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Post by abacus9900 on Nov 30, 2010 15:16:08 GMT 1
No, the point is (as far as I know but I am not a physicist) you have to use entangled pairs because otherwise quantum objects interact with the environment (i.e. are effectively 'measured') and so, therefore, become integrated with classical physics. It still remains though that 'spooky action at a distance' cannot be reconciled with classical physics since all the evidence so far produced refutes this idea. If you know of any experiment that undermines this conclusion then please, do tell. Perhaps it is the non-intuitive nature of QM that is giving you trouble accepting it but ultimately we must rely on the science, not our preconceptions.
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Post by robinpike on Nov 30, 2010 15:35:50 GMT 1
Perhaps it is the non-intuitive nature of QM that is giving you trouble accepting it but ultimately we must rely on the science, not our preconceptions. When you say 'the science', it means two things to me: i) the results of an experiment, ii) the conclusion of an experiment. i) is easy to rely on, ii) is not. Returning to the EPR experiment, the experimental apparatus can be performed with either a) entangled pairs or b) non entangled pairs (but in the same polarisation). By performing the experiment for b) as well as a), it would confirm that it is not the experimental apparatus that is creating results that agree with QM regardless of whether QM is involved or not.
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Post by robinpike on Nov 30, 2010 15:55:51 GMT 1
Just to make it clear, the point that I am making about theories and proofs (by using the EPR experiment as an example), is that the statement: QM has been proved beyond any doubt; cannot be taken as valid if the classical version of the experiment has not been performed as well (to prove that the apparatus is capable of giving the classical prediction when QM is not involved).
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Post by abacus9900 on Nov 30, 2010 16:28:01 GMT 1
Perhaps it is the non-intuitive nature of QM that is giving you trouble accepting it but ultimately we must rely on the science, not our preconceptions. That is rather an odd point of view because there would be little point in conducting experiments if people could not infer conclusions from them. I think you may be a bit confused here. Entangled pairs do not possess any definite polarisation before a measurement is made - this is the point about 'spooky action at a distance.' It is only AFTER a measurement is made on one of the pair that its 'twin' exhibits a complimentary polarisation (regardless of their relative locations) but it is impossible to predict what the specific polarization will be among a number of possibilities before actually making a measurement.
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Post by speakertoanimals on Nov 30, 2010 16:50:23 GMT 1
The point to note here is that as far as I know, Bell himself was hoping that quantum theory would be proved wrong, hence he devised an inequality that made only the most general assumptions about what constraints a classical theory had to have.
Indeed, he showed that various previous proofs of the supposed impossibility of hidden-variables theories were incorrect. It is quite clear what answer he was hoping for, so I expect that he made the classical door as wide as he possibly could. If there had been SOME way to get those values using a classical explanation, I'm sure he would have included that in his work.
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Post by Progenitor A on Nov 30, 2010 16:51:46 GMT 1
Returning to the EPR experiment, the experimental apparatus can be performed with either a) entangled pairs or b) non entangled pairs (but in the same polarisation). By performing the experiment for b) as well as a), it would confirm that it is not the experimental apparatus that is creating results that agree with QM regardless of whether QM is involved or not. This is interesting Robin. How do you propose that the experiment is conducted with non-entangled pairs? To my mind the 'entanglement' refers to two particles that are tied to one another somehow so that what happens to one, invariably affects the other. How can that happen with non-entangled pairs?
