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Author Topic: What triggers decoherence?  (Read 8669 times)

Offline JP

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Re: What triggers decoherence?
« Reply #25 on: 21/05/2014 12:50:21 »
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What I am assuming is that quantum mechanics underlies the behavior of quons.

I imagine it would be difficult to do anything in QM without this assumption.
There are legitimate physicists questioning this assumption.  In particular, they question whether there is some transition when you collect enough mass which causes quantum mechanics to break down.  Roger Penrose is one of the major proponents of this idea.  I'd say it is definitely one of the more out-there ideas in modern physics, but it is still an interesting idea.


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although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

Would this reflect the difference between waveform collapse and decoherence? 
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

There's another interesting theorem in quantum mechanics related to this: the no cloning theorem.  What this says is that if you want to make a perfect copy of quon 1, you can't do it by measuring it and creating a second quon.  The reason is that measurement will collapse its state.  So if it is X% in state A and Y% in state B, and you take a measurement and see state A, you've lost information about the percentages, so you can't make a perfect copy.   Decoherence may put the quon into state A, but the percentages are somehow stored in the environment.

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Decoherence is based on a quantum version of statistical mechanics........
 

I know I've said this before, but if you have not already written a pop sci book, you should; and if you have, I want it!

The reason I included the “Schrödingcat” in the OP is that most of the pop sci explanations I have seen have been attempts to describe why the cat would not be in superposition.  How far off track would it be to ask if the cat might not be in superposition, but the individual quons of which the cat is composed might be?


Thank you.  I'll send you a free copy if I ever do write such a book.  ;)

If the quons making up the cat are in superpositions, the cat itself will be, assuming that the laws of quantum mechanics don't break down at cat-scale.   

Where decoherence is often misrepresented as explaining collapse is that if the cat interacts with its environment, it may be the case that the cat assumes the "alive" state.  But the quantum information about the superposition of alive and dead is still in the environment. 

In collapse, the cat would suddenly assume the "alive" state and the information about the superposition would be gone. 

So decoherence can push the question about collapse off to the environment rather than the quon itself.  But at some point, you'll have to come back and address the fundamental question of whether the environment collapses or not.

My own guess is that collapse is probably the wrong way to think about things and that even we as observers are part of the environment.  If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.  As far as I know, we can't answer this until we can relate our conscious experience to the laws of physics, which is way beyond current science.  This line of thought is studied by some physicists, but it is also fairly out-there as far as theories go.  If nothing else, it appears to be far out from current testability.
 

Offline yor_on

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Re: What triggers decoherence?
« Reply #26 on: 22/05/2014 19:54:18 »
There is also an idea of thoughts having its origin as a quantum phenomena, if I remember right? As if we in some weird manner became quantum computers, but as our 'questions' fail to be perfectly formulated? Also a philosophical aspect isn't it? Why the world is arranged so that you only get that perfect answer if your question is perfectly formulated. You might think that it is the same macroscopically, but practically speaking we get along just fine without those 'perfect questions' it seems? Alternatively that it is a form of decoherence that produce consciousness, or maybe both rather? A interesting thread this one.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #27 on: 23/05/2014 15:18:41 »
Quote from: JP
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

That makes sense to me, but still leaves one point to be cleared up. 

In current scientific wisdom, are wavefunction collapse and decoherence considered as two separate things, either of which might happen; or is wavefunction collapse and outdated idea that has largely been replaced by decoherence?

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There's another interesting theorem in quantum mechanics related to this: the no cloning theorem.  What this says is that if you want to make a perfect copy of quon 1, you can't do it by measuring it and creating a second quon.  The reason is that measurement will collapse its state.  So if it is X% in state A and Y% in state B, and you take a measurement and see state A, you've lost information about the percentages, so you can't make a perfect copy.   Decoherence may put the quon into state A, but the percentages are somehow stored in the environment.

That’s fantastic; an explanation of the no cloning theorem that even I can follow! 

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If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.

So, as I sit “observing” my computer screen, I am in quantum superposition with my computer, but cannot be aware of that??

I look forward to my “free copy”, but at my age I have to suggest you don’t wait too long before making a start.  :)
 

Offline JP

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Re: What triggers decoherence?
« Reply #28 on: 23/05/2014 16:42:06 »
Quote from: JP
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

That makes sense to me, but still leaves one point to be cleared up. 

