What makes schrodinger cat quantum?

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That is still not the complete story.

First of all, the SIZE has nothing to do with all of this. There are every indication to show that "macro"-sized objects can, in fact, exhibit quantum properties. The Delft/Stony Brook experiments involved 10^11 particles exhibiting such properties. We have seen this size gets progressively bigger all the time.

The issue here is how large of a length scale and how long of a time scale can one maintains the coherence of the system. That, to me, is the first and foremost fundamental criteria of observing quantum properties. It is why superconductivity plays a central role in this because no other system can show quantum phenomena in a clearer fashion at a macroscopic scale.

Now, having said that, at what point, and why, do we lose such observation? Decoherence? Sure, but even that isn't sufficient, or at best, incomplete, and this is NOT just from the observational point. It is also from the theoretical standpoint. We have seen that even ONE, single interaction can https://www.physicsforums.com/showpost.php?p=1498616&postcount=55". So it doesn't even require a gazzillion interactions, which would make it even infinitely WORSE to try and model.

Zz.
I agree that size isn't the whole story. Also, all of us will be familiar with the twentieth century version of Young's double slit experiment (I think courtesy of Albert Michelson) when very little interference lost the pure quantum state. It is of note that most of the large scale examples of superposition are in the low temperature arena; SQUIDS & super-conduction (of course the S in the abbreviation SQUID does stand for that). Moreover, if there was no chance of macro scale superposition, that is bad news for quantum computing. However, as has been mentioned already in this thread, thermal agitation is a potent destroyer of the pure quantum state & the cat at 310 K has very little chance (a cat in hell’s chance even) of staying in any pure quantum state.
 
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The challenge isn't about QM being valid at a large scale. The challenge was your decision that you CAN write THAT particular wavefunction out of thin air.
It is the most general wavefunction for an isolated system. Of course the assumption that the system can be isolated at all is not realistic. But that doesn't change anything qualitatively, as all you get is an entangled superposition with the rest of the universe.
 

ZapperZ

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It is the most general wavefunction for an isolated system. Of course the assumption that the system can be isolated at all is not realistic. But that doesn't change anything qualitatively, as all you get is an entangled superposition with the rest of the universe.
You have no justification at all for being able to write a wavefunction that can contain the COMPLETE description of the coin-tossing system. Just because you can write one, doesn't mean that it is appropriate for THAT system. You haven't given any such justification.

Furthermore, such wavefunction should give indications of the existence of such superposition, the same way we detect the coherence gap in the SQUID experiment AND the way we detect bonding-antibonding in chemical bonds. Where are they in YOUR system? This is something I asked for very early on, and something you completely side-stepped. Not only that, you then turned around and somehow admitted that there's no way to carry forth the unitary time evolution of QM into the classical realm of coin-tossing. Yet, this is RIGHT AFTER you had just written the wavefunction for it! To me, these are completely nonsensical and contradictory statements.

And why did it take you THIS long to respond to my last posting. The case has gone very cold!

Zz.

Zz.
 

ZapperZ

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"To me, when you CAN write such wavefunction then there are observational consequences that can be checked."ZapperZ

and what are those observational consequences might be?
Ask the person who wrote it for this coin-tossing system. I'd like to know as well.

The observational consequences for other quantum systems are well-known : bonding-antibonding, coherence gap in the Delft/Stony Brook experiments, etc.. etc. Read the Leggett paper that I had referred to on here.

Zz.
 

Hurkyl

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Can you point out to me the effect of superposition on coin tossing?
Have I misunderstood something? Isn't "the effect of superposition on coin tossing" supposed to be "quantum mechanics reproduces classical results"?

The thread partially reads as if you're trying to debunk people claiming that QM can reproduce classical observations by challenging them to show that classical observations aren't reproduced.
 
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ok look sir ZapperZ, i understand you are in this heated debate. but for me who just study the QM at the very surface, none of your 'evidence' (bonding-antibonding, coherence gap in the Delft/Stony Brook experiments) actually help my understanding.

for me to understand the implication of superposition, (a topic which happen to be introduced together with QM even in high school textbooks) I need a simple example that i can understand with ease in my current state of knowledge. I had a wishful thinking that by assuming superposition principle does apply in coin tossing, I can actually understood superposition and thus the essence that make a Schrodinger cat quantum instead of a just another coin tossing.
 

