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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 b

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, you're obviously a pretty smart guy or gal. But your nit-picking here over 'observing' vs 'happening' is so far from your original very bold claim that the arrow of time is not thermodynamic but quantum. You even went so far to say the original article was 'misleading' because of this. I had to call you on this. Bold claims require strong confirmation. And even the arxiv paper you linked to makes the same statement that I defended - that irreversibility of purely quantum processes is still due to the macroscopic (ie large) system size, which immediately implies the same broad thermodynamic (and statistical ensemble) considerations we've been assuming for the past century.
      - lisper 12y ago
        We may just have to agree to disagree on this. IMHO the difference between the arrow of time being due to entanglement and the arrow of time being due to the 2nd law is analogous to the difference between gravity being a force and it being an artifact of the curvature of spacetime. Not a big difference as a practical matter but it's a huge difference conceptually. And in an article entitled "The quantum mechanics of fate" this difference matters. Also, this is HN comments, not Physics Review A. Not everything that gets said here needs to pass the highest standards of peer review.

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, you're obviously a pretty smart guy or gal. But your nit-picking here over 'observing' vs 'happening' is so far from your original very bold claim that the arrow of time is not thermodynamic but quantum. You even went so far to say the original article was 'misleading' because of this. I had to call you on this. Bold claims require strong confirmation. And even the arxiv paper you linked to makes the same statement that I defended - that irreversibility of purely quantum processes is still due to the macroscopic (ie large) system size, which immediately implies the same broad thermodynamic (and statistical ensemble) considerations we've been assuming for the past century.
    - lisper 12y ago
      We may just have to agree to disagree on this. IMHO the difference between the arrow of time being due to entanglement and the arrow of time being due to the 2nd law is analogous to the difference between gravity being a force and it being an artifact of the curvature of spacetime. Not a big difference as a practical matter but it's a huge difference conceptually. And in an article entitled "The quantum mechanics of fate" this difference matters. Also, this is HN comments, not Physics Review A. Not everything that gets said here needs to pass the highest standards of peer review.