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Cosmology, Quantum Mechanics & Consciousness Message Board › Back to some Real Quantum Mechanics - Quantum Interference - The Movie...

Back to some Real Quantum Mechanics - Quantum Interference - The Movie...

lan B.
user 10895495
London, GB
Post #: 157

(From Andrew):

>Referring to the other thread, I too discovered the Stanford Encyclopedia of Philosophy after googling a reference from Camilla by the name of Stanford Enc. I agree it is excellent. In particular it has a terrific section on decoherence in quantum menachics, within which is stated very clearly that decoherence is not a complete explanation of the quantum measurement problem

True .. not a complete one!

>the problem of how the physics suddenly decides to stop obeying one kind of law (eg Schrodinger's equation) and act in accordance with another law (concerning the probability of a particular measurement on an observable).

Sorry, that’s not true! The whole point about decoherence is that there is no “collapse of the wavefunction”. You’re taking more of a Roger Penrose-type position, Andrew – one in which there is so-called objective state-reduction. (Ghirardi-Rimini-Weber-type theories.)

>But when coupled with some other ideas (as to which there is no consensus) then decoherence can provide a complete explanation.

As said, Fay Dowker in particular is a strong opponent. It is true that any of the real-number diagonal elements within the density matrix describing some quantum state or other could describe “the state of the actual set of could-be measurements which constitutes the world", but in fact – for “historical reasons” – the system has always been nearer to exemplifying some of these classical-like variables rather than others, and for energy-minimisation reasons, it is not likely to “jump away from” them.

Of course, I’ve just invoked probability, but decoherence doesn’t deny probability! (Just as it doesn’t deny the logical possibility that the ubiquitous non-local entanglements of QM determine (non-causally) the basis of such “probability”. (Which probability is an inescapable phenomenological feature of QM!)


There is no disagreement about the basic equations and formalism of quantum theory which have been confirmed by experiments to hold with an extraordinary degree of accuracy. These equations tell us how to build silicon chips, they yield the Heisenberg uncertainty principle as a consequence, not a reason. And as a bonus, they will show why an electron can go through two slits while an elephant cannot.

According to the decoherence interpretation there is no quantum-classical cut-off point. It is energetic considerations which preclude the elephant’s passage. (It would be obliged in any case to “unbind” and remain in that state for a very long time -- > 10^25 years! – in order to pass through multiple openings!)

At the end of the day, all understanding of quantum behaviour is fully explained by these equations. The mystery, which is why the subject is so interesting to discuss, is the measurement

The decoherence interpretation was developed as a putative solution to the measurement problem!

.. Because the measurement problem – which I agree is the fundamental problem within QM – is the problem of reconciling the classical world with the quantum one: whether or not there is any “quantum/classical watershed”



A former member
Post #: 106
Ian: I have some issues with what you say there!

You say: “..... the measurement problem .....is the problem of reconciling the classical world with the quantum one: whether or not there is any “quantum/classical watershed”
I don’t think that is wholly accurate. Traditionally the problem was posed as I described it. It remains the case today, as far as I know, that the absolute basis of quantum theory amounts to the mathematical descriptions of (a) the evolution of quantum state over time and space and (b) the probability of any particular value of an observable. So I am merely referring to the actual terms in which the theory is expressed and the calculations are done, not to any particular interpretation or position. The question of whether or not there actually is any ‘wave function collapse’ is a refinement in the search for an understanding of what is really going on. Thus Wikipedia describes the measurement problem in terms of how or if wave function collapse occurs.

Secondly my understanding of decoherence theory is that it is about explaining how coherent quantum states dissipate into the environment. This is about the time evolution of quantum states and is therefore fully described by the Schrödinger (etc) equation, but since the ’environment ‘ normally means something a bit complex and messy, statistical techniques may be involved. This theory, at least as originally formulated, was not about re-writing the rule book of quantum theory.

