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#51997 05/13/14 04:31 PM
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Decoherence

The Schrodinger’s Cat thought experiment is, unless you are a supporter of Eugene Wigner, generally solved by citing Decoherence. The reasoning goes something like this: Instead of arguing that the intervention of a conscious observer causes the wave function of the quon to collapse; the interaction of the quon with its environment brings about Decoherence. Typically, this involves interaction with other quons and brings about a situation similar to the older idea of a collapsing wave function. No conscious observer is necessary for this, so the cat is not put into a state of superposition. However, cats the world over cannot heave a collective sigh of relief, as Brian Clegg suggests they might, because once in this box they will be either alive or dead at the end of the process.

Decoherence, it appears, can solve the cat problem, but does it raise another question of its own? It would seem to. Quons have been around for billions of years, presumably interacting with other quons in their environment. Why, then, has decoherence not caused the wave function collapse of every quon in the Universe, long ago?

NB. Quon = quantum particle. http://en.wikipedia.org/wiki/Quon


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Bill, just what is it you are trying to figure out? I am having a little problem trying to understand your question. Let me ramble a little bit. I will look at a photon. Specifically I will look at the polarization. That is a well studied area. A given photon has an indeterminate polarization until something occurs that causes it to select one polarization. Which polarization will be selected is purely random. We can state the odds as to which polarization will be selected. Then if we look at a lot of photons we can predict how many will have each polarization. However, for an individual photon it is totally indeterminate. In this case I am not talking about entangled photons, I am talking about individual photons. Now once the photon has interacted with whatever, does the same photon still exist? In the case of a photon it has probably caused a change in energy in whatever it interacted with and has disappeared. Then if the particle with which it interacted releases the energy it received from the photon it will release a new photon which will once more have an indeterminate polarization. So the indeterminacy is carried on for all time, at least for photons. For other particles, such as electrons, the story may be slightly more complex, but at each interaction the result will still have an indeterminacy in it.

And of course we can point out that Indeterminacy is applied to all particles at all times. We can never state with full precision just what is the state of any particle. Even if we can determine the polarization of a photon, there are other parts of the photon that cannot be measured at the same time.

Now if you are as confused by this answer as I am, then my job is done.
Bill


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Quote:
Now if you are as confused by this answer as I am, then my job is done


Thanks Bill, you did a great job!

Seriously, though; I'm going to have to give a bit more thought to my question. I know what I mean, all I have to do is make it make sense to others. smile


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I suspect I may not have expressed my original question very well, so I’ll give it another shot.

1. An unobserved quon is in an indeterminate state of superposition. When it is observed, measured or interacts with anything in its environment it assumes definite characteristics. This change is thermodynamically irreversible.

2. If assertion 1 is correct, once a quon has decohered, there is no going back. Photons may appear to be a special case, but I believe that is not so.

3. Decoherence can occur when one quon interacts with another. No intelligent observation or measurement is needed.

4. Given that 1-3 are correct; why have not all the quons in the Universe already succumbed to decoherence? Have they?
Are all quons that are in a state of quantum superposition artificially generated in experiments etc?


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Well, I think we need Orac on this one, but I will try one more thing.

Consider a helium atom. It consists of a nucleus with 2 electrons in 'orbit' around it. I emphasize orbit because of course it isn't strictly speaking an orbit, but we can use that for this discussion. An electron has a spin, which can be either up or down. However, there cannot be 2 electrons in the same state, so one of the 2 electrons in a helium atom must be up and the other down. But we have no way of knowing which is up and which is down. In fact electron spin is indeterminate, it could be either up or down, until we measure it, or until it in some way interacts with another particle and decoheres. I will talk in terms of measurement. So we measure the spin of one of the 2 electrons, and now know whether it is up or down, and we know that the other one is the opposite. But we look away for a second, then go back and measure the spin of the same electron again. But when we quit looking at it, it becomes indeterminate again. So we we look back at it and we don't know which way it is spinning. When we measure it again, we may find it in either of the 2 spin states, at random. So we can't have full decoherence, because indeterminacy is always there. Observing an indeterminate state doesn't lock it in, when we look away it becomes indeterminate again.

