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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...

A former member
Post #: 100
PS Peter I didn't specifically refer to your latest message as I found it a bit strange. But also you were talking about a single entity "interacting with itself" and "creating an interference pattern", which doesn't make sense to me. An interference pattern is the statistical outcome from a large number of such events, not one event. In the case of a single electron (or molecule in that video) there is no interference pattern, there is just one outcome in term of where that particle hits the detector. But, theoretically speaking, there is a probability distribution associated with the position of the hit. I guess that is what you really meant.

Whilst there has been much confusion about the idea of a particle 'being observed', I think it is widely accepted these days that 'observation' by humans has nothing to do with the physics going on here. The results seen in these experiments are strictly the physical consequences of the interaction between the electron or molecule and its environment. Human interaction (and conscious human interaction whilst on that topic) are relevant only to the extent that we set up the environment, as in the choice between one slit or two.
A former member
Post #: 97
Hi Andrew,


You are right, in that you cannot get an interference pattern from a single event, and right in your surmise as to what I actually meant. The particle certainly does "intact with itself" and thereby create a probability distribution which has the same values as the interference pattern, and which , given a large number of events, causes the interference pattern to emerge.


I was not specifically aware of the confusion around the idea of "being observed", but it comes as no great surprise. My understanding is that if any piece of equipment is in place (and a human might well count as a piece of equipment in this context) that is capable of determining which slit the particle passes through, the particle's waveform collapses, and Classical Physics type behaviour arises.

I term any such piece of equipment as an observer. As particle sizes get bigger, it seems inevitable that observers will increasingly abound, and I am suggesting that this is the mechanism which causes the transition from QM behaviour to Classical Physics behaviour.

Intuitively, I feel that my explanation will cause a more rapid transition, but one that starts later (i.e. at larger particle sizes) than other possible explanations, which means that it may be possible for experiments to support or refute this idea.

Obviously, if this hypothesis has any support at all, the next step would be to develop it into a quantitative theory, which would make the experimental evidence much more compelling (whether in favour or against). I doubt I can do that on my own, but I'd like to think this group could make some progress in that direction....



Peter

A former member
Post #: 102
Probably the transition from quantum behaviour to classical behaviour happens in three different ways: (a) the decoherence of a quantum system via contact with its environment, (b) the Heisenberg uncertainty principle which relates fuzziness to the fixed dimensions of the Planck scale and (c) the law of large numbers.

Of these, (a) is a transition over time, (b) is a transition over scale (so I don't expect that ever larger molecules will go through two slits at once) and (c) is purely statistical (so you don't seem fuzzy even though you are made of a lot of fuzzy particles).

I don't think any new computational theory is required: quantum theory will do it all.

PS Best to avoid talking about observers. That's the term used in the Copenhagen interpretation which opened the door to linking consciousness with quantum theory. Apparently it's not very polite to link the two these days, people might talk, you never know what might happpen........

A former member
Post #: 98
Hi Andrew,


Probably the transition from quantum behaviour to classical behaviour happens in three different ways: (a) the decoherence of a quantum system via contact with its environment, (b) the Heisenberg uncertainty principle which relates fuzziness to the fixed dimensions of the Planck scale and (c) the law of large numbers.

Of these, (a) is a transition over time, (b) is a transition over scale (so I don't expect that ever larger molecules will go through two slits at once) and (c) is purely statistical (so you don't seem fuzzy even though you are made of a lot of fuzzy particles).


I am reading up on Quantum Decoherence, and I agree it may offer an explanation; but I do have some problems with that - maybe the issue is my lack of understanding of QM, of course. I will continue to discuss this (see below) on the grounds that if my objections are false, I will eventually learn something, but if valid we need to find some other explanation for the transition from QM to Classical Physics.

I fail to see however, how the uncertainty principle or the law of large numbers can provide an explanation. Indeed, the uncertainty principle is the underlying justification for perceiving a waveform as a probability distribution, and the law of large numbers is precisely the statistical basis by which probability distributions can be added to form interference patterns such as those exhibited by the Young's Slits experiment. I don't see how the uncertainty principle can prevent the Sacred Elephant, a London Bus, this planet or even an entire Galaxy from passing through two slits simultaneously. All that happens is that as the "particle" gets bigger the necessary separation between the two possible paths increases (= the width of the particle approx) with the result that we are permitted to determine the velocity of the "particle" more accurately. The law of large numbers as you say makes large fuzzy things seem unfuzzy, but we are seeking the opposite effect - the transition from QM to Classical Physics makes a very clear phenomenon (i.e. the interference pattern) fuzzy so that superimposition washes it out.