In current scientific wisdom, are wavefunction collapse and decoherence considered as two separate things, either of which might happen; or is wavefunction collapse and outdated idea that has largely been replaced by decoherence?

It's not my particular field of expertise, but wavefunction collapse is a feature of one particular interpretation of quantum mechanics.  It is probably not something to get hung up on, since the same mathematics has alternative explanations.  This is one place where I prefer the many-worlds interpretation.  Nothing collapses, it's just that the world we are experiencing gets only one version of the quantum state, [it]i.e.[/it] when we open the box in our universe, the cat is dead.  There's a separate universe that branched off from ours when we opened the box in which it is alive and the two universes can't talk to each other.  It's easier to see in this interpretation how the two-state-ness of the cat prior to opening the box has been lost: there was a potential for us to go into either universe initially, but once we "choose" a path, we're stuck there.

Decoherence is a separate phenomenon that says that although the cat may be dead, it could pass this two-state-ness into something else so this hasn't been lost. 

On a global scale, if you could write down the quantum state of the entire universe, collapse would cause it to change irreversibly whereas decoherence would not.

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If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.

So, as I sit “observing” my computer screen, I am in quantum superposition with my computer, but cannot be aware of that??
This is something neither I, nor anyone really has the answer to because we have almost no idea how our conscious experience can be explained scientifically.

If all objects are quantum at their core, then you'd expect that when we observe anything it is fundamentally quon interactions and we should be able to describe ourselves as in some sort of quon superposition.  Why, after all, should we be special and not subject to the same laws as all other objects in the universe?  The problem is that we don't experience this in any obvious way.  Could decoherence explain this?  Maybe, but it's hard to say anything without having any idea how conscious experience works from a scientific standpoint.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #29 on: 23/05/2014 22:32:45 »
Quote from: yor_on
A interesting thread this one.

There's certainly a lot of information in it, and a lot to think about.  I have this feeling that the OP of a thread like this should, at some point before it dies quietly, write a brief account of what he/she has gained from it.  I intend having a shot at that, but given that I have some dozen pages of closely typed notes to work through, and an extra busy period coming up, it may not be very soon.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #30 on: 26/05/2014 13:55:53 »
Before even attempting a summary I would like to present a refined version of the Schrödinger thought experiment.

Everything is as in the original, except that the box has room for food, water and a litter tray.  The mechanism that releases the poison also stops a clock.  The cat is fitted with a heart monitor, also linked to a clock.  The experiment is set up for 48 hours. 

At the end of the time the box is opened, and the cat is found to be dead.  The clock shows that the poison was released at hour 3.

Does the heart monitor show that the cat died at hour 3, or at hour 48?

If the cat was in a live/dead superposition, would the live version have eaten or visited the tray?

I realise that these questions are based on classical/intuitive thinking, but they are the sort of questions people ask.


 

Offline JP

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Re: What triggers decoherence?
« Reply #31 on: 26/05/2014 16:30:17 »
Ok, the proper way to treat that problem is to write out the possible states of the system and then ask which is consistent with a measurement of "dead cat and clock stopped at hour 3."  When you measure and "collapse" a wavefunction, you leave the system in a superposition of all its states that are consistent with that measurement. 

So back to this specific case, if the heart rate monitor is interacting with the cat all along, it will show that it stopped at hour 3.  The cat would not have visited the tray. 

What a rigorous quantum treatment will tell you is that the instant before you opened the box, the cat was in state A and B simultaneously with

A) Dead cat, clock stopped at hour 3, heart rate monitor showing dead at hour 3, no food eaten after hour 3.
B) Alive cat, clock not stopped, heart rate monitor showing living cat, food eaten all the way up to box being opened.

In the Copenhagen interpretation of quantum mechanics, opening the box forces the system to choose one of these states.  The big question in this interpretation is what makes the "observer" more special than other, quantum interactions, which don't force collapse.

It's a little less (to me at least) confusing in the many-worlds interpretation which says that in one universe, you have state A and in one you have state B.  Opening the box entangles you (the observer) with the cat in such a way that in one universe you are observing an alive cat and in another you are observing a dead cat (moreover, these two universes can't talk to each other).  The big question here is why your consciousness inhabits only one of these universes. 

