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Quantum Entanglement Drives the Arrow of Time

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Re: Quantum Entanglement Drives the Arrow of Time

#111

Post by loup-vaillant »

shasta wrote: post #83
loup-vaillant wrote: post #79

Earlier quoted context omitted.

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 e…

> 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…

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.

Re: Quantum Entanglement Drives the Arrow of Time

#113

Post by SoftwareMaven »

clavalle wrote: post #110
SoftwareMaven wrote: post #106

Earlier quoted context omitted.

That seems like a pretty reasonable analogy. It raises the question, though: why is the "player character" so important in the real world?

I was thinking, initially, that the 'player characters' where just previously entangled systems. Anytime anything that has previously decohered interacts with a system still in quantum superposition a new 'player character' would be created. But that seems like an exponential process so it immediately makes me ask: Why isn't everything entangled at this point? Is there that much in the universe still in superposition…

If you want to twist your mind over this subject, read Quantum Enigma by Bruce Rosenblum and Fred Kuttner[1]. It's written by a couple of quantum physics researchers who spend some time thinking about WTF quantum reality means. It's one of the few metaphysical books on quantum physics I've found that sticks to the real science (and, as a result, has no conclusions).

The new 'player character' on decoherence is, essentially, the many worlds interpretation. That is a valid interpretation, and while it matches the data (and may not be falsifiable), I have a hard time buying it (where does all that energy come from??).

1. http://www.amazon.com/Quantum-Enigma-Physics-Encounters-Cons...

Re: Quantum Entanglement Drives the Arrow of Time

#114

Post by fspeech »

jerf wrote: post #88
mbq wrote: post #64

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 bo…

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 ti…

Classical physics does not presume time arrow either. The equations are all symmetric if you reverse time. Classical physics adequately explains time arrow through entropy. What is missing is decoherence or resolution of the classic/quantum dichotomy.

Re: Quantum Entanglement Drives the Arrow of Time

#115

Post by fspeech »

The title of the article is unfortunate. Classical physics adequately addresses the arrow of time through thermodynamics (the second law in particular). What is missing is so called "decoherence". In other words, quantum physics is supposed to explain everything but when we interpret results we divide the world into classic (the observer) and quantum (the observed) parts. The answer, from reading the article, seems to be that even if the world is in a pure state (quantum) a large part of it could behave like a mixture state (classic observer/environment) through entanglement. This makes it easier to have a coherent mental picture of quantum physics.

Re: Quantum Entanglement Drives the Arrow of Time

#116

Post by shasta »

loup-vaillant wrote: post #111
shasta wrote: post #83

Earlier quoted context omitted.

> 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…

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…

> 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.

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.

Re: Quantum Entanglement Drives the Arrow of Time

#117

Post by pygy_ »

It reminds me of another paper that was discussed here earlier (I can't find the submission, though):

A quantum solution to the arrow-of-time dilemma—Lorenzo Maccone

    The arrow of time dilemma: the laws of physics are
    invariant for time inversion, whereas the familiar
    phenomena we see everyday are not (i.e. entropy
    increases). I show that, within a quantum mechanical
    framework, all phenomena which leave a trail of
    information behind (and hence can be studied by
    physics) are those where entropy necessarily increases
    or remains constant. All phenomena where the entropy
    decreases must not leave any information of their
    having happened. This situation is completely
    indistinguishable from their not having happened at
    all. In the light of this observation, the second law
    of thermodynamics is reduced to a mere tautology:
    physics cannot study those processes where entropy has
    decreased, even if they were commonplace.
Phys. Rev. Lett. 103, 080401 – Published 17 August 2009

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103... – http://arxiv.org/abs/0802.0438

Re: Quantum Entanglement Drives the Arrow of Time

#118

Post by arh68 »

tim333 wrote: post #8

The reasoning sounds a bit iffy as in: “Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.” I think you can understand coffee cooling quite well without any quantum stuff - the atoms in the coffee are moving faster than those in the room. There will be a tendency…

I think the author pretty clearly explained the connection:

    the arrow of time does not seem to follow from the underlying
    laws of physics, which work the same going forward in time as in
    reverse. By those laws, it seemed that if someone knew the paths
    of all the particles in the universe and flipped them around,
    energy would accumulate rather than disperse: Tepid coffee would
    spontaneously heat up
What he's saying is a bit unclear: we know the how, which is even a kind of plausible explanation as to why. What the author says we couldn't explain before is the converse: why does the opposite never happen? So I interpret that quote more like "Finally, we can understand why a cup of coffee only equilibriates..."

Re: Quantum Entanglement Drives the Arrow of Time

#119

Post by fspeech »

sheerun wrote: post #69

I love following article: http://www.flownet.com/ron/QM.pdf It basically shows that observation (measurement) and entanglement are the same things. Think about it: particles are not magically going out of superposition as we observe (measure) them. We (our atoms) become entangled with those particles, we become superposition. It's just propagation of entangled state. Why we don't perceive ourselves as in superpositio…

"Entanglement propagates with speed of light"

Hmm, never heard of this one before. "Entanglement" is a mathematical concept, why does it have anything to do with speed of light?

Re: Quantum Entanglement Drives the Arrow of Time

#120

Post by eli_gottlieb »

denom wrote: post #89

In the article the author describes the notion of a "pure state" which is something that has independently evolving probability. Individual 'units' lose their pure state and become part of an entangled ensemble--move to equilibrium. How is the evolution of biological organisms and technological systems explained in this sense? Played backwards, evolution would fit this and traditional notions of thermodynamic entropy…

>How is the evolution of biological organisms and technological systems explained in this sense? Played backwards, evolution would fit this and traditional notions of thermodynamic entropy. Is evolution a kind of de-entangling?

Evolution and technological change are completely different from the physical arrow of time. Optimization processes (evolution, technology) cannot, as far as we know, actually disentangle themselves on-net. What they can do is move the waste-heat/entropy/entanglement into concepts they don't care about, or entangle themselves with some radiating source of "fuel".

(Shout-outs to everyone who thinks the concept of an "optimization process" is total hokum, as I'd like to hear alternate explanations for the apparently similar behavior of so many things that seem to share no purely physical properties at all, and yet all seem to function to shift entropy from some things into other things according to a computable ordering.)

Or in other words, yes, all Earthly life actually lives by converting sunlight into a combination of life and waste, with the "waste entropy" often being radiated off as waste-photons into space, which we don't care about.

An interesting question, I conjecture, is how this conception of entropy/entanglement ties into energy, which apparently remains necessary for the whole process to occur, and yet is conserved in all physical processes.

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