By contrast, Quantum Decoherence does potentially provide an explanation. The problem is that the Young's Slit experiment clearly demonstrates that each particle interacts with itself. Variants of the experiment firing one particle at a time still show the interference pattern. So, we require that Quantum Decoherence allows for a single particle to decohere from itself. This appears at first sight to be a ridiculous proposition, an oxymoron, but it is not, because the interference pattern is caused by the interaction of two waveforms which can in principle decohere even though both are associated with the same particle.

Even so, I'm unclear how Quantum Decoherence can provide for a single particle to decohere with itself in a way that can explain the transition from QM to Classical Physics in an experiment that depends only on the interaction of a single particle with itself.

http://en.wikipedia.o...­
In quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior. Quantum decoherence gives the appearance of wave function collapse (the reduction of the physical possibilities into a single possibility as seen by an observer) and justifies the framework and intuition of classical physics as an acceptable approximation: decoherence is the mechanism by which the classical limit emerges out of a quantum starting point and it determines the location of the quantum-classical boundary. Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way. This prevents different elements in the quantum superposition of the system+environment's wavefunction from interfering with each other.

So if anybody can properly explain it to me I'd be most grateful...


Thanks


Peter
A former member
Post #: 99
Hi Andrew,

PS Best to avoid talking about observers. That's the term used in the Copenhagen interpretation which opened the door to linking consciousness with quantum theory. Apparently it's not very polite to link the two these days, people might talk, you never know what might happpen.......

I talked about observers after defining the term, not about observers; but if you object to this please suggest an alternative - detector perhaps?

I completely agree that the risk that people might talk is unacceptable, lol: they might then start to think and even (heaven forbid) have ideas or form opinions of their own. No well ordered society can permit such subversive behaviour, I agree: it might cause decoherence.




Peter
A former member
Post #: 104
You said, Peter:

I don't see how the uncertainty principle can prevent the Sacred Elephant, a London Bus, this planet or even an entire Galaxy from passing through two slits simultaneously. All that happens is that as the "particle" gets bigger the necessary separation between the two possible paths increases (= the width of the particle approx) with the result that we are permitted to determine the velocity of the "particle" more accurately.

The uncertainty principle prevents certain pairs of physical measurements being simultaneously measured to any degree of accuracy. The best known example of such a pair is position and momentum. The product of their standard errors must be at least half Planck’s constant which is incredibly tiny. Since an electron or not-too-large molecule does not have much mass, its momentum going though the slit is pretty small and so, by the uncertainty principle, its location must be highly uncertain. That’s how it can sort of ‘go through both slits’. But if you scale up to an elephant then the momentum will be relatively gigantic and (probably with some help from the law of large numbers) the uncertainty in its position so tiny as to be classically in one place only.

By contrast, Quantum Decoherence does potentially provide an explanation. The problem is that the Young's Slit experiment clearly demonstrates that each particle interacts with itself. Variants of the experiment firing one particle at a time still show the interference pattern. So, we require that Quantum Decoherence allows for a single particle to decohere from itself. This appears at first sight to be a ridiculous proposition, an oxymoron, but it is not, because the interference pattern is caused by the interaction of two waveforms which can in principle decohere even though both are associated with the same particle.

I don’t think it is helpful to say that a particle interacts with itself. That’s why you arrive at that ‘ridiculous proposition’. Better to say that the particle behaves according to its environment (eg one slit or two) and in accordance with the laws of quantum mechanics. The interference pattern that results from repeating the one-particle experiment many times is mathematically equivalent to the interference between two waveforms, but (with the exception of one interpretation of quantum physics) I don’t think it is physically the result of any such interference. It is what the theory predicts and the experiment shows. If you want to understand it any deeper than that then you’ll have to be patient because as yet no-one understands this. But instead of wasting your time reading up on philosophy you could do better by reading about the different interpretations of quantum theory. There are many, some of which are more or less bonkers and some of which involve the conscious mind. (If asked, I am not prepared to say whether I think the latter are also bonkers.)


A former member
Post #: 102
Hi Andrew,

Yes... but:

1.

Scaling up the momentum of the Sacred Elephant does not necessarily scale up the error in the determination of his momentum: so the uncertainty in his location can remain arbitrarily large. I'm sorry, but I do not believe that the Uncertainty Principle, per. se. can prevent the Elephant from passing through the two slits simultaneously.



2.

If you dislike the phrase "particle interacts with itself" I'm sure I can change it: it is after all merely a linguistic device - we have no real vision as to what is going on in QM as you so rightly point out, so we are forced to import language from the macro world.

But changing the language cannot and will not change the sense of what I am saying.

In particular the description of Quantum Decoherence seems principally concerned with the loss of phase synchronization between one particle and another. This has little relevance to the processes causing the interference pattern in the Young's Split experiment. I accept I need to understand Quantum Decoherence better, but the more I read the less plausible it seems to me that Quantum Decoherence can explain the transition from QM to Classical Physics behaviour in the case of the Young's Slit experiment.