And I think that is how it works and is the answer to your question.

Bill Gill


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Thanks Bill, I thought the answer was something like that until I saw:

“Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way". (Wiki). This seems to suggest that once decoherence has happened, it would require an input of energy to reverse it.


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Sorry I have been busy but Bill gave you the correct answer.

Decoherence does not generate actual wave function collapse that is a very strict old school interpretation which we know is wrong and refers back to QM pre 1980. The answer echos back to the "infinite dimension" problem your observation can not cause a total collapse because you can not observe every dimension or quantum statistic specifically you are observing a limited number on quantum statistics. So the whole original post is out of date and rather silly.

To show the problem with the pre 1980 answer look at the work with macro object entanglement. These are large objects you can hold in your hand so in the original concept they must be fully decohered and yet they can still to be shown as quantum active and this is a large version of Bill's atom/electron example of observation

So specifically that leaves you with only two options

1.) QM coherence was never lost by observation just suppressed
2.) Things can randomly gain coherence when not fully observed

Long story short ... as we can show QM information is energy the two statements above have very real world implications. The first one describes faithfully the laws of energy conservation the second describes a very different universe where energy can randomly jump into objects even macro objects.

Therefore by logic we accept answer 1 and view observation as a simple leak of information to the environment which includes you.

A layman way to perhaps create an analogy is with strings on a guitar by placing you finger at different points you observe a different note but the fundamental note of the string without a finger in place remains.

Last edited by Orac; 05/15/14 05:09 AM.

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Thanks Orac.
Silly questions are a speciality of mine. smile

I’m going to have to give this some thought. I suspect it may have answered the original question, but I have to be sure I understand before I can let it go.


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I didn't really do much Bill answered the question perfectly I just extended it, thank Bill.

There is a thing I thought you might pick up I was sort of expecting you would, so I will pose the question to you.

I want you to read the wiki entry on Quantum Decoherence again and I want you to consider carefully this part

Originally Posted By: http://en.wikipedia.org/wiki/Quantum_decoherence
Specifically, decoherence does not attempt to explain the measurement problem. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".


So my question .... what defines reality and how would we test it?

Last edited by Orac; 05/15/14 05:50 PM.

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I believe it was Philip Dick who said that “Reality is that which, when you stop believing in it, doesn’t go away.”
Heisenberg, on the other hand, said: “…what we observe is not nature in itself but nature exposed to our method of questioning”.

Both of these suggest, I think rightly, that the concept of reality is metaphysical, rather than scientific. Ultimately all we can test is whether or not our ideas about reality accord with the answers we obtain when we carry out experiments.

I had not overlooked the bit you quote, but I was taking one thing at a time and trying to sort out if decoherence was linked to thermodynamics in such a way that would let us consider decoherence as a concrete feature in what we perceive as reality; or if it had to be thought of as some sort of quantum superposition of done and not done.


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Originally Posted By: Bill S.
Both of these suggest, I think rightly, that the concept of reality is metaphysical, rather than scientific. Ultimately all we can test is whether or not our ideas about reality accord with the answers we obtain when we carry out experiments.

I personally don't accept that at all, I definitely only experiment and test real things in my reality and my reality rarely changes and most of the time it matches your reality smile

The real question your answer sidesteps is there only one reality and the answer is definitely no and that is what causes the problem. You dance around the core of that answer by turning that it into something metaphysical. The problem with your answer is you have now a whacky philosophical imaginary universe that at it's core still obeys a central hard set answer that the sum of the squares of the probability amplitudes equals 1. It is a very strange and wonderful metaphysical world you have which suddenly decides it will create such a strange rule and obey it without exception.

Sascha did a nice article on the issue which should be understandable to you (http://www.science20.com/alpha_meme/why_energy_seems_quantized_especially_to_crackpots-136282)

Originally Posted By: Bill S.
I was taking one thing at a time and trying to sort out if decoherence was linked to thermodynamics in such a way that would let us consider decoherence as a concrete feature in what we perceive as reality;

It is and has to be linked to thermodynamics in a very concrete way both have very concrete links to energy.