ZapperZ

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ok look sir ZapperZ, i understand you are in this heated debate. but for me who just study the QM at the very surface, none of your 'evidence' (bonding-antibonding, coherence gap in the Delft/Stony Brook experiments) actually help my understanding.
How was I to know what your level of "understanding" is? You asked, I answered. You are more than welcome to look them up yourself. If you are really just about to study QM at the very surface, then you should also consider that maybe, many of these things will require a bit more background prerequisites for it to sink in. Furthermore, if you do a search on here, I've posted many lengthy explanation on the implication of the Delft/Stony Brook SQUID experiments. So I'm not just throwing things out without making any effort at explaining why and how of the experiments.

And normally, I would, but I'm on vacation right now, and you'll understand why I don't wish to take a lot of time to make lengthy responses, especially when I've done it many times before.

for me to understand the implication of superposition, (a topic which happen to be introduced together with QM even in high school textbooks) I need a simple example that i can understand with ease in my current state of knowledge. I had a wishful thinking that by assuming superposition principle does apply in coin tossing, I can actually understood superposition and thus the essence that make a Schrodinger cat quantum instead of a just another coin tossing.
Unless someone can show me the effect of superposition on coin-tossing, the way we can with quantum systems, then claiming that coin-tossing can be described via QM wavefunction is unverified and unjustified. It is as simple as that.

Zz.
 

ZapperZ

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Have I misunderstood something? Isn't "the effect of superposition on coin tossing" supposed to be "quantum mechanics reproduces classical results"?

The thread partially reads as if you're trying to debunk people claiming that QM can reproduce classical observations by challenging them to show that classical observations aren't reproduced.
Under SOME conditions, QM can reproduce classical results (eg: harmonic oscillator). However, this is not true in general, and certainly hasn't been shown via First Principles in systems in which there are a gazillion couplings to the environment resulting in purely classical systems. It is why we still have a major debate on the classical-quantum transition. If this is a done deal, why bother with more studies on the mesoscopic scale studies? The Penrose suggestion on superposition with mirrors would have been a waste of time if this issue is settled!

Classical systems looks utterly different than QM systems. It exhibits no superpositions, no anti-correlation, no anti-bunching, etc. How does it gets that way from a starting point of QM description? We have no universal agreement on this issue, even if decoherence is the leading candidate. Do YOU know of any? And do you actually find nothing wrong with that "wavefunction" for coin-tossing, which is the major part of my disagreement in this thread?

Zz.
 

Hurkyl

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Under SOME conditions, QM can reproduce classical results (eg: harmonic oscillator). However, this is not true in general, and certainly hasn't been shown via First Principles in systems in which there are a gazillion couplings to the environment resulting in purely classical systems.
...
Classical systems looks utterly different than QM systems.
You didn't mean that literally, did you? Is there really an (empirically verified) classical result for which is known that it cannot be reproduced by QM?

I'm going to assume not -- that you just meant that it is just not known whether QM reproduces classical results. However, settling that is a matter of theory not experiment. (put differently, we already have the experimental results for this question)


And do you actually find nothing wrong with that "wavefunction" for coin-tossing
The relative state of the coin-dog system is surely going to be a mixed state, not the pure state that was proposed.

I suppose it's a little optimistic to assume the coin system and the dog system are crisply defined and disjoint, and that all of the states of those systems are cleanly categorizable into head/tail and alive/dead.

But other than that, I confess it looks like the natural conclusion from the hypothesis that quantum mechanics is valid on these scales.
 
Unless someone can show me the effect of superposition on coin-tossing, the way we can with quantum systems, then claiming that coin-tossing can be described via QM wavefunction is unverified and unjustified. It is as simple as that.
How else would you describe it? The wavefunction I wrote down earlier in the thread was in a hypothetical setting where you would be able to keep everything in the box isolated from the environment. But, in general, one can assume that it is entangled with the rest of the universe.

A classical description is ruled out a priori, because classical mechanics is known to be false. Pretending that the generic state does not involve some complicated superposition (involving the coin and the rest of the universe), is very strange.


