4 ms·

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 thin

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 13y 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 13y 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 13y 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 13y ago
  I'm on it. I've placed a broken egg on my table. I'll let you know the results.
  - ars 13y ago
    It's been 6 days - how's your egg doing? :)
    - gameshot911 13y ago
      At one point a breeze did blow two pieces of shell closer to one another, but beyond that entropy has appeared to increase quite a bit. At this point my highest hopes are that the mold growing on it will evolve into an intelligent species capable of manually reconstructing the egg.
- SoftwareMaven 13y ago
  Isn't that exactly what the paper this article references is stating a proof of: that the reforming is physically impossible?
- loup-vaillant 13y ago
  You do not understand. Let me rephrase. You go on a space ship, on your way to the edges of the universe. Your buddy goes on a space ship, on his way to the other side of the universe. You will soon be outside each other's observable universe. Now, if you drop something in your ship (it spins, so you have gravity), you can "reverse time", and pull it back up. Can you do the same to you buddy's ship, should something ever fall there? The simple answer is no. You can't. He's on his own. Now there _is_ a way I haven't spoken of: non-causal interaction. You and your buddy could agree on some things before you depart. For instance, you could agree to pull back up whatever falls. With Everett branches, it's even easier: you pre-commit to reversing your own Everett branch, whatever it is, so all your selves do it. If successful, the worlds should merge back together, at least locally. Just one catch: all your other selves must successfully reverse time locally. It only takes one failure for the plan to fail. But if you want to reverse time after the fact, say because you happen to be in an Everett branch you don't like (you lost a bet about which way the photon will go), you won't be able to reverse time here, because your other self certainly will not (he won the bet, so…). Maybe, just maybe, you could use the vanishingly small entanglement left with the other Everett Branch to directly communicate with your other self. I'm not even sure it can be done in principle. For practical purposes, it should be forever beyond reach, even if you have a super-intelligent AI to help you.

jules 13y 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 13y 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 13y 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 13y ago
I'm on it. I've placed a broken egg on my table. I'll let you know the results.
- ars 13y ago
  It's been 6 days - how's your egg doing? :)
  - gameshot911 13y ago
    At one point a breeze did blow two pieces of shell closer to one another, but beyond that entropy has appeared to increase quite a bit. At this point my highest hopes are that the mold growing on it will evolve into an intelligent species capable of manually reconstructing the egg.
SoftwareMaven 13y ago
Isn't that exactly what the paper this article references is stating a proof of: that the reforming is physically impossible?
loup-vaillant 13y ago
You do not understand. Let me rephrase. You go on a space ship, on your way to the edges of the universe. Your buddy goes on a space ship, on his way to the other side of the universe. You will soon be outside each other's observable universe. Now, if you drop something in your ship (it spins, so you have gravity), you can "reverse time", and pull it back up. Can you do the same to you buddy's ship, should something ever fall there? The simple answer is no. You can't. He's on his own. Now there _is_ a way I haven't spoken of: non-causal interaction. You and your buddy could agree on some things before you depart. For instance, you could agree to pull back up whatever falls. With Everett branches, it's even easier: you pre-commit to reversing your own Everett branch, whatever it is, so all your selves do it. If successful, the worlds should merge back together, at least locally. Just one catch: all your other selves must successfully reverse time locally. It only takes one failure for the plan to fail. But if you want to reverse time after the fact, say because you happen to be in an Everett branch you don't like (you lost a bet about which way the photon will go), you won't be able to reverse time here, because your other self certainly will not (he won the bet, so…). Maybe, just maybe, you could use the vanishingly small entanglement left with the other Everett Branch to directly communicate with your other self. I'm not even sure it can be done in principle. For practical purposes, it should be forever beyond reach, even if you have a super-intelligent AI to help you.