The important point to note is that for now, there is no accepted way to distinguish between these two interpretations of the mathematics of quantum mechanics.  This may get resolved when we know more about how consciousness works or it may not.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #32 on: 27/05/2014 14:36:19 »
Thanks again, JP.

We seem to be moving towards the multiverse, which probably needs a new thread.  I try to keep an open mind on the multiverse, but have lots of questions/misgivings.  I think I'll try to pull together the decoherence information before getting into anything else.
 

Offline JP

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Re: What triggers decoherence?
« Reply #33 on: 27/05/2014 14:58:29 »
The multiverse may or may not need a new thread, depending how seriously you take it.  If you think of all the interpretations as just ways of trying to make sense of the mathematics of quantum mechanics, then you can pick or choose which makes the math the clearest for you.  If you take one particular interpretation seriously as physical reality, then you've gone beyond science and into the realm of philosophy in my opinion, since all are equally well supported by both theory and experiment.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #34 on: 27/05/2014 18:04:08 »
Quote from: JP
If you think of all the interpretations as just ways of trying to make sense of the mathematics of quantum mechanics, then you can pick or choose which makes the math the clearest for you.

Strange, perhaps, that I had reached that conclusion with string theory, but not the multiverse. 
This must raise the question as to whether any scientists (apart, possibly, from David Deutsch) actually believes in the physical reality of any sort of multiverse.   

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  ....since all are equally well supported by both theory and experiment.

I assume you mean that there are experiments the results of which are consistent with the multiverse theories; rather than that there are experiments that could establish the existence of other universes.

OK, I accept that that is a silly question, in view of your comment about the "realm of philosophy", but it highlights the possible naivety of us hitch-hikers. 
 

Offline JP

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Re: What triggers decoherence?
« Reply #35 on: 27/05/2014 19:28:10 »
We could dive into philosophy of science now, but it suffices to say that physics is about modeling nature, generally through mathematics.  Usually there is an obviously superior interpretation of this mathematics, typically because that interpretation is the simplest or most intuitive.  In the case of quantum mechanics, the what the mathematics says is so bizarre and counterintuitive that there are several interpretations for it, each of which has a good case for being the "best" interpretation.  Until we have a real reason to pick one over another, it's up in the air. 

Of course, you'll find ardent supporters for each one, but the consensus is that all are worthwhile interpretations.

As for who takes many-worlds seriously: I tend to lean that way, but I'm open to all the interpretations.  I simply find the least bizarre interpretaion to be that there are many universes, each evolving independently and deterministically than that the wavefunction collapses or that there are pilot waves.  The problem gets pushed onto how we get stuck in one particular universe, but that's less troubling to me at least than the idea of wavefunction collapse.

Having said that, I tend to pick and choose interpretations based on what I find least confusing for the problem at hand.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #36 on: 27/05/2014 21:09:27 »
This is a first shot at a summary of my understanding of the answer to the twofold original question.

What triggers decoherence?

Why, after almost 14 billion years, have not all the quons in the Universe decohered?

It seems that decoherence is triggered when a quon interacts with its environment.  This may mean interacting with just one other quon, or with complex system.

The question as to why there are plenty of quons in the Universe that are still in superposition when quons have been interacting for billions of years is perhaps rooted in a confusion between wave function collapse and decoherence. 

Wave function collapse involves a process which is irreversible under the second law of thermodynamics, and entails the permanent loss of “quantumness”.  Once collapsed, the object involved can be totally described in terms of classical physics.   

Decoherence, on the other hand, is a less cut-and-dried concept in that, although the process that triggers it may be thermodynamically irreversible, decoherence itself is not so easy to pin down.  It appears to be much more observer specific.  If, for example, I observe a quon, I will not see it in a superposition.  As far as my observation is concerned, the wave function has collapsed, but in a broader context, the quon retains its “quantumness” and may be in states of superposition that I am unable to observe.  If I stop observing the quon; then observe it again, decoherence will happen again, and I will observe something of which the wave function has just apparently collapsed.

Decoherence brings about an apparent wave function collapse only in the frame of reference of the observer.   
 

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Re: What triggers decoherence?
« Reply #36 on: 27/05/2014 21:09:27 »

 

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