Last edited by Orac; 05/16/14 05:10 AM.

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Quote:
I personally don't accept that at all, I definitely only experiment and test real things in my reality and my reality rarely changes and most of the time it matches your reality


I have always assumed you were a person who valued proof. No doubt you test things that exist in your reality. In order to live a meaningful life in society we have to assume that others share our reality, but we have no proof of that. It is not my intention to get into a discussion about solipsism, but statements about reality are based on assumptions that are best addressed before slipping into dogmatic pronouncements.


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Not hard to test if a mass of people share a reality it has been done in a number of wars and conflicts ... unless you are saying they didn't die they just all just jumped to the metaphysical place in the sky ... Rev and Paul will be pleased.

Scientifically reality is easy anything you can do a scientific test that is falsifiable is by definition a reality. You can try layering all the philosophical, religious and just plain layman crazy junk you like over the answer but reality is defined as that which exists and exists only has one real test. You can do all sorts of interesting tests on reality and you can even map it's edges but that is another story smile

I do get a lot of amusement out of people who get themselves all bent out of shape over reality like it is some complex abstract idea. I could tell you a story about how long it took a group of us to get wikipedia entry to reflect the simplicity

Here is the statement that cause all the problems with all the crazies and whackjobs out there. We were willing to compromise and put scientifically at the front of the statement but in the end they accepted the argument as is.

Originally Posted By: http://en.wikipedia.org/wiki/Reality
The truth refers to what is real, while falsity refers to what is not. Fictions are considered not real.

Last edited by Orac; 05/16/14 06:12 PM.

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John Barrow, who was not referring to religion, philosophy or “just plain layman crazy junk”, says: “Mathematical ‘existence’ meant only logical self-consistency and this neither required nor needed physical existence to complete it. If a mathematician could write down a set of non-contradictory axioms and rules for deducing true statements from them, then those statements would be said to ‘exist’.”

Perhaps a simplistic approach to existence is not always the most useful.

I said: “In order to live a meaningful life in society we have to assume that others share our reality”. You do that; I do that, and for all practical purposes that is fine.

However, assuming you read this post; when you do, you could be sitting at your computer reading something I wrote, or you could be lying in bed dreaming the whole thing. You cannot prove that life, as you experience it is not just in your head, like the dream.

No amount of scathing comments about “crazies and whackjobs” actually constitutes proof of anything, but you know that; I think you just enjoy trying to wind people up. I admit it's fun, I have been known to do it myself, but it's not really what I look for on a science forum - well not always.


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I have gathered quite a lot of information about decoherence, both here and elsewhere, I'm going to try to find a bit of time to pull the threads together - see how much sense I can make of it.


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Since this thread seems to be running into a fight over philosophy I figure this article by Ethan Siegel might be of interest. What Scientific Arrogance Really Looks Like

Bill Gill


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What can I say guilty as charged ... Ethan has to be a nice guy it helps his site rating and lets face it the huge number of layman rather than scientists are his target audience.

I am firmly with Lord Kelvin if you can't measure it and test it in a hard unbiased and stark way then it is unsatisfactory. This is not some personal preference or something I like it how one has to act in my field because there are no way to know the "right answer" or the "right direction". Mathematicians suffer the same problem you can only trust what you can prove because the why it all is like that is not clear.

Ethan like you Bill thinks there is middle safe ground you can run along and put things in little boxes of what is "right", "probably right", "probably wrong" and "definitely wrong" but that won't work for fundamental science because it overlooks the case that you can have a whole large section built on a wrong with more wrongs piled on top. History is full of such folly like classical physics itself as a prime example.

I should say I don't doubt most of the fields value in what they do. I definitely think they have merit but I always treat any conclusions they come to with absolute doubt because I know how their truth is built. The ultimate end product of this is "the medical report", the one that goes "x" is bad for you until next year when it's good for you and on and on it goes.

So from the arrogant out here we apologize but sorry no we aren't going to tow the political nice guy line anytime soon and it is necessary you know what they say .... IN GOD WE TRUST, ALL OTHERS BRING DATA.

