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coherence - silver Harloe

About coherence

Previous Entry coherence Dec. 15th, 2004 @ 03:18 pm Next Entry
In recent years, I've spent far too much time grappling with the concept of quantum coherence - trying to understand it.

My primitive, layman's understanding of it has to do with all possibilities simultaneously existing until a measurement forces one to become "what really happened."

Unto recently, I thought "wouldn't that mean decoherence could only happen in a universe of one particle, because if anything else existed, would not any two particles constantly 'observe' each other (though gravity, if nothing else)?"

I would also chastise people who acted like sentience (in the science fictional use of the word) was some magic pill required to make coherence "go". Since I don't believe in "external" sentience, clearly I barely believe in sentience at all - it's more of a convenient fiction (like center of gravity) which helps us understand some things better (read anything/everything by Daniel Dennett for more on that). I believe the particles in my head work just like the particles in this table here, and if I can cohere quantum states by observing them, so clearly must the table be able to because there's nothing magic about the state of my head.

Recently, two thoughts/ideas have come up which challenge or expand some of my notions. I'm not sure which.

First, on the philosophical/metaphysical side, I made the argument of 2 paragraphs above to someone and they scoffed "if it wasn't a sentient observer, how would anyone know the observation took place?" This question is very similar to the old question about the tree falling in the forest with no sentient listener about. I'm not sure it means, though, that only sentient observers can force coherence. In part, because I still think the tree vibrates the air in a way which would be observable as sound were anyone to happen along (suppose you showed up after the tree fell - supposedly soundlessly - but still within time to hear the echoes. do you? of course you do. it did make a sound originally, which is still affecting the rest of world as such).

Second, on the technical side, I read the words "thermodynamically irreversible measurement" in relation to quantum coherence. I want to understand those words completely. They seem somehow key to understanding what might be happening in my head (but not - necessarily - the table) which coheres possibilities into a single past behind me.

Finally, I want to better understand quantum computing. My cheap, layman's understanding of quantum computing has to do with setting up a system of decohered "thingies" which cover all possible states of, say, a huge encryption key. You decide to observe the reality where they found the correct encryption key and thus they cohere into that state and you've got your answer. Weird, eh?
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From:cratermoon
Date:December 15th, 2004 02:05 pm (UTC)

Feynman

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suppose you showed up after the tree fell - supposedly soundlessly - but still within time to hear the echoes. do you? of course you do. it did make a sound originally, which is still affecting the rest of world as such

Richard Feyman uses exactly that example in one of his lectures on quantum mechanics (I think it's in Vol 3), but takes it another step. Suppose there was a thorn that was touching a leaf near the tree when it fell. Long after any sound had died down, if you looked closely, you could perhaps see a scratch on the leaf which you could not explain in any other way than the sound of the tree falling causing the thorn or the leaf to vibrate.

I'm sure you know quantum coherence is part of the Copenhagen Interpretation, and it's not the only way to semantically view quantum theory, just the most accepted. In accordance with this, Feynman insists very strongly that the coherence of quantum states does not represent a real thing and should not be treated as real.

Some of the most recent developments in string theory and experiments involving quantum entanglement suggest other ways to interpret coherence.
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From:silverharloe
Date:December 15th, 2004 02:25 pm (UTC)

Re: Feynman

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Defining the many worlds as real or a mathematical shorthand does not seem to affect my questions/thoughts and I imagine the "thermodynamically irreversible measurement" is still something I need to understand.
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From:cratermoon
Date:December 16th, 2004 07:44 am (UTC)

Re: Feynman

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I'm not sure what the question is. Anyway, on to thermodynamically irreversible measurement. In the macro world, there is clearly a direction to time. Film someone falling, then run the film backwards, and it's funny, but everyone can tell it's backwards. It's the second law of thermodynamics at work. On the subatomic level, where things vibrate and interact, if you could to film it and run the film backwards, no one could point out an event and say that's backwards, that can't happen. So a thermodynamically irreversible measurement would be one where you can tell which way time is flowing, and the measurement results in some change in entropy that would violate the 2nd law if it were reversed.

Maybe I'm being too simple. The point is that a measurement is something that results in a one-way transfer of energy. Like the tree falling, at a macro level, energy is transferred, the system is disturbed in a way that can't be restored without putting more energy back into the system.
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From:hillgiant
Date:December 16th, 2004 08:32 am (UTC)

excessive adjectives

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Another way to look at it: Thermodymically reversible measurements (or processes for that matter) happen only under very special circumstances. Engineering discussions about reversible processes usually involve words like "inelastic spring", "frictionless", "perfect black body", and "perfect"-anything. They are theorical shortcuts to make the analysis simpler and are extremely difficult to even approximate in practice. On the quantum level, collisions are frictionless and inelastic, but that scale is hardly what I would consider thermodynamic. In practice all measurements are thermodynamically irreversible.
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From:hillgiant
Date:December 16th, 2004 08:09 am (UTC)
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One of the kids in my high school physics class asked the obvious though naive question, "What do photons look like". The teacher did not miss a beat. He asked, "What would I look at them with?" In order to observe a quantum state, you have to interact with it. Since the scale is so small the act of observation has a significant impact on the state observed.

I agree that some people get too hung up on the idea that these "observations" must have an "observer". This is the worst sort of strong anthomorphic priciple. Worse, it is a distortion of the idea of a thought experiment. Just because our imaginary sandbox exists inside our imagination does not mean that the universe requires some intereaction with an intellect to function. My word. How ever did the universe get along before we came along to listen to trees and look at boxed cats?
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