[HN Gopher] Emergence of a second law of thermodynamics in isola...
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Emergence of a second law of thermodynamics in isolated quantum
systems
Author : westurner
Score : 129 points
Date : 2025-02-02 22:15 UTC (1 days ago)
(HTM) web link (journals.aps.org)
(TXT) w3m dump (journals.aps.org)
| westurner wrote:
| ScholarlyArticle: "Emergence of a Second Law of Thermodynamics in
| Isolated Quantum Systems" (2025)
| https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuan...
|
| NewsArticle: "Even Quantum Physics Obeys the Law of Entropy"
| https://www.tuwien.at/en/tu-wien/news/news-articles/news/auc...
|
| NewsArticle: "Sacred laws of entropy also work in the quantum
| world, suggests study" ... "90-year-old assumption about quantum
| entropy challenged in new study"
| https://interestingengineering.com/science/entropy-also-work...
| wills_forward wrote:
| This paper is basically statistical mechanics with a quantum
| veneer. Two major issues:
|
| 1. Scale: They're simulating just 13 qubits with QuTiP and making
| grand claims about quantum thermodynamics. The computational
| complexity they're glossing over here is astronomical. Anyone
| who's actually worked with quantum systems knows you can't just
| handwave away the scaling problems.
|
| 2. Measurement Problem: Their whole argument about instantaneous
| vs time-averaged measurements is just repackaging the quantum
| measurement problem without actually solving anything. They're
| doing the same philosophical shell game that every "breakthrough"
| quantum paper does by moving around where they put the observer
| and pretending they've discovered something profound.
| whatshisface wrote:
| Quantum mechanics is statistical mechanics in the complex
| numbers.
| carlob wrote:
| Quantum mechanics is Markov chains in imaginary time.
| teamonkey wrote:
| Can you explain that?
| diegoperini wrote:
| State transitions are probabilistic and operators have
| complex coefficients.
| tsimionescu wrote:
| State transitions are deterministic, it's only
| measurement that is probabilistic.
| Filligree wrote:
| Even that is arguable. Subjective experience is
| probabilistic... kinda.
| guntars wrote:
| Do atoms decay deterministically?
| wfewras wrote:
| I saw the best minds of my generation pithposting on hn.
| Filligree wrote:
| Yes, aka. continuously. Interactions with larger systems
| makes it appear discontinuous.
| tsimionescu wrote:
| As long as they are isolated, their state is a
| superposition of all possible states, and evolves
| determinsitically, with the amplitude of each of these
| "sub-states" evolving perfectly determinsitically. If you
| want to perform a measurement, you choose a possible
| decomposition of the superposition state and measure
| along that axis, and you'll get one of the values along
| that axis, with a probability that is the modulus of the
| square of the (complex) amplitude of that value.
| zifk wrote:
| I disagree with you on both fronts.
|
| 1. The main underpinning of this article is the analytical
| theory they come up with independent of their simulation. The
| fact that it explains a few qubits well is exactly why this is
| interesting. If you were to scale up their model - a spin-1/2
| ising model, you would effectively get a classical magnet,
| which is obviously well described by classical thermodynamics.
| It's in limit of small systems that quantum mechanics makes
| thermodynamics tricky.
|
| 2. Their time averaging is just to remove fluctuations in the
| state, not avoid the measurement problem. They're looking at
| time averages of the density matrix, which still yields a
| quantum object that will collapse upon measurement. And as
| their mathematical model points out, this can be true for
| arbitrary time averaging windows, the limits just change
| respectively as smaller time averages allow for larger
| fluctuations. There's nothing being swept under the rug here.
| readthenotes1 wrote:
| "The second law of thermodynamics states that the entropy of an
| isolated system can only increase over time. "
|
| Isn't there a difference between "can only increase" and "cannot
| decrease"?
| goatlover wrote:
| Over long enough time, fluctuations to lower entropy states
| will happen, so the jaw is statistical.
| 2rsf wrote:
| Well, it's the an equal sign missing from one. For the later it
| can stay the same while it cannot stay the same for the former.
| n00b101 wrote:
| The trusty laws of thermodynamics strike again
| timewizard wrote:
| > This implies that for macroscopic systems, the expected time
| one would be required to wait to observe such a decrease in
| entropy occurring is unobservably large.
|
| Yea but we have virtual particles and the Casimir effect. Am I
| wrong or isn't this these perturbations evidencing themselves on
| a macroscopic scale?
| whatshisface wrote:
| Perturbations can mean either analogical reasoning (something
| is similar to something that it could come from with a small
| change) or actual perturbation (the effect of Venus on the
| orbit of the moon). Virtual particles are perturbations in the
| former sense, while quantum fluctuations are a small
| perturbation in the latter.
| 11101010001100 wrote:
| The signal to noise at the union of QM and thermodynamics on HN
| is evidence of regression to the mean.
| abyssin wrote:
| Why?
| 11101010001100 wrote:
| The comments on HN are always full of ostensibly deep quips
| or questions but the work to connect them to scientific or
| philosophical questions is absent. That's the part that
| doesn't scale.
| nhatbui wrote:
| So true, tale as old as time. Someone "raises doubts" based
| on partial knowledge of the subject, they go back and forth
| with someone, and then finally someone comes in with
| conversation-killing "what is consciousness anyways" type
| comment
| lisper wrote:
| > Even for isolated systems where, in principle, we could have
| zero surprisal and access to all possible information...
|
| This makes no sense. How could you have access to "all possible
| information" in an isolated system? You obviously can't make any
| measurements, and if the system is prepared, then it's entangled
| with the system used to prepare it and again cannot be isolated.
| The whole notion of "an isolated system" is a theoretical fiction
| that doesn't actually exist in physical reality, but even in
| theory one cannot access all of the information in an isolated
| system because of the no-cloning theorem. So this really feels to
| me like the old joke about spherical chickens.
|
| Furthermore, this seems like an already-solved problem.
| Constructing classical reality requires copying classical
| information, and the only way to make that happen is to discard
| quantum information [1]. That is the source of the Second Law and
| the arrow of time [2].
|
| [1] https://arxiv.org/abs/quant-ph/9512022
|
| [2] https://blog.rongarret.info/2014/10/parallel-universes-
| and-a...
| 11101010001100 wrote:
| As you know, all possible information for an isolated system is
| obtained via solutions to the Schrodinger equation. This is
| standard many-body physics.
| lisper wrote:
| Well, yeah, but that seems like a vacuous observation to me.
| In order to find solutions to the SE you have to know the
| initial conditions. How are you going to obtain those for an
| isolated system? You haven't solved the problem, you have
| just pushed it backwards in time.
| 11101010001100 wrote:
| Thermalization (excluding systems which exhibit many body
| localization and the like) will occur regardless of initial
| conditions.
| lisper wrote:
| That is manifestly untrue because the SE is time-
| reversible. That is the whole problem.
| 11101010001100 wrote:
| Yes, and at the same time, the statistics of the system
| can still satisfy thermalization.
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