7 ms·

This is nonsense; entropy and the arrow of time are essentially a many-body effects and require no quantum effects to occur. A simplest way to see it is to make

by mbq 12y ago

This is nonsense; entropy and the arrow of time are essentially a many-body effects and require no quantum effects to occur. A simplest way to see it is to make small simulation of a, say, 1000 gas particles with only classical bouncing in a one side of a box partitioned in half with a barrier, obviously with a time-reversible numerical method -- after the removal of the barrier the gas will evenly spread over the box without any entanglement.

deleted 12y ago
[deleted]
jpeterson 12y ago
I had the same thought. For any combined system, i.e. a situation where you've combined two systems such as the hot cup of coffee and the cool room, the number of "dispersed" states is far greater than the number of non-dispersed states. So any change of state is far more likely to be in the dispersed direction. The article didn't make it clear what role entanglement plays in this.
loup-vaillant 12y ago
yet, once the particles are spread, if you somehow reverse their velocities, they will concentrate back to one half. Yet we can never do that. Why do you think? The obvious answer in a classical universe is that we simply don't know the velocity and position of each particle, so we can't just reverse them. To pull such a feat, we'd have to be incredibly lucky, as in "winning every lottery for a century" lucky. With entanglement and de-coherence however, such reversal becomes impossible even in principle: see, when the universe splits through de-coherence, you no longer have access to the other half. Even if you manage to reverse your half of the universe, you need the other half to be reversed too, or they'll never merge back together. And not just the other half, since de-coherence happens all the time. You need all the Everett branches to be reversed. No. Way. So it does look like a better candidate for the arrow of time. (Of course, a better candidate still would be collapse interpretation, since that one is not time reversible in the first place. But this interpretation is ridiculous to begin with, so let's ignore it.) --- Read this to have an idea of how time could work in a timeless universe: http://lesswrong.com/lw/qr/timeless_causality/ http://lesswrong.com/lw/qr/timeless_causality/
- jules 12y ago
  > So it does look like a better candidate for the arrow of time. I don't fully agree. You seem to agree that even in a fully classical world, you would experience physics as irreversible on a macro level. So we cannot dismiss subjective uncertainty (lack of knowledge that could in principle be known) a priori as an explanation for the apparent irreversibility of physics. Certainly quantum uncertainty is an additional component of uncertainty, but most likely it's not only quantum uncertainty that causes our experience of irreversibility. For instance if we create a gas of very heavy particles, we would still experience irreversibility, but the quantum effects would be negligible. It seems to me therefore that it would be a good idea to try to investigate to which extent our experience of irreversibility is due to quantum uncertainty and to which extent it is due to subjective uncertainty.
- mbq 12y ago
  Assuming that we can breaks nothing; particles will concentrate, but without the barrier they will spread again and this would be a disturbance that simply averages out.
- shasta 12y ago
  > The obvious answer in a classical universe is ... Since there is an obvious answer in classical physics, it's a bit disingenuous to claim that this solves a long-standing problem in classical physics, no? > With entanglement and de-coherence however, such reversal becomes impossible even in principle [...] So it does look like a better candidate for the arrow of time. That would be true if you could demonstrate by experiment that a broken egg springing back up onto the table and reforming is physically impossible instead of just unfathomably unlikely. Can you demonstrate that?
  - gameshot911 12y ago
    I'm on it. I've placed a broken egg on my table. I'll let you know the results.
    - ars 12y ago
      It's been 6 days - how's your egg doing? :)
jerf 12y ago
Here's a crazy thought... the really stonking smart physics PhDs who have spent their whole lives working on this problem are perfectly aware of the classical 18th-century physics you are referencing, and they've found it an unsatisfactory explanation. Perhaps you might like to dig in a bit further before being so dismissive. (As a bit of a hint, your argument circularly assumes the existence of a forward arrow of time to demonstrate the forward arrow of time. Your "simulation" snuck a forward arrow of time into its definition, then proceeded to prove it exists. This is not satisfactory.)
- rwallace 12y ago
  Then again, if one is going to argue from authority, could one not equally well cite the equally stonking smart physics PhD's who dismissed the argument with "there is no physics in this paper"? And does the argument about quantum entanglement not also circularly assume the existence of a forward arrow of time?
  - jerf 12y ago
    'dismissed the argument with "there is no physics in this paper"?' Since retracted by essentially the same authorities and it's now a bustling field, so that doesn't work very well as an argument. And I'm pretty sure the entire point here is that the arrow falls out of the entanglement process itself, not that we first assume temporal ordering in the physics. Remember that we do get to assume the existence of time in general in this argument; the article may not have spelled it out as clearly as it could have but it did in fact observe this still doesn't solve "time" in general. It's a big result, though.
    - guscost 12y ago
      Is the entanglement "process" theoretically reversible in the same way that classical physics is theoretically reversible? Is it possible to describe the idea without using words that implicitly include the arrow of time? It's possible that I'm simply not understanding the concepts well enough, but I don't see how the "process" of entanglement is any less dependent on temporal ordering. Why wouldn't running it backwards make physical sense as a "disentanglement" phenomenon? For that matter, why is it right to conclude that the coffee cup has only become entangled with its exterior after it has cooled down? Is this only because the matter in the cup is supposedly the result of disentangled quantum fluctuations from the distant past? I realize that the coffee cup example is an imperfect one, but can someone explain why the process of entanglement is special in this regard where the process of increasing entropy is not? I'm sure that many smart people have been thinking about these questions, but the interpretations of QM still seem to be stuck in the realm of philosophy.
snarfy 12y ago
It depends which point of view. If I made a movie of the simulation and then played it backwards, the gas would go into the box. There is nothing in your experiment that says this is wrong. In your experiment, you must wait, and therein lies the problem. You've implicitly put a forward arrow in there.
SoftwareMaven 12y ago
This implies that atoms behave like little billiard balls, but nearly a century of quantum physics has shown us very clearly that belief is false. Unless you are looking at it, atoms exist as their wave function (and recent experiments have shown that a collapse won't occur while you are watching so e.g. a radioactive isotope will not decay while being observed), so any description of behavior needs to use their wave function to describe it.
SilasX 12y ago
The problem is that doesn't distinguish the positive time direction, since flipping the direction and running it back past zero would produce the same evenly spread gas. The resolution that Gary Drescher gives in Good and Real is to stop thinking in terms of a positive and negative time direction, but instead define an away-from-order direction. This is the same direction in which memories (like "wakes" that follow a moving object in the ocean) form, so we will only have memories of things in a pastward, higher-order state. This holds true whether you record the memory in a brain, wake, hard drive, film, or notches on a log: they are all entropy increasing processes, and so all observations will align with the increase in entropy. It's very similar to the resolution linked in this comment https://news.ycombinator.com/item?id=7605595 https://news.ycombinator.com/item?id=7605595 However, I agree that you don't need quantum-specific effects for the explanation; the same thing happens in a classical world. The article and the one linked in the above comment, are wrong in this respect, as you say. Even so, decoherence can be regarded as a special case of entropy increasing.
duaneb 12y ago
The arrow of time develops from moving particles? You sir just begged the question.