Last edited by Orac; 05/17/14 04:35 AM.

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Orac, you certainly don’t need to apologise for your confidence in yourself and your knowledge. I know I couldn’t do your job if they offered me ten times your salary.

Unfortunately your propensity for launching vitriolic attacks on others, and persistently putting down people with lesser/different qualifications hints at an underlying insecurity. This is a shame, because you have a lot to offer, and the fact that you post on SAGG suggests that you are prepared to offer that to “hitch-hikers”. Just don’t expect an old nit-picker like me to sit at your feet accepting every word. That would be tantamount to pretending I understood something when I didn’t. That’s not me.

Quote:
History is full of such folly like classical physics itself as a prime example.


I have to wonder how arrogant it is to describe classical physics as “folly”. Surely folly would be believing something when you had evidence that it was wrong. Shouldn't classical physics be seen as providing giants on whose shoulders future generations can stand to see greater things.


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Originally Posted By: Bill S.
Unfortunately your propensity for launching vitriolic attacks on others, and persistently putting down people with lesser/different qualifications hints at an underlying insecurity.

I don't launch at people because they are unqualified I launch at illogical and clearly wrong things. I have never once used any sort of qualification as a justification of anything I am happy to be a janitor.

There have been positions which I totally disagree with but they were entirely logical I know Paul and Rev have both done that. My issue with Paul always comes when he tries to say something about science that either isn't true or is a total misrepresentation.

I have clashed with Bill for example over wanting to rule stuff in and out of an ultimate theory of anything. I called it what is was stupid because we don't have a clue what drives QM and we have yet to understand gravity. If he wanted to rephrase it as "likely" or "probably" I had no issue but "definitely" get the hell out of here.

I am entirely consistent with what annoys me, I don't expect people to agree with me and I don't mind if they have a different view but please be logical and truthful with arguments.

Originally Posted By: bill.s
I have to wonder how arrogant it is to describe classical physics as “folly”. Surely folly would be believing something when you had evidence that it was wrong. Shouldn't classical physics be seen as providing giants on whose shoulders future generations can stand to see greater things.

I have no issue if we were living in the 18th century but classic physics has been dead in the water over 100 years now and still people want to argue and try and return to it and think what amounts to millions of observations are somehow wrong.

In the past we have been lazy, classic physics could cover 99.99% of anything in the world and so we never bothered to correct it and hence here we are 100 years down the track. That is no longer true at all, everything in computers and half the new electronic devices are relying on QM. Next year we will have an entangled communication link from a satellite in space to earth. The Higg's discovery will change a whole pile of things over the coming years.

So exactly how much further do we have to go before we are allowed to get arrogant? The term Flat Earther's comes to mind which a term a certain president likes.

My problem with Ethan's article is it whats wrong with the 100 years the lets not upset anyone or anything. Everything has changed QM and the Higgs isn't going to go away and it's not going to blend neatly into classical physics. Everything from classic physics needs to be retested and that means every other field relying on classic physics needs to recheck implications it is a fact ... call us arrogant.

Last edited by Orac; 05/18/14 08:38 PM.

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Originally Posted By: Orac
In the past we have been lazy, classic physics could cover 99.99% of anything in the world and so we never bothered to correct it and hence here we are 100 years down the track. That is no longer true at all, everything in computers and half the new electronic devices are relying on QM. Next year we will have an entangled communication link from a satellite in space to earth. The Higg's discovery will change a whole pile of things over the coming years.

It is true that QM explains things more than classical physics does. But it is also true that classical physics still works wonders, in the area in which it is applicable. That is in any area where we don't need the extreme accuracy of QM. It is also true in any area where we don't know how to get from the QM explanation of what is happening to the classical explanation. In many cases the difficulty is working with QM in large groups of particles. When I say large groups I am talking about billions of particles, such as in a glass of water. There is, at present, no way to extend the QM description of the individual atoms of water to the way the whole glass acts. Therefore classical physics still has a major part to play in working with the world around us.

Bill Gill


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C is the universal speed limit.
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