The effects of any such superposition must be the same as what classical mecanbics predicts, just like General relativity reduces to classical mechanics at weak fields and low speeds. It doesn't mean that at low velocities and weak fields particles do not move along geodesics.

In case of quantum mechanics, the classical limit is a far more complicated issue, one of the reasons being precisely that you do not get rid of global superpositions in the classical limit.
 
Consider a universe containing one or more coins. For one particular coin we can using some convention say that it is in the "tails" or "heads" state. So, there exists an observable A with eigenstates |heads> and |tails>. We can then find a complete set of commuting observables that includes A, for the whole universe. This requires extending A so that the result of an observation of the coin can include the result that the coinn isn't actually there.

This then amounts to expanding the wavefunction in the form:


|psi> = |psi_1>|head> + |psi_2>|tail> + |psi_3>|no coin>

where the |psi_i> contains the information you need in order to specify the state of the universe apart from specifying that the coin is/isn't there and if it is there the head/tail state, so it includes the state of the atoms in the coin as well.

If the initial state is |psi>|head> then under time evolution, the state will evolve to be in some superposition of |head>, |tail>, unless the information present in |psi> yields a zero amplitude for one of the head or tail sectors. One can argue that this is unlikely if the initial state contains a coin thrower who will decide to throw the coin a few days in the future. You would then expect that the exact way he will throw the coin a few days later will not be sharply enough defined for the outcome to be deterministic.
 
If you are really just about to study QM at the very surface, then you should also consider that maybe, many of these things will require a bit more background prerequisites for it to sink in.
I do consider that and my knowledge will eventually evolve be at that state. But for now I am not and I hope through this forum I could achieve firm basic understanding of the foundational principle of quantum mechanic, such as superposition, with my current technical and mathematical skill.

So far, solving all the pde for 1D quantum system and hydrogen atom and doing that in terms of operators or Hilbert space or matrix have not actually help my basic understanding. Or at least I feel, or as you can see, I am a bit lost in the interpretation.
 
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I do consider that and my knowledge will eventually evolve be at that state. But for now I am not and I hope through this forum I could achieve firm basic understanding of the foundational principle of quantum mechanic, such as superposition, with my current technical and mathematical skill.

So far, solving all the pde for 1D quantum system and hydrogen atom and doing that in terms of operators or Hilbert space or matrix have not actually help my basic understanding. Or at least I feel, or as you can see, I am a bit lost in the interpretation.
ArielGenesis, no one understands quantum mechanics. Stick with the math which gives numerous predictions that have been tested and never falsified. Forget about trying to understand what it all really means (at least for now). It will drive you crazy. This thread is an example.
 
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ZapperZ

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Consider a universe containing one or more coins. For one particular coin we can using some convention say that it is in the "tails" or "heads" state. So, there exists an observable A with eigenstates |heads> and |tails>. We can then find a complete set of commuting observables that includes A, for the whole universe. This requires extending A so that the result of an observation of the coin can include the result that the coinn isn't actually there.

This then amounts to expanding the wavefunction in the form:


|psi> = |psi_1>|head> + |psi_2>|tail> + |psi_3>|no coin>

where the |psi_i> contains the information you need in order to specify the state of the universe apart from specifying that the coin is/isn't there and if it is there the head/tail state, so it includes the state of the atoms in the coin as well.

If the initial state is |psi>|head> then under time evolution, the state will evolve to be in some superposition of |head>, |tail>, unless the information present in |psi> yields a zero amplitude for one of the head or tail sectors. One can argue that this is unlikely if the initial state contains a coin thrower who will decide to throw the coin a few days in the future. You would then expect that the exact way he will throw the coin a few days later will not be sharply enough defined for the outcome to be deterministic.
Then show the effect of such superposition to VALIDATE that such superpostion exists.

You will note that not only are many measurements that we have validates such superposition, all this rigorous theoretical and experimental verification were done JUST to show the existence of such superposition. In other words, these were not simply accepted just because theory, or someone, says so!

So we know that by looking at the coin itself will NOT tell you that before the act of measurement, the coin was in a superposition of state. Come up with a measurement similar to the coherent gap in the SQUID supercurrent, or the bonding-antibonding in chemical bonds, that clearly show the effect of such superposition. Till then, you wavefuction is a speculation.

