
Thermodynamics constrains interpretations of quantum mechanics - evanb
http://physicstoday.scitation.org/do/10.1063/PT.5.7331/full/?utm_source=Physics+Today&utm_medium=email&utm_campaign=7862648_The+week+in+Physics+%E2%80%94+December+12-20+quarantine&dm_i=1Y69%2C4OIUW%2CE1N7PA%2CHHKXZ%2C1&
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lisper
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.

~~~
SomeStupidPoint
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
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.)

~~~
SomeStupidPoint
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.

~~~
lisper
Did you not bother to read the original article?

"Now an international collaboration led by Adán Cabello has invoked a
fundamental thermodynamics result, the Landauer erasure principle, to show
that systems in hidden-variable theories must have an infinite memory to be
compatible with quantum mechanics."

All I'm saying is that you don't need thermodynamics to show this.
Conservation laws are enough.

~~~
SomeStupidPoint
My point was your argument doesn't hold, because you cant actually construct a
situation in which the system doesnt already have sufficient storage capacity:
the inability to require those bits without already having enough bits in the
system to store them is pretty important. I don't believe that it can require
QM to account for arbitrary memory, without presupposing we have it to
construct the device. Hence, impossible constructions dont matter.

I have some similar objections to the summary of their paper, but I haven't
had time to read it yet. It's possible they address some of them.

In particular, measuring from an arbitrary location and an arbitrary number of
times sounds like the introduction of the infinity by assumption, rather than
by consequence.

~~~
lisper
I think you're missing the point. It is not that all that information _exists_
, it is that all that information has to exist in a _single particle_. This is
not impossible, but it is implausible.

~~~
SomeStupidPoint
Correction: it has to be accessible to a single particle, but the information
could be non-localized across the whole system, since your particle is itself
fairly non-localized, and arguably has weakly interacted with the various
devices along the way, meaning that Im not sure we can fully disentangle the
wave-equation for the photon from the equations for the splitters. This could
easily be the photon "borrowing" bits from the splitter, by sharing a non-
local value with it, which carries the split path information for both.

Which is just to say that the system (in full) has to be able to contain a
number of bits sufficient for that, which your setup does.

~~~
lisper
> Im not sure we can fully disentangle the wave-equation for the photon from
> the equations for the splitters

That doesn't matter. The two sides of the experiment are space-like separated,
and the splitters on each side are light-like separated. There is no place the
information can possibly be other than in the photons.

~~~
SomeStupidPoint
...except non-local variables. Which is why I keep mentioning non-locality.

I don't think words are working and I'm not sure I understand your point
fully. Could you draw a diagram? Specifically, the apparatus and where you
think the requirement for information occurs.

I really do want to understand your point, because it's an interesting
argument, even if I'm not sure I agree.

~~~
lisper
> I don't think words are working

Indeed. I have no idea what you mean by "non-local variable."

You should read this:

[http://www.scottaaronson.com/democritus/lec11.html](http://www.scottaaronson.com/democritus/lec11.html)

just to make sure we're on the same page about the basics. (Pay particular
attention to the part starting with "No-go theorems galore".)

~~~
SomeStupidPoint
Could you point out what you think that has to do with locality?

It mentions it once, to assert it as a desired axiom (and incorrectly assert
that QM can be local, which it can't be). I also would argue that GR supports
non-local theories well enough, in that the actual geometry can appear macro-
Euclidean (or other nice space) while containing micro-bridges which break the
locality structure for the "nice" macro space.

A non-local value is any value which can impact things at super-luminal speed
(generally, instantaneously). Of course, I would argue that the non-locality
is only apparent, and we're simply seeing the shape of things very wrong
because of how brains work, the information reaches us, etc.

~~~
lisper
> A non-local value is any value which can impact things at super-luminal
> speed

Ah. Those are ruled out by relativity. If you can send information faster than
light, then you can send information backwards in time and violate causality.

------
squozzer
> 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.

------
ivan_ah
arXiv link:
[https://arxiv.org/abs/1509.03641](https://arxiv.org/abs/1509.03641)

~~~
T-A
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.

------
hprotagonist
Leibniz would be thrilled.

