  - lisper 10y ago
    You're missing the point. Locality, topology, and even geometry are irrelevant. What matters is how many bits of information you would need to specify the path that the photon takes. And the answer is: you need an infinite number of bits. (OK, well, actually you need an unbounded number of bits, which is not quite the same thing. But it's equally implausible.)
    - T-A 10y ago
      So if you use an infinite amount of matter to create an infinite number of beam splitters, an infinite amount of energy to move them to an infinite number of positions (which I guess you keep track of using a computer with infinite memory), over (I guess again) an infinite amount of time, then... ... it turns out that you also need an infinite number of hidden variables for the resulting setup to work according to QM? Shocking! :)
      - lisper 10y ago
        Yes. But this specious argument is exactly why I corrected myself and said "unbounded" instead of "infinite."
    - SomeStupidPoint 10y ago
      I mean, your argument supposes that the system can't have that many bits -- but doesn't your device to generate arbitrary bits require more bits to be generated than that amount? Your argument sounds like "5 bits is too many bits to have in a system, and my 10 bit device can generate 5 bits!" I guess? But that doesn't really seem interesting to me: impossible outcomes from impossible to construct situations don't matter. Also, any value which can take on an arbitrary real (or even rational) number has arbitrary storage capcity -- and so far as I know, lots of quantum values do.
ivan_ah 10y ago
arXiv link: https://arxiv.org/abs/1509.03641 https://arxiv.org/abs/1509.03641
- T-A 10y ago
  Note the first assumption: "The choice of which measurement is performed can be made randomly and independently of the system under observation". In the Physics Today article, this becomes "if an experimenter is free to make spin measurements anywhere". Either way, the conclusion that "systems in hidden-variable theories must have an infinite memory" only holds if you assume free will.
deleted 10y ago
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35 points by evanb 10y ago

> Cabello and colleagues show, however, that if an experimenter is free to make spin measurements anywhere in the xz-plane, the heat generated per measurement is unbounded—obviously, an unphysical result. First thing that popped into my head was "ultraviolet catastrophe" -- and saw that one of the scitation.org commentors had beaten me to punch.

hprotagonist 10y ago

Leibniz would be thrilled.

lisper 10y ago

There is a much simpler argument to be made here: send a stream of entangled particles through a polarizing beam splitter. At each branch, put another polarizing beam splitter oriented at 45 degrees to the previous one. You can continue this indefinitely to split the beam an arbitrary number of times. QM predicts that entangled particles must remain correlated throughout the splitting no matter how many splits there are. If there were only a finite number of hidden variables the correlation would have to stop after a finite number of splits.

platz 10y ago
unless the hidden-variables are non-local/global, though
- lisper 10y ago
  What would that make a difference? You still need a potentially unbounded amount of information to specify which path the particles take.
SomeStupidPoint 10y ago
Why can't splitting just be creating a new non-local hidden variable by the very action of splitting, carrying the information for the split pair? (And really, several consecutive splits is just defining the shape/topology of the single, original split non-local variable.)
- lisper 10y ago
  You're missing the point. Locality, topology, and even geometry are irrelevant. What matters is how many bits of information you would need to specify the path that the photon takes. And the answer is: you need an infinite number of bits. (OK, well, actually you need an unbounded number of bits, which is not quite the same thing. But it's equally implausible.)
  - T-A 10y ago
    So if you use an infinite amount of matter to create an infinite number of beam splitters, an infinite amount of energy to move them to an infinite number of positions (which I guess you keep track of using a computer with infinite memory), over (I guess again) an infinite amount of time, then... ... it turns out that you also need an infinite number of hidden variables for the resulting setup to work according to QM? Shocking! :)
    - lisper 10y ago
      Yes. But this specious argument is exactly why I corrected myself and said "unbounded" instead of "infinite."
  - SomeStupidPoint 10y ago
    I mean, your argument supposes that the system can't have that many bits -- but doesn't your device to generate arbitrary bits require more bits to be generated than that amount? Your argument sounds like "5 bits is too many bits to have in a system, and my 10 bit device can generate 5 bits!" I guess? But that doesn't really seem interesting to me: impossible outcomes from impossible to construct situations don't matter. Also, any value which can take on an arbitrary real (or even rational) number has arbitrary storage capcity -- and so far as I know, lots of quantum values do.

ivan_ah 10y ago

arXiv link: https://arxiv.org/abs/1509.03641 https://arxiv.org/abs/1509.03641

T-A 10y ago
Note the first assumption: "The choice of which measurement is performed can be made randomly and independently of the system under observation". In the Physics Today article, this becomes "if an experimenter is free to make spin measurements anywhere". Either way, the conclusion that "systems in hidden-variable theories must have an infinite memory" only holds if you assume free will.

deleted 10y ago

[deleted]