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Post by Progenitor A on Nov 30, 2010 17:07:56 GMT 1
I recently attended a lecture by Alain Aspect, who talked about his EPR experiment involving entangled pairs of polarised photons. The results agreed perfectly with the QM prediction and not the classical prediction. Does QM actually predict the outcome of the (modified) EPR experiment? This raises a number of problems with me Firstly EPR predicted the possibility of two results, one of which would have violated th euncertainty principle and th eother involving 'spooky action at a distance' (In Einstein's words) They were not using QM theory to arrive at these possible outcomes , but simply 'classical' logic Secondly, to say that QM 'predicts the outcomes of this expeiment, gives, I think, the entirely false impression that they (the QM physicists) know what is going on. They do not. They know the outcome of the experiment simply by measurement. Not much prediction is involved if you know (from experiment) what is going to happen. Rather like me 'predicting' the sun will rise tomorrow. So why my concern? Because the experiment was also repeated where the entangled photons were let fly for 30 kilometers before their polarisation was measured, and exactly the same results were found - agreement with QM and disagreement with classical theory. Does 'classical' physics have anything at all to say that disagees with the result? So you might say, that is even further proof that QM is the right explanation But there is no explanation - but to me it is the opposite. I could understand if at such a distance the results failed to agree with QM, but since the distance had no impact on the results, it suggests to me that distance is a red herring and therefore interaction over distance is also a red herring. But that is surely the importance and mystery - action at a distance, the instant communication between particles with a large physical separation? The so-called 'non-locality'? So it leads me to consider this: although it seems impossible, is there any way that a classical explanation could give the found results? No. But that is not surprising. QM cannot explain it either
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Post by nickcosmosonde on Nov 30, 2010 17:26:32 GMT 1
"Spooky action at a distance"? You're referring to Bell's Theorem? How can a notion of "hidden variables" be "conclusively shown" to be incorrect? What are you talking about?
These are not the "non-intuitive results" that he didn't accept. The Copenhagen Interpretation of them is what he didn't accept. Neither did a host of other quantum theorists at the time, including Schrodinger, de Broglie and Planck. Moreover, after the war fewer and fewer physicists accepted it, and even fewer do so today. To the extent that there exists a consensus at all about the matter, most physicists choose to see the matter as Schrodinger did, with another sizeable body of support for Bohm's Implicate Order theory. The "hidden variables" conjecture you claim, despite the logical impossibility of the idea, to have been shown to be incorrect.
I'm being irrational? This from the man who claims there is no such thing as an independent "nature" other than our "ideas" of it?
Of course science reveals nature. As StA argued, it increases our ability to predict a wider and wider range of observations. It broadens the quantity and quality of the facts we know. These are the "truth" that science discovers - or, rather, our propositions that express them - not some mystical predicate that our theories approximate. (Science is not the only such activity that does this, incidentally.)
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Post by nickcosmosonde on Nov 30, 2010 17:33:12 GMT 1
Are you sitting somewhere typing this nonsense? At a computer terminal. Located somewhere in Britain, perhaps? A sizeable land mass on the surface of the Earth? A planet orbiting the Sun somehwere bwteen the orbits of Venus and Mars? In turn, revolving around the Milky Way? Etcetera etcetera?
Anyone who seriously believes that a conclusion such as "statistically, the notion of locality is a dead duck" has anything to do with science or reason let alone finally settled knowledge is a loon.
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Post by robinpike on Nov 30, 2010 17:33:26 GMT 1
That is rather an odd point of view because there would be little point in conducting experiments if people could not infer conclusions from them. People can of course infer a conclusion from an experiment, what is hard, is knowing whether the conclusion is correct or not. I think you may be a bit confused here. Entangled pairs do not possess any definite polarisation before a measurement is made - this is the point about 'spooky action at a distance.' It is only AFTER a measurement is made on one of the pair that its 'twin' exhibits a complimentary polarisation (regardless of their relative locations) but it is impossible to predict what the specific polarization will be among a number of possibilities before actually making a measurement.[/quote] Here is a description as to how Alain Aspect's experimental apparatus could be checked to confirm that it is capable of giving non QM results for non entangled pairs of photons. Attached is a diagram of Alain Aspect's experimental set-up. The modification needed is to add two additional polarisers [say A1 and A2] after the polariser, say the one on the left a). To enable simultaneous timings, the polariser b) on the right would have to be moved an appropriate distance to the right. Now in this set-up, the photon that goes left, will meet the polariser a), causing its wave function to collapse, which also causes the other photon's wavefunction to collapse, both photons now being dis-entangled although in the same polarisation. The photon on the left then goes through either polariser A1 or A2, and the photon on the right goes through polariser b). In this set-up, the polarisation of the photon pair being measured is when they are no longer entangled, and so the results of the experiment should give results in exact agreement with the classical prediction (and not QM). Attachments:
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Post by abacus9900 on Nov 30, 2010 18:09:51 GMT 1
"Historically, in physics, hidden variable theories were espoused by a minority of physicists who argued that quantum mechanics is 'incomplete'... Einstein, Podolsky, and Rosen argued that 'elements of reality' (hidden variables) must be added to quantum mechanics to explain entanglement without action at a distance. Later, Bell's theorem would suggest (in the opinion of most physicists and contrary to Einstein's assertion) that local hidden variables are impossible."en.wikipedia.org/wiki/Hidden_variable_theoryThere it is, in black and white. 'Truth' is, or at least should be, a definition of how much information is available to us. Why are you disputing this?