You refer however to the ‘decoherence interpretation’ of quantum physics. You also posit that a particle such as an electron does not exist per se, but is always to be found in the form of a wave, however closely it may resemble a particle when it hits a measuring target. I’m not sure where you get these assertions from, plausible though they seem. I can see that they might appear to give an interpretation in which ‘wave collapse’ does not occur. But:


  • Wikipedia on quantum interpretations does not explicitly include decoherence in its comprehensive looking list of alternatives. Why not? Your description would seem to be a complete solution.
  • The Stanford Enc, says “Unfortunately, naive claims of the kind that decoherence gives a complete answer to the measurement problem are still somewhat part of the ‘folklore’ of decoherence, and deservedly attract the wrath of physicists ....... and philosophers”
  • You agree that decoherence is not the complete answer without explaining why, given that your version looks as if it is a complete answer.
  • I believe that QED ( quantum electro-dynamics) which is relativistic quantum theory applied to fields and particles, asserts that a quantum field is mathematically identically equivalent to a set of quantum particles (including the potentially infinite array of associated virtual particles and force messenger particles). Thus it does assert the existence of a quantum particle as a particle of infinitesimal size, which seems to contradict you on that point (pun intended).

No need to respond if you feel this is digging too deep!
lan B.
user 10895495
London, GB
Post #: 160


Andrew, l'll get back to you but pressed at the moment. Sorry.

Meanwhile, next Monday's Aristotelian Society talk has been cancelled:




Issue No. 3
The 133rd Session



30 April 2012 | 16.15 - 18.00

Choice and Voluntary Action

Maria Alvarez
Kings College London

The Woburn Suite
Senate House
University of London

[b]TALK CANCELLED
A former member
Post #: 125
Ian
Thanks again for the rapid response. I’ll split the topic into two and here I’ll discuss the question of ‘what is a photon?’ Of course the question has no answer at present. But, to recap, you said: “...... there is no supportive physical phenomenology for anything except waves. That is, nothing else on a sufficiently small scale has been observed to be particulate (atomic nuclei being an arguable exception).” I can’t agree with such casual dismissal of 100 years of wave/particle duality so I just responded with sarcasm. You fired back with the higher authority of Wikipedia who say: “... photons are not thought of as "little billiard balls" but are rather viewed as field quanta – necessarily chunked ripples in a field, or "excitations", that "look like" particles...”. Since I rather doubt that the Wiki description is anything more than a metaphor but you seem to treat it as a description of reality I offered the challenge: prove it, give any reference to the form of the quantum wave that the photon is, or its mathematical formula. To which you immediately said: E = hc/λ., adding that “the smaller the wave, the more massive and hence more particle-like it is!”.

Your equation is basically how Einstein introduced the photon concept and it relates the energy of a photon to its wavelength. X-rays are more energetic than visible light and so on. All good stuff, but where is the little wave, the ‘chunked ripple in a field’ with which the Wiki quote appears to sustain your belief? You have appealed to that equation because it does describe the photon and it contains the term λ which is wavelength. But that equation does not tell you that λ represents wavelength. That’s a bit of understanding that you have imported from elsewhere, knowing that photons can be wave-like. And there is nothing in that equation about the distribution of the photon or of its ‘chunked ripple’ over space and time, nothing ‘back and forth’ to imply wave-like motion. To be blunt, your equation is not that of a wave, is it? So regrettably I cannot accept your equation as a validation of the Wiki metaphor.

Actually I was expecting you to produce a sine wave description of the photon. But, to save going through that hoop later, let’s do it now. A sine wave does the job but the job it does is to describe the photon path through space-time. Thus X rays are represented by a more bunched up sine wave than visible light. But the question is: what does one photon of the light look like? One full cycle of the sine curve? Half a cycle? All the cycles? Or just a dot on the sine curve when you take a snapshot at a moment of time? I suggest that the wave doesn’t actually describe the photon itself – what exactly it is. Nothing does, that we know of. A photon is a quantum and we have no way of understanding it in any more depth than that. Therefore the Wiki description is, I say, purely a metaphor and perhaps a misleading and unnecessary one at that. Therefore it doesn’t support your idea. I much prefer the Stanford discussion to which I referred earlier.
A former member
Post #: 126
Ian
Now I’ll respond to your belief that decoherence explains all of quantum physics without any 'collapse of the wave function'. I challenged you to post a reference to a single piece of work that shows how the Penrose process U (ie the progress of a quantum state over time as in the Schrodinger equation) determines the probability of transition from one physical state to another (such as for example the probability that an electron in higher atomic orbit drops to a lower orbit during a fixed period of time). In other words how U can substitute in precise calculations for the other process R. In reply you said, referring to the U and R terminology: “The labelling was purely logico-schematic. Penrose was illustrating the central, essential mystery of QM by showing that the 2 are in any algorithmic sense totally disparate, disconnected.”

