Thermodynamical cost of some interpretations of quantum theory | Hacker News Reader

Thermodynamical cost of some interpretations of quantum theory

arxiv.org

49 pointsn4r918 comments

To save others some time.

Input 1: authors check out (PRL, Nature pubs). Not a one-off crazy posting to arXiv.

Input 2: Unless this your field, I'd be surprised if you get much from the paper other than interpretation of quantum mechanics is still an active field and information theory / thermodynamics has an important role in that field.

Input 3: After a few reads, I don't like the phrasing the authors put on Type-II interpretations. They say "Another class sees quantum probabilities as not directly dealing with intrinsic properties of the world but with relational experiences between an observer and the world." Another way to say it may be that it's impossible to separate observation from the intrinsic properties of the world. That is, reality is tightly bound up with observation and trying to separate the two is artificial. But I'd be happy to hear what others think about it.

> Another way to say it may be that it's impossible to separate observation from the intrinsic properties of the world.

I think it is more parsimonious to say that there are no intrinsic properties of the world that are not relational, not that they are impossible to separate from observation. It defeats the purpose of interpretations like relational quantum mechanics to hold on to the concept of an intrinsic reality when the interpretation makes such a concept superfluous.

I would say that it is impossible to disentangle the intrinsic properties of the world from observations in the same way that going north of the North Pole is impossible.

"no intrinsic properties of the world that are not relational." I like that better. Thanks!

I agree on the phrasing issue. Both there "types" seem to require Bohr's classical observer, but as soon as you assume that observers are classical you've swept the big question under the carpet, which is, "Why is there a classical world at all?" Decoherence and similar approaches at least try to address this question, and don't fit at all well with their two-Type scheme.

Or to put it another way: observers are intrinsic to reality as well as the systems under observation, and any attempt to treat them separately will fail. But this is uninteresting, because any interpretation is going to have to acknowledge this at some level (as Bohr correctly pointed out quite a long time ago.)

If you've got Many Worlds at the back of your mind there, rest assured (!) that the lead author considers it to be a Type-I interpretation See http://arxiv.org/pdf/1509.04711v1.pdf

Why are the quantum measurements the focus of this paper? The real source of entropy is the random bits being generated to decide which measurement to perform; the measurement actually being performed is just an after-show.

Regardless, I think it's clear that assumption `(i)` is the one that breaks: the system affects the decision of what to measure. The stream of random bits will consume all the neg-entropy in the system, cause heat death, and thereby break whatever mechanism was doing the measuring (e.g. if it was you, then you would die).

I haven't read the paper, nor am I an expert, but the assumption (i) sounds fishy "the decision of which measurement is performed on a quantum system can be made independently of the system". By fishy, I mean obviously false. Which means being incompatible with this assumption is OK. I'm probably missing something...

What don't you like about that? Physicists go to great lengths to make sure that random decisions are not accidentally influenced in unexpected ways, for example by basing the decision on light from quasars at the other end of the universe [1].

[1] http://www.nature.com/news/cosmic-light-could-close-quantum-...

The influence may be negligible, but it's theoretically there. The decision-maker is part of the same system (decision is happening within a system of neurons augmented with electronic tools, all of which exist in the same physical reality).

There are no neurons involved, essentially random number generators are making the decisions and in such experiments usually in the nanosecond range while particles are in flight between source and detector to ensure that the decision can not influence the creation and preparation process. And by using a source of randomness billions of light years away it becomes very unlikely that the particle source and the source of randomness could have interacted in an interesting way because the universe is to young to allow interactions due to the finite speed of light. There is of course still a possibility that an interaction happened when the universe was still young and tiny but you can probably imagine what kind of conspiracy the universe would have to have going on for that to work.

Isn't that just determinism followed to its logical end?

Some physicists argue that there might not be such thing as independence even with "quasars which are located diametrically opposite to one another"[1].

[1] http://arxiv.org/abs/1112.1811 - page 12

"Independently" is also a loaded word - one could mean that there's no direct causal connection between the state of the system and the state of the measurement apparatus, or one could mean that those two states are (in ensemble experiments) independent in the statistical sense.

In order for Bell's theorem (and a lot of subsequent quantum theory) to work, we have to interpret causal and statistical independence as being one and the same. But the more one learns about the theoretical justification for this conflation, the shakier it seems. Bell, in particular, justified it by claiming that it was unscientific to assume that experimenters didn't have free will. Okay, sure.

Any universe where the assumption that experimental measurement settings are always already statistically correlated with the state of the system being measured (even if that correlation would have to have been established millennia ago) is fundamentally weird in certain ways - these kinds of theories are sometimes called 'conspiracy theories' as it sort of seems like all the information is there and nature is conspiring to hide it from us. But there's a whole branch of super-determinist interpretations of quantum mechanics motivated by the sense that this kind of weirdness is not as bad as the kind of weirdness we'd have to otherwise accept. This kind of work has been out of fashion for a while, but it seems to be gaining a certain amount of momentum in the last decade or so.

"Bell, in particular, justified it by claiming that it was unscientific to assume that experimenters didn't have free will. Okay, sure." You make some good points. I am not sure how Bell meant this, but read out of context, this is obviously false. Deterministic modelling (and therefore lack of absolute free will of the components of the system) is a long tradition of science.

For various reasons, in quantum mechanics, one typically excludes the observer from the system being modeled. This is obviously problematic, especially if you're doing large scale cosmology. But that's the measurement problem for you.

Regarding the context, the quote I'm thinking of comes from one of the essays in Speakable and Unspeakable in Quantum Mechanics, which is an enlightening read, both in terms of content and for its historical value. The issue, I think, is that the kind of 'conspiracy' implied by superdeterminist theories, wherein nature sort of guides our hand in picking certain experimental settings so as to hide information seems troubling to many scientists. See Zeilinger's quote at https://en.wikipedia.org/wiki/Superdeterminism

Science aside, I found the authorship fascinating and encouraging; it is composed of individuals from five different countries and cultures (Sweden, Germany, UK, Spain, and China).

Is this typical for science?

Not that uncommon in quantum foundations, where researchers are often few and far between (partly due to the competitive academic job market).

... and yet they are entangled ;-)

Reply on news.ycombinator.com