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Post by abacus9900 on Nov 30, 2010 18:16:00 GMT 1
The point about science is that it continually performs experiments which either support scientific theories or otherwise. Why is there a problem?
I will allow STA to comment on this idea since she is a professional physicist.
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Post by speakertoanimals on Nov 30, 2010 18:20:20 GMT 1
Yes, otherwise there would be little point! In one sense, a supposed theory that could not be applied to this situation and give a prediction of the result could be judged as useless before you started.
You don't have to 'understand' quantum theory to make a prediction, you just have to apply the same rules as you did making previous predictions. And if quantum theory was incapable of making any predictions (like the energy levels of the hydrogen atom), then why would anyone have ever bothered with it? (Lets leave aside the issue of string theory and the seeming inability of it to make any testable predictions.................).
As regards scientific truth -- whatever philsophers of science may be blathering on about, most working physicists take a much more pragmatic approach -- if a theory makes better predictions that a previous theory, and explains more than previous theories, then it is a better theory. Science does after all care about disproof rather than proof, but even so, a theory that explains more and has so far not been shown to give incorrect predictions could loosely be considered as being 'closer' to the truth than one which has been disproved! Most scientists are realists then who don't have much time for antirealists.
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Post by nickcosmosonde on Nov 30, 2010 18:37:35 GMT 1
It's a logical point about any axiomatic system, including but not limited to maths.
No, you could not. You could determine that a particular geometry does not apply, that's all. There will always remain an infinite number of possible other geometries that could equally as well predict your measurement.
There is no such thing as an abstract measurement. What are you measuring? How do you interpret your reading? In this case, how are you trying to measure "absolute time"? So that your measurement shows it doesn't exist?
Science is a great deal more than measurements. It's a great deal more than mathematics. It's fundamental feature is the creative use of freely constructed concepts to co-ordinate, determine, interpret, and direct such measurements and mathematical representations in an attempt to explain our observations. Now in this case our observations - our measurements - can only be of time intervals. We accept provisionally a theory that accounts for our observations that all such intervals are relative to comparative velocity. This theoretical explanatory scheme does not and can not in the least imply that time is therefore not absolute, beneath all our relative measurements. Rather, it would seem if we wish to remain Realists about our attempts to understand our observations, it would seem to imply just the opposite.
You are defining "classical" in contrast to "quantum" physics. There remains the possibility that with greater understanding what you take to be "quantum" is seen to be a special case of what you take to be "classical". Similarly, how you are defining "classical" in referring to our theory of gravity may also reaidcally change, once this "best theory" is superceded. It's highly probable, inductively, from the history of science so far, that neither of these defining terms will remain appropriate, and that both will be seen to be but partial views of a deeper structure yet to be understood.
Hence one at least and probably both of these theories is wrong.
The GUESS is that our understanding of what "quantum" means and refers to will remain the same or even translatable when these two cuirrently incompatible theories are eventually superceded. The same could be said for what you understand by "gravity".
Almost certainly it will be both.
You're erecting a straw man.
That's according to your definition of what "classical" means. Even accepting this, it remains logically possible that all such experimental results will be conformable to a wider broader deeper theory that in all respects satisfies what you take to be "classical". Sorry, that's just a logical truth.
Such as?
