Quantum Entanglement Drives the Arrow of Time • Hacker News Board

Re: Quantum Entanglement Drives the Arrow of Time

#121

Post by TTPrograms » Thu, Apr 17, 2014, 7:47 PM UTC

arh68 wrote: post #118

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…

Statistical mechanics doesn't say why a coffee cup never heats up spontaneously - rather, it says that the probability of that happening is incredibly small. You end up with probabilities like 10^-100000. This doesn't require quantum at all, and is effectively the same.

Re: Quantum Entanglement Drives the Arrow of Time

#122

Post by guscost » Thu, Apr 17, 2014, 8:19 PM UTC

jerf wrote: post #100

rwallace wrote: post #99

Earlier quoted context omitted.

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?

'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…

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.

Re: Quantum Entanglement Drives the Arrow of Time

#123

Post by johnbm » Thu, Apr 17, 2014, 8:24 PM UTC

cromwellian wrote: post #3

This Google Talk https://www.youtube.com/watch?v=dEaecUuEqfc uses entanglement and quantum information theory in a clear and understandable way to explain 'spooky' quantum phenomena, like the quantum eraser, de-coherence, the aspect experiment, and the measurement problem. Even if you don't know any QM, just basic algebra and calculus, it's really approachable. I used to be a fan of the Many Worlds interpretation, bu…

I like it too, because it's the first explanation of QM I've heard that isn't just about minuscule systems floating in a void.

I did the double slit experiment in uni, and it involved a real screen and a real divider with real slits, made out of 10^n atoms. Feynman's line integrals say that particles explore all possible paths. This would seem to include possibly interacting with all those atoms that are nearby. Yet we pretended none of that mattered and could just talk about the photons.

QIT seems entirely unsurprising to me. Especially the idea about measurement and two opposite interference patterns.

This also isn't the first time I've heard software guys take to it. I think it's because programmers deal with much larger scale than most occupations. We do not delegate to junior aides, we construct our own robot aides, and then deploy them on a massive scale.

It always surprises me when mathematicians and physicists are crappy programmers who fail at making basic complex systems work. If you can't handle complexity, how can you handle the universe?

Re: Quantum Entanglement Drives the Arrow of Time

#124

Post by andy112 » Thu, Apr 17, 2014, 9:06 PM UTC

clavalle wrote: post #101

andy112 wrote: post #96

Earlier quoted context omitted.

Can you give a concrete example of what you mean?

Sure (as far as my layman's understanding of quantum physics goes). So, you have independent physical system (say, a photon). Before it interacts it exists in quantum superposition. It has the potential to exist in any possible configuration and, in fact, seems to exist in all of them simultaneously. Almost like a variable of a certain type but this type is kind of special in that some possible values are more likely…

Very cool way of thinking about it! Thanks for that elaboration.

Re: SoftwareMaven's "why is the 'player character' so important?", I don't think the player character we follow is necessarily any more important than the other player characters. You can say every individual character (or particle in the real-world side of the analogy) has an importance only within its own reference frame and I think it still works.

Just like you can have many players playing the MMO game and "collapsing the state" of different things from their own reference frames at different times, you can have the same be true for the particles in the real-world analogy. No one player of the game is more "important" than any others. The only requirement is that it all stays self-consistent in the backend and across everyone's individual points of view at all times.

But (for better or worse) I don't think what we're talking about now is science, really, unless there's some way to test it.

Maybe you could try to detect a "lag" by doing something that causes an especially large number of states to collapse across an especially large number of frames of references all at once, but I'm not sure if you could do anything that would detect this lag because all of our ways of detecting would be lagging too. (This goes along the same lines as trying to tell if the computer you're using is running within a VM or running natively.)

Any ideas?

Re: Quantum Entanglement Drives the Arrow of Time

#125

Post by SilasX » Thu, Apr 17, 2014, 9:20 PM UTC

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…

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

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.

Re: Quantum Entanglement Drives the Arrow of Time

#126

Post by wmnwmn » Thu, Apr 17, 2014, 9:34 PM UTC

It's never been clear to me how deeply the thermodynamic quantities are really connected to time. For example there could be a state in which entropy (or entanglement) increased from left to right in space, yet it wouldn't mean that time flows from left to right.

Re: Quantum Entanglement Drives the Arrow of Time

#127

Post by duaneb » Thu, Apr 17, 2014, 10:29 PM UTC

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…

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

Re: Quantum Entanglement Drives the Arrow of Time

#128

Post by api » Thu, Apr 17, 2014, 11:14 PM UTC

cromwellian wrote: post #3

This Google Talk https://www.youtube.com/watch?v=dEaecUuEqfc uses entanglement and quantum information theory in a clear and understandable way to explain 'spooky' quantum phenomena, like the quantum eraser, de-coherence, the aspect experiment, and the measurement problem. Even if you don't know any QM, just basic algebra and calculus, it's really approachable. I used to be a fan of the Many Worlds interpretation, bu…

You would probably enjoy Dr. Chris Adami's Reddit AMA:

http://www.reddit.com/r/science/comments/236ap1/science_ama_...

There seems to be a growing understanding that our universe is actually made of three things, not two: matter, energy, and information. Informatics is joining with thermodynamics and matter physics / chemistry as a fundamental science of "stuff."

Re: Quantum Entanglement Drives the Arrow of Time

#129

Post by im3w1l » Fri, Apr 18, 2014, 12:24 AM UTC

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 of the arrow of time like this. We started in an extremely exceptional state of very low entropy. The state space is very high dimensional. Therefore the entropy is much higher, pretty much everywhere we go. In this space we do a random walk.

Now, the mystery that remains to be solved is: Why did we start with such an exceptional (low entropy) state?

Re: Quantum Entanglement Drives the Arrow of Time

#130

Post by cousin_it » Fri, Apr 18, 2014, 12:41 AM UTC

im3w1l wrote: post #129

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 of the arrow of time like this. We started in an extremely exceptional state of very low entropy. The state space is very high dimensional. Therefore the entropy is much higher, pretty much everywhere we go. In this space we do a random walk. Now, the mystery that remains to be solved is: Why did we start with such an exceptional (low entropy) state?

One possible answer is based on complexity: we were more likely to start in a low entropy state because it had a description with low Kolmogorov complexity, or something like that. (K-complexity induces a probability distribution on the state space that is very different from the uniform distribution.) If that reasoning works, it seems to reduce the question "why did we start in a low entropy state?" to the question "why do we often observe hypotheses with low complexity to be true?", which is pretty much the problem of induction. I'd love to know the answer to that.

Another possible answer is anthropic: you see an ordered universe because otherwise it wouldn't create an ordered mind like you. Unfortunately, the anthropic answer seems to predict that we're in a tiny ordered bubble that will be swallowed by chaos any moment now, so the complexity-based answer looks more promising to me.