Hence my request for guidance from one who understands.



Peter
lan B.
user 10895495
London, GB
Post #: 156


From Andrew:

>PS Peter I didn't specifically refer to your latest message as I found it a bit strange. But also you were talking about a single entity "interacting with itself" and "creating an interference pattern", which doesn't make sense to me. An interference pattern is the statistical outcome from a large number of such events, not one event. In the case of a single electron (or molecule in that video) there is no interference pattern, there is just one outcome in term of where that particle hits the detector. But, theoretically speaking, there is a probability distribution associated with the position of the hit. I guess that is what you really meant.

Right. Obviously, “one ‘particle’, one event”, but of course the destination of that ‘particle’ – i.e. which precise pixel on the screen that it eventually ‘hits’ – is not arrived at in a classically deterministic way. On any interpretation, the buckyball (say) becomes entangled with its immediate environment, which happens to be the slits and the detector/screen in this case (assuming that just about all the gas has been evacuated from the enclosure beforehand). Sticking my neck out just a little – but not too far, and I would say that day by day experimental confirmation is beginning to make my protuberant posture look increasingly normal – the decoherence interpretation will claim that the composite quantum system decoheres such that the superposition collapses, and position now becomes a determined measurable. (Subject to the limitation of the Uncertainty Principle, but that of course applies to everything in any case.)

The superposition collapses, but there is no classical state. That is, the original wavefunction has changed its form, but not ‘vanished’. Entanglement is ‘gregarious’ and the diagonal component of the wavefunction – expressed in terms of probability density functions (pdfs) whose real-number or quasi-classical measurable diagonal elements within the corresponding density matrix, a generalisation of the notion of wavefunction -- becomes progressively more localised. This entails an increase in the degrees of internal freedom of the composite system, analogously to the classical case of an increase in the entropy of some closed system. These minute, multifarious local entanglements have the consequences of producing future, persisting interference effects – and so it’s not after all true that “the wavefunction has collapsed” – but in order to detect these interference effects (and, the most shocking consequence of the otherwise objectively realist and unspooky approach which characterises the decoherence interpretation, in order to detect the alternative past histories whose measurement would entail that they, too have happened!) it would be necessary to construct a second-order quantum measurement device, one which exhaustively ‘interrogates’ the quantum state of the first-order quantum measurement device – i.e. in this case the supercooled, evacuated 2-slit interferometer – and such a device would even according to a conservative estimate be so huge that it would have a radius approximately 10^20 times that of the radius of the observable universe.

All of this is calculable, and the techniques can be found within works by Joos and Zech, and elegantly laid out within Roland Omnès’ 1991 book – now out of print – The Interpretation of Quantum Mechanics. (Some physicists such as Eugene Lamb after visiting the CPNSS at the LSE and talking there took umbrage at Omnès grandiose self-assurance at this point!)

If I revert to being little more conservative in sticking to the theoretical interpretation of what’s happened according to QM under the rubric of any interpretation whatsoever, then no-one disagrees that “in transit through the slits”, the ‘particle’ is not localised, but rather its quantum state – really Feynman’s path integral encompassing all of the space between the emitter and the screen, including the slits, but ‘edited down’ by the fact that the only way to the screen is via the slits, because everywhere else has been blocked off – has been reduced, and this process of channeling and selecting can go on indefinitely, but as said the classical components of the wavefunction such as its centre-of-mass merely become increasingly localised, i.e. ‘classical-like’.

Again, the decoherence interpretation mandates that at no time is there such an entity as a ‘particle’. All that is ever happening is varying degrees of localisation of the quantum state. (Which is encoded by the wavefunction. One should avoid confusing the physical state with the mathematical apparatus which describes it.)


>Whilst there has been much confusion about the idea of a particle 'being observed', I think it is widely accepted these days that 'observation' by humans has nothing to do with the physics going on here. The results seen in these experiments are strictly the physical consequences of the interaction between the electron or molecule and its environment. Human interaction (and conscious human interaction whilst on that topic) are relevant only to the extent that we set up the environment, as in the choice between one slit or two.

Dead right. So-called observer-dependent interpretations are FAPP (For All Practical Purposes; John Bell) just about as dead as their originator, John von Neumann. (Mathematical Foundations of Quantum Theory, 1932). The philosopher Karl Popper was highly effective in combating these during the mid-20th century, so, well done, Karl!

Ian


A former member
Post #: 103
Hmmmm


OK


I certainly have to change my language.



But I need to think more about whether the underlying ideas are invalidated by what you say.



Peter
A former member
Post #: 105
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 - 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). But when coupled with some other ideas (as to which there is no consensus) then decoherence can provide a complete explanation.

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. 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 problem.
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