OK, but you have not actually met the challenge of showing how U can substitute in precise calculations for the other process R. And I think you agree that it’s not possible to do so. Evidently you think it’s not necessary to do. But what is absent from your comment is any recognition that U and R are integral to the present mathematical formulation of quantum physics. They were called U and R by Penrose for his books but by other names they are essential elements of any textbook on quantum theory. We need both or we don’t have a working theory. Your ‘decoherence interpretation’ seems to imply that quantum physics can get along fine without R, in effect that the decoherence processes (which are described purely by U) can substitute for R and give us the same theory. But so far you have not justified this very strong assertion.

Indeed, much of quantum behaviour is completely explained in terms of U processes, whether the evolution of a quantum state before a physical event or the decoherence of a quantum state into its environment. But not all of what happens is described by U. We need the occasional bit of R as well. I cited the electron dropping orbit. I suppose you will say that the electron is never in a pure state of one orbit or the other; that its state shifts very quickly owing to some entanglement with the environment. Nice idea but where’s the theory? What could cause the quantum state of an electron to be nearly an eigenstate but not quite? (Note: it’s the R process that enforces pure eigenstates, the states that correspond to physical reality.) Where is the calculation that shows how the probability of changing orbit emerges from U processes alone? I don’t think the theory exists as such, does it? It would be like re-writing nearly a century of quantum theory. That presumably is why Stanford Enc comes down hard on your idea.

So it looks as if you have more work to do, to maintain your claim. I look forward to hearing about it!
A former member
Post #: 131
Ian
On 26 April you said you would respond to my message, which you have not done. And my subsequent challenge has now expired without a response to my two points above. So I presume that you are forced to concede my points of disagreement with you on a couple of basic and related points about quantum mechanics. It is an unjustified simplification for you to say that on the smallest scales all quantum behaviour is wave-like, not particle-like. And it is incorrect to say that decoherence fully explains quantum behaviour.
lan B.
user 10895495
London, GB
Post #: 204


Andrew and anyone else following this:

I have responded to these "accusations" -- wink -- on the The nature of freewill thread.

lan B.
user 10895495
London, GB
Post #: 211


For those who might be personally averse to themselves making the effort to explore the immense informative reserves of the internet in order to investigate leads, words and phrases, and heavily dropped hints, this link might be useful. (Posted also on the The nature of "freewill" thread):


Roland Omnès
From Wikipedia, the free encyclopedia
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Roland Omnès
Born Roland Omnès
18 February 1931 (age 81)

Residence France

Citizenship France

Nationality France

Fields Theoretical physics
Quantum Mechanics

Institutions Université Paris-Sud XI

Roland Omnès (18 February 1931) is the author of several books which aim to close the gap between our common sense experience of the classical world and the complex, formal mathematics which is now required to accurately describe reality at its most fundamental level.

Contents

• 1 Biography
• 2 Philosophical work
• 3 The new 'Copenhagen Interpretation'?
• 4 Bibliography

Biography
Omnès is currently Professor Emeritus of Theoretical Physics in the Faculté des sciences at Orsay, at the Université Paris-Sud XI. He has been instrumental in developing the consistent histories and quantum decoherence approaches in quantum mechanics.

Philosophical work
In his philosophical work (especially in Quantum Philosophy), Omnès argues that:

1. Until modern times, intuitive, rational thought was sufficient to describe the world; mathematics remained an adjunct, simply helping to make our intuitive descriptions more precise.