Zz.
 
Then show the effect of such superposition to VALIDATE that such superpostion exists.

I think this is an unreasonable demand, because what you are effectively asking is a deviation from classical theory at the macro level which one would not expect to exist, except in exceptional cases.
And the seemingly exceptional case of the flux qubit is actually not as macroscopic as it seems to be, as explained here:

http://arxiv.org/abs/quant-ph/0609007

ZapperZ's demand is a bit like someone making the claim that objects moving under gravity always move along geodesics and not according to a Newtonian gravitational potential, even in cases where gravity is very weak. Then a ZapperZ like sceptic comes along demanding experimental proof for that in the weak gravity limit.

Clearly in the case of general relativity, this would be unreasonable because the idea of a dualistic description of Nature is a priori not seen to be tenable. In case of quantum mechanics, however, this is atitude is different, because quantum mechanics is tradionally formulated as a dualistic theory in which classical concepts play an important role.

But then, this is an effective formulation of a true quantum theory at best. The classical world has to arise from quantum dynamics, the precise way in which that happens is still under discussion. But this lack of understanding does not always preclude one from setting up a general argument, like in this case one that shows that a superposition exists in the macro-world.
 

Demystifier

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ArielGenesis, it seems to me nobody answered your question in a way that would satisfy you. So let me try.

How do we know that Schrodinger cat is quantum and not classical? From the experimental point of view, the only way to determine whether it is quantum or classical is to try to measure interference. If there is interference, then it is quantum. If there is no interference, then it is either truly classical or apparently classical due to decoherence that destroys interference.

Now, the fact is that there is no interference. This is actually expected, because the theory predicts that decoherence should take place for macroscopic objects such as cats. To conclude, from the experimental point of view it appears classical, but from the theoretical point of view there is a good reason to believe that it is actually quantum. In fact, from the theoretical point of view everything is believed to be quantum, including coins.
 

ZapperZ

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I think this is an unreasonable demand, because what you are effectively asking is a deviation from classical theory at the macro level which one would not expect to exist, except in exceptional cases.
And the seemingly exceptional case of the flux qubit is actually not as macroscopic as it seems to be, as explained here:

http://arxiv.org/abs/quant-ph/0609007

ZapperZ's demand is a bit like someone making the claim that objects moving under gravity always move along geodesics and not according to a Newtonian gravitational potential, even in cases where gravity is very weak. Then a ZapperZ like sceptic comes along demanding experimental proof for that in the weak gravity limit.

Clearly in the case of general relativity, this would be unreasonable because the idea of a dualistic description of Nature is a priori not seen to be tenable. In case of quantum mechanics, however, this is atitude is different, because quantum mechanics is tradionally formulated as a dualistic theory in which classical concepts play an important role.

But then, this is an effective formulation of a true quantum theory at best. The classical world has to arise from quantum dynamics, the precise way in which that happens is still under discussion. But this lack of understanding does not always preclude one from setting up a general argument, like in this case one that shows that a superposition exists in the macro-world.
Then you are also arguing that all of our current efforts at trying to detect QM effects at the macro scale is worthless because there's nothing to prove here.

What you are trying to hide is the FACT that you cannot show such effects. Again, using size as the excuse for not being able to show such a thing doesn't wash anymore.

There is a continual work on testing the "limits" of QM and testing to what extent QM effects can be detected with not only large scale, but also longer time scale. This would be a silly thing to do if this is a done deal. The FACT that work, both experimental and theoretical, continues in this area is MY proof that there's a lot more to understand and to VERIFY the question on at what point to we lose the "weird" effects of QM.

And let me correct the misconception here. I have NEVER, ever said that QM is wrong, and that it is wrong at the classical scale. I had always stressed that both the QM world and the classical world look DIFFERENT, and at some point, the QM effects are no longer evident. That's why your writing of the wavefunction of a classical system is bogus in my book, because such a system cannot be tested, whereas the waverfuction for QM system can!

