(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.)
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.
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.
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.
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.)
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Read this to have an idea of how time could work in a timeless universe: http://lesswrong.com/lw/qr/timeless_causality/
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?
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.
My understanding is that this would violate a physical law that we still believe to hold.
You're right though, that I don't understand. At least I thought I understood your last post. With this one, I don't follow the connection to the OP.
I was replying mainly to mbq: https://news.ycombinator.com/item?id=7602812 My understanding is, with QM, is is even harder or even impossible to reverse time, even with perfect knowledge of your reachable surroundings.
In a classical universe however, it looks much easier, so in such a universe, the explanation for time is less satisfactory.
:)
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.
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
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.
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.