2. In the late 19th and early 20th centuries, we arrived at a Fracture between common sense and our best descriptions of reality. Our formal description became the truest picture (most consistent with how things are, experimentally) and common sense was left behind. Our best descriptions of reality are now incomprehensible to common sense alone, and our intuitions as to how things are, are often negated by experiment and theory.

3. However it is, finally, possible to recover common sense from our formal, mathematical description of reality. We can now demonstrate that the laws of classical logic, classical probability and classical dynamics (of common sense, in fact) apply at the macroscopic level, even in a world described by a single, unitary wavefunction. This follows from the fundamental principles of quantum mechanics, with no need for extralogical constructs such as wavefunction collapse.

We will never, Omnès believes, find a common sense interpretation of quantum law itself. Nevertheless, it is now possible to see that common sense and quantum reality are compatible with each other: we can enter the world at either starting point, and we will find that each leads to the other: experiment leads to theory, and the theory can now recover the common sense framework in which the experiment was conducted (and in which our lives are lived).

The new 'Copenhagen Interpretation'?
Omnès' work is sometimes described as an updating of the Copenhagen Interpretation of quantum mechanics. This is somewhat misleading. The relationship between the two accounts is as follows:

The Copenhagen Interpretation of quantum mechanics (argued for most centrally by Niels Bohr) tells us to "shut up and calculate". It says that there are certain questions we simply cannot ask, and that there are inexplicable rules which we have to apply in order to get from a quantum description of reality (which we know is experimentally correct to at least 10 decimal places of accuracy) to the reality of our day-to-day, common sense lives (which seems self-evidently correct, and yet is apparently in contradiction with quantum law).
Omnès tells us that we no longer have to shut up in order to calculate: there is now a self-consistent framework which enables us to recover the principles of classical common sense - and to know, precisely, their limits - starting from fundamental quantum law.

Bibliography
The work Omnès presents in his books was developed by Omnès himself, Robert B. Griffiths, Murray Gell-Mann, James Hartle, and others.

The Interpretation of Quantum Mechanics (Princeton University Press, 1994) - a technical exposition of Omnès's account, for physicists.
Understanding Quantum Mechanics (Princeton University Press, 1999) - a somewhat less technical revision and updating of the above work, also intended for physicists.
Quantum Philosophy: Understanding and Interpreting Contemporary Science (English Edition - Princeton University Press, 1999); (French Edition - Gallimard, 1994)
Converging Realities: Toward a Common Philosophy of Physics and Mathematics (Princeton University Press, 2004) - Here Omnès presents, in detail, his position on the relationship between mathematics and reality which he started to develop in Quantum Philosophy.

Hope this helps! I will address current concerns/misgivings in due course, when I have the time and energy, but there unfortunately does seem to be some mileage in the adage: "You can take a horse to water, but you can't make him drink."

Incidentally, here is an excellent professional mathematician's review of Omnès' first significant blockbuster The Interpretation of Quantum Mechanics and it is surprisingly good at capturing the main theoretical thrust both of Omnès' general Weltanschauung and his specific proposals to harmonise the orthodox, bare-bones postulatory structure and mathematical apparatus of QM with the classical-like appearance of the world of our everyday, "commonsense", macroscopic experience. The reviewer makes it clear that significant problems remain, but as many of you (the few who are reading this!) must by now realise, I think that some variant of the decoherence approach holds out the best prospect for reconciling this fundamental schism within current, "fundamental" physics, and Omnès remains a highly significant figure in the field's development. Work still in progress, then!

The web address of this review:


http://www.ams.org/no...­.

Regards

Ian



lan B.
user 10895495
London, GB
Post #: 212



(Hmm .. I just clicked on this link, and all it did was send me to the American mathematical Society Home Page. here's the address infull once more. (Fingers crossed!)):


http://www.ams.org/no...­



Camilla M.
user 7151822
London, GB
Post #: 39
Don't be silly, message boards are usually set to refuse to take addresses for advertising reasons. You'll have to write it with the word "dot" for "." Or else just give the addr ending part minus slashes put hyphens or something instead.

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