by mbq 12y ago

deleted 12y ago

[deleted]

jpeterson 12y ago

I had the same thought. For any combined system, i.e. a situation where you've combined two systems such as the hot cup of coffee and the cool room, the number of "dispersed" states is far greater than the number of non-dispersed states. So any change of state is far more likely to be in the dispersed direction. The article didn't make it clear what role entanglement plays in this.

loup-vaillant 12y ago

yet, once the particles are spread, if you somehow reverse their velocities, they will concentrate back to one half. Yet we can never do that. Why do you think? The obvious answer in a classical universe is that we simply don't know the velocity and position of each particle, so we can't just reverse them. To pull such a feat, we'd have to be incredibly lucky, as in "winning every lottery for a century" lucky. With entanglement and de-coherence however, such reversal becomes impossible even in principle: see, when the universe splits through de-coherence, you no longer have access to the other half. Even if you manage to reverse your half of the universe, you need the other half to be reversed too, or they'll never merge back together. And not just the other half, since de-coherence happens all the time. You need all the Everett branches to be reversed. No. Way. So it does look like a better candidate for the arrow of time. (Of course, a better candidate still would be collapse interpretation, since that one is not time reversible in the first place. But this interpretation is ridiculous to begin with, so let's ignore it.) --- Read this to have an idea of how time could work in a timeless universe: http://lesswrong.com/lw/qr/timeless_causality/ http://lesswrong.com/lw/qr/timeless_causality/