What you and others commonly take the "tests of Bell's Theorem" to show is excessively theory-laden. This happens time and time again in physics, especially when there is a paradigm crisis. Michelson-Morley is a classic case in point. It would take pages to try to get this well proven fact of scientific history to someone ignorant of philosophy of science.
Ahh, I wondered how long it would be before the old "I know more physics than you, therefore you're wrong" argument came along. This is so tiresome and childish. How is it that you have managed to obtain some sort of qualification to be a professional scientist when you display such ignorance of the valid rules of argument?
Go back to the playground.
All right, I'll amplify my statement, as it seems to send you into such a pre-adolescent confusion. David Bohm's Implicate Order theory explains all those "quantum" experimental results you refer to in a hypothetical framework that you would term "non-quantum". My point is that it is not for you or anyone else to arbitrarily dictate that this theory is mistaken just because it does not preserve the ontic reality of your interpretation of what "quantum" must mean. Whether it's mistaken or a better explanation than, say, the Copenhagen Interpretation, is a matter to be decided by experiment and observation or, failing that, by reason.
I don't think I'll lose, which is why I'd be happy to bet. By "classical" loophole what exactly do you mean? One that preserves locality, causality, spatio-temporal continuity, onti reality, what?
And? Why does this mean that Time and Space are not absolute? All it shows is that measurement of velocities is relative to an inertial frame. This is a modification of Newton's notions of Time and Space, not a contradiction. The standard realist interpretation of such differing measurements relative to observers is that the orientation of the time and space axes have been rotated. There remains some ground through which they have been rotated, SpaceTime. But they always remain orthogonal - the dimension of Time is distinct and invariant in itself and in relation to Space. This interpretation is perfectly concordant and comprehensive to all possible observations related to SR - as Minkowski logically demonstrated in 1906. It's also concordant with Lorentz's Theory of the Ether, an absolutist ontology, because the mathematical content of that theory if identical to Einstein's SR.
He agreed with Galileo. There was no reason not to.
This is seriously confused, and mistaken. Observations accord with Lorentzean relativity. Lorentz devised his transformation equations precisely in order to preserve absolute time and space, given the contravention of Galilean relativity implied by Clerk Maxwell. There is no difference mathematically between Einstein's SR and Lorentz's aether theory. Aghain, you're not appreciating how any measurement is and must be theory-laden - what it is taken to be a measurement of, what it means, is determined by the theory that directs you to make it.
For the simple reason that the role of this notion in the theory is not as an inductive generalisation, it's an axiom.
No, it's not. It requires elaboration of the theorems deduced from those axioms. Lorentz did this satisfactorily.
Yes, I remember this argument from our previous discussion. The simple fact is you're mistaken. The Twins paradox is a direct result of Einstein's 1905 Special Relativity paper. He explicitly states SR does not apply to accelerated or curvilinear motion. That's what's "special" about it. There are additional effects separating the twins due to GR acceleration effects, but these are additional to those predicted and implied by SR.
Read your physics. I suggest Einstein's SR paper, where he outlines the paradox in just these terms. Nothing to do with accelerated motion.
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Post by Progenitor A on Nov 30, 2010 18:40:12 GMT 1
Yes, otherwise there would be little point! ;D Aspect conducted his experiments to test the contradictions encapsulated in the EPR 'thought' experiment. He had no idea what the outcome of those experiments would be. To evreyone's surprise they confirmed 'spooky action at a distance'. That was not a prediction of QM, it was an inexplicable experimental result. In one sense, a supposed theory that could not be applied to this situation and give a prediction of the result could be judged as useless before you started. Your English is iincomprehensible here. You don't have to 'understand' quantum theory to make a prediction Precisely! Once a number of experiments have indicated an outcome, no matter how weird the outcome, then the outcome of similar experiments can be 'predicted' with unerring accuracy, just as I can predict with unerring accuracy that the sun will rise tomorrow The meaning of 'prediction' is lost, unfortunately And if quantum theory was incapable of making any predictions (like the energy levels of the hydrogen atom), then why would anyone have ever bothered with it? No-one has suggested that QM cannot make predictions in general. It is simply the case that no-one predicted the outcome of the EPR experiment (except that it had only one of two possible outcomes)
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