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Sorry, your grand statement matter-of-factly dismissing thermodynamic arrow of time needs a much better source than some computer scientist's armchair blog abou

Sorry, your grand statement matter-of-factly dismissing thermodynamic arrow of time needs a much better source than some computer scientist's armchair blog about QM. I know it's been recently trendy for armchair physicists to post grand philosophical claims about the universe from some vague understandings of Bell's Inequality and/or entanglement, and blurring the line between philosophy of physics with physics. But your casual dismissal of entropic thermodynamic arrow-of-time time in favor of purely increasing entanglement ranks way high on the BS meter. For context - I have a PhD in experimental condensed-matter physics, and TA'd thermodynamics and statistical mechanics for several years. I left the field of physics many years ago so maybe there were some notable recent developments in quantum information theory that I missed. But I skimmed your linked article and can't take it seriously. Would love a better source of you can provide one, eg peer-reviewed (in a respectable physics journal) or written by an actual career physicist at a reputable institution.

lisper 12y ago
http://arxiv.org/abs/quant-ph/9605002 http://arxiv.org/abs/quant-ph/9605002 This doesn't discuss the arrow-of-time directly, but it's a pretty straightforward corollary.
- jeffwass 12y ago
  Cheers for the article, quite interesting. I'd still maintain the original comment in the original article is correct, that the arrow of time is still driven by 'thermodynamic' processes, where I'm allowing thermodynamic really means statistical mechanical process, and statistical mechanics includes (as it always has) quantum statistics of quantum processes. Ie, your arxiv link gives further quantum detail of the underlying macroscopic measurements (eg, the schrodinger cat example). But the irreversibility and arrow-of-time come from the enanglement of the particle with the 10^23 atoms of the macroscopic observer system. And in that sense the irreversibility is more due to the large size of the system, just like a classical example (such as shaking a bottle of originally separated black/white marbles) than the fact that there is quantum entanglement between the particle and the measurement system. So while Cerf and Adami identified interesting quantum processes to describe underlying interactions, the irreversibility still comes from a statistical consideration of the interaction with a macroscopically large system. Just like the traditional arrow-of-time statement that you had originally refuted.
  - lisper 12y ago
    I respectfully disagree. I maintain there is a fundamental difference between the Cerf & Adami process (which is really just decoherence) and classical thermodynamic processes. That difference is: for ordinary thermodynamic processes, the odds of observing a decrease in entropy with increasing time is very small but nonetheless non-zero. In the decoherence model, the odds of observing a reversed entanglement is exactly zero because measurement is entanglement. As a practical matter it amounts to the same thing, but in principle it is as different as GR is from Newtonian mechanics.
    - jeffwass 12y ago
      Your arxiv link disagrees with you. From the paragraph after equation 6.6 : >This irreversibility is completely equivalent to the irreversibility in classical mechanics. Indeed, classically, to reverse the microscopic time evolution, it is necessary to invert the velocity of all the particles, the practical impossibility of which gives a macroscopic irreversible aspect to time evolution. In quantum mechanics, it is necessary to undo any unitary evolution associated with all interactions that particles have undergone, so that reversibility is practically impossible if a macroscopic number of particles have been involved. We are led to conclude that irreversibility is not an inherent feature of quantum mechanics.
      - lisper 12y ago
        I know that looks like a disagreement, but it's actually not. Note that I said the odds of observing a reversed entanglement is exactly zero, not that the odds of it actually happening were exactly zero. The odds of it happening are comparable to the odds of thermodynamic reversal. The difference is that if a thermodynamic reversal were to happen, it could be observed. If a quantum reversal were to "happen" (I put "happen" in scare quotes because the whole concept of something "happening" becomes a little murky here) it could not possibly be observed, because any attempt to observe it would "prevent" it from "happening."
        jeffwass 12y ago

lisper 12y ago

http://arxiv.org/abs/quant-ph/9605002 http://arxiv.org/abs/quant-ph/9605002 This doesn't discuss the arrow-of-time directly, but it's a pretty straightforward corollary.

jeffwass 12y ago
Cheers for the article, quite interesting. I'd still maintain the original comment in the original article is correct, that the arrow of time is still driven by 'thermodynamic' processes, where I'm allowing thermodynamic really means statistical mechanical process, and statistical mechanics includes (as it always has) quantum statistics of quantum processes. Ie, your arxiv link gives further quantum detail of the underlying macroscopic measurements (eg, the schrodinger cat example). But the irreversibility and arrow-of-time come from the enanglement of the particle with the 10^23 atoms of the macroscopic observer system. And in that sense the irreversibility is more due to the large size of the system, just like a classical example (such as shaking a bottle of originally separated black/white marbles) than the fact that there is quantum entanglement between the particle and the measurement system. So while Cerf and Adami identified interesting quantum processes to describe underlying interactions, the irreversibility still comes from a statistical consideration of the interaction with a macroscopically large system. Just like the traditional arrow-of-time statement that you had originally refuted.
- lisper 12y ago
  I respectfully disagree. I maintain there is a fundamental difference between the Cerf & Adami process (which is really just decoherence) and classical thermodynamic processes. That difference is: for ordinary thermodynamic processes, the odds of observing a decrease in entropy with increasing time is very small but nonetheless non-zero. In the decoherence model, the odds of observing a reversed entanglement is exactly zero because measurement is entanglement. As a practical matter it amounts to the same thing, but in principle it is as different as GR is from Newtonian mechanics.
  - jeffwass 12y ago
    Your arxiv link disagrees with you. From the paragraph after equation 6.6 : >This irreversibility is completely equivalent to the irreversibility in classical mechanics. Indeed, classically, to reverse the microscopic time evolution, it is necessary to invert the velocity of all the particles, the practical impossibility of which gives a macroscopic irreversible aspect to time evolution. In quantum mechanics, it is necessary to undo any unitary evolution associated with all interactions that particles have undergone, so that reversibility is practically impossible if a macroscopic number of particles have been involved. We are led to conclude that irreversibility is not an inherent feature of quantum mechanics.
    - lisper 12y ago
      I know that looks like a disagreement, but it's actually not. Note that I said the odds of observing a reversed entanglement is exactly zero, not that the odds of it actually happening were exactly zero. The odds of it happening are comparable to the odds of thermodynamic reversal. The difference is that if a thermodynamic reversal were to happen, it could be observed. If a quantum reversal were to "happen" (I put "happen" in scare quotes because the whole concept of something "happening" becomes a little murky here) it could not possibly be observed, because any attempt to observe it would "prevent" it from "happening."
      - jeffwass 12y ago