jules 12y ago
> So it does look like a better candidate for the arrow of time. I don't fully agree. You seem to agree that even in a fully classical world, you would experience physics as irreversible on a macro level. So we cannot dismiss subjective uncertainty (lack of knowledge that could in principle be known) a priori as an explanation for the apparent irreversibility of physics. Certainly quantum uncertainty is an additional component of uncertainty, but most likely it's not only quantum uncertainty that causes our experience of irreversibility. For instance if we create a gas of very heavy particles, we would still experience irreversibility, but the quantum effects would be negligible. It seems to me therefore that it would be a good idea to try to investigate to which extent our experience of irreversibility is due to quantum uncertainty and to which extent it is due to subjective uncertainty.
mbq 12y ago
Assuming that we can breaks nothing; particles will concentrate, but without the barrier they will spread again and this would be a disturbance that simply averages out.
shasta 12y ago
> The obvious answer in a classical universe is ... Since there is an obvious answer in classical physics, it's a bit disingenuous to claim that this solves a long-standing problem in classical physics, no? > With entanglement and de-coherence however, such reversal becomes impossible even in principle [...] So it does look like a better candidate for the arrow of time. That would be true if you could demonstrate by experiment that a broken egg springing back up onto the table and reforming is physically impossible instead of just unfathomably unlikely. Can you demonstrate that?
- gameshot911 12y ago
  I'm on it. I've placed a broken egg on my table. I'll let you know the results.
  - ars 12y ago
    It's been 6 days - how's your egg doing? :)

jerf 12y ago

Here's a crazy thought... the really stonking smart physics PhDs who have spent their whole lives working on this problem are perfectly aware of the classical 18th-century physics you are referencing, and they've found it an unsatisfactory explanation. Perhaps you might like to dig in a bit further before being so dismissive. (As a bit of a hint, your argument circularly assumes the existence of a forward arrow of time to demonstrate the forward arrow of time. Your "simulation" snuck a forward arrow of time into its definition, then proceeded to prove it exists. This is not satisfactory.)

rwallace 12y ago
Then again, if one is going to argue from authority, could one not equally well cite the equally stonking smart physics PhD's who dismissed the argument with "there is no physics in this paper"? And does the argument about quantum entanglement not also circularly assume the existence of a forward arrow of time?
- jerf 12y ago
  'dismissed the argument with "there is no physics in this paper"?' Since retracted by essentially the same authorities and it's now a bustling field, so that doesn't work very well as an argument. And I'm pretty sure the entire point here is that the arrow falls out of the entanglement process itself, not that we first assume temporal ordering in the physics. Remember that we do get to assume the existence of time in general in this argument; the article may not have spelled it out as clearly as it could have but it did in fact observe this still doesn't solve "time" in general. It's a big result, though.
  - guscost 12y ago
    Is the entanglement "process" theoretically reversible in the same way that classical physics is theoretically reversible? Is it possible to describe the idea without using words that implicitly include the arrow of time? It's possible that I'm simply not understanding the concepts well enough, but I don't see how the "process" of entanglement is any less dependent on temporal ordering. Why wouldn't running it backwards make physical sense as a "disentanglement" phenomenon? For that matter, why is it right to conclude that the coffee cup has only become entangled with its exterior after it has cooled down? Is this only because the matter in the cup is supposedly the result of disentangled quantum fluctuations from the distant past? I realize that the coffee cup example is an imperfect one, but can someone explain why the process of entanglement is special in this regard where the process of increasing entropy is not? I'm sure that many smart people have been thinking about these questions, but the interpretations of QM still seem to be stuck in the realm of philosophy.

snarfy 12y ago

It depends which point of view. If I made a movie of the simulation and then played it backwards, the gas would go into the box. There is nothing in your experiment that says this is wrong. In your experiment, you must wait, and therein lies the problem. You've implicitly put a forward arrow in there.

SoftwareMaven 12y ago

This implies that atoms behave like little billiard balls, but nearly a century of quantum physics has shown us very clearly that belief is false. Unless you are looking at it, atoms exist as their wave function (and recent experiments have shown that a collapse won't occur while you are watching so e.g. a radioactive isotope will not decay while being observed), so any description of behavior needs to use their wave function to describe it.

SilasX 12y ago

The problem is that doesn't distinguish the positive time direction, since flipping the direction and running it back past zero would produce the same evenly spread gas. The resolution that Gary Drescher gives in Good and Real is to stop thinking in terms of a positive and negative time direction, but instead define an away-from-order direction. This is the same direction in which memories (like "wakes" that follow a moving object in the ocean) form, so we will only have memories of things in a pastward, higher-order state. This holds true whether you record the memory in a brain, wake, hard drive, film, or notches on a log: they are all entropy increasing processes, and so all observations will align with the increase in entropy. It's very similar to the resolution linked in this comment https://news.ycombinator.com/item?id=7605595 https://news.ycombinator.com/item?id=7605595 However, I agree that you don't need quantum-specific effects for the explanation; the same thing happens in a classical world. The article and the one linked in the above comment, are wrong in this respect, as you say. Even so, decoherence can be regarded as a special case of entropy increasing.

duaneb 12y ago

The arrow of time develops from moving particles? You sir just begged the question.