And since you tried to paint a caricature of me, I'll do the same of you. Your point is similar to someone who claims that the Higgs is already a done deal, since based on the Standard Model, it should be there, and the Standard Model is such a successful theory that there's no reason to argue that cannot be found. So asking for experimental verification of such a thing is unreasonable.

The FACT remains that you made an untestable speculation.

So there!

Zz.
 
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I do consider that and my knowledge will eventually evolve be at that state. But for now I am not and I hope through this forum I could achieve firm basic understanding of the foundational principle of quantum mechanic, such as superposition, with my current technical and mathematical skill.

So far, solving all the pde for 1D quantum system and hydrogen atom and doing that in terms of operators or Hilbert space or matrix have not actually help my basic understanding. Or at least I feel, or as you can see, I am a bit lost in the interpretation.
I’m going to stick my neck out here and offer ArielGenesis what I think is an acceptable handle on quantum mechanics. As a disclaimer I agree with the succeeding comment by SW VandeCarr that essentially QM is a mathematical theory that gives precise predictions. Therefore any interpretation is just that and as such is a mental construct to make sense of what appears counterintuitive.

My interpretation is that since the mathematical formulation of QM is based on interacting wave equations and since wave interference is experimentally verified, the basic building blocks (the quanta) are wave packets and not point particles. Moreover, my additional suggestion is that any particle associated with this quantum wave is an illusion created by our experimental design. When we detect a particle we have no idea what this particle is except for an interaction between something, particle or wave & our detecting apparatus. What we see is a blip on a screen, or a tract on a photographic film or even, now, a computer generated graphic but what we don’t see is an actual particle.

Furthermore, nature gives us the result we are looking for. If you design an experiment to look for presumed particles that is usually what you find. The problem comes when you try to ask where exactly is the point particle because it doesn’t exist. Of course we do detect something and since we are often dealing with single quanta we only get one interaction. If we repeat the process we may find the particle somewhere else but it isn’t it’s just a different aspect of the same wave.

For those of you who like experimental verification for any outlandish idea, can I say that a recent experiment trying to show interference and particles at the same time would back up my interpretation. I say my interpretation but I have to admit that there is one paragraph in ‘The Brief History of Time’ by Prof Stephen Hawking that would suggest he beat me to this idea by decades
 
Thank you to SW VandeCarr, Demistifier and Adrian59, really appreciate that.

So a particle is basically a solution to a Schrodinger equation, which is a wave or a wave function. It simply for some strange experimental reasons that what we observe is actually a blimp on a screen which looks more like a particle instead of a wave.

And the only thing that shows that this wave is a probability amplitude wave is the interference effect which usually vanishes via de-coherence in a large scale or a long run.

I hope that I am not asking too much but what is the simplest example regarding probabilistic interference. I don't really care if it exist or possible or simply theoretical or speculative or purely mathematical with no physical meaning. I just want to get the concept.

Thank you again to three of you, really appreciate that.
 
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The simplest example of probabilistic interference is the double slit experiment.
 
Then you are also arguing that all of our current efforts at trying to detect QM effects at the macro scale is worthless because there's nothing to prove here.

What you are trying to hide is the FACT that you cannot show such effects. Again, using size as the excuse for not being able to show such a thing doesn't wash anymore.

There is a continual work on testing the "limits" of QM and testing to what extent QM effects can be detected with not only large scale, but also longer time scale. This would be a silly thing to do if this is a done deal. The FACT that work, both experimental and theoretical, continues in this area is MY proof that there's a lot more to understand and to VERIFY the question on at what point to we lose the "weird" effects of QM.

And let me correct the misconception here. I have NEVER, ever said that QM is wrong, and that it is wrong at the classical scale. I had always stressed that both the QM world and the classical world look DIFFERENT, and at some point, the QM effects are no longer evident. That's why your writing of the wavefunction of a classical system is bogus in my book, because such a system cannot be tested, whereas the waverfuction for QM system can!

And since you tried to paint a caricature of me, I'll do the same of you. Your point is similar to someone who claims that the Higgs is already a done deal, since based on the Standard Model, it should be there, and the Standard Model is such a successful theory that there's no reason to argue that cannot be found. So asking for experimental verification of such a thing is unreasonable.

The FACT remains that you made an untestable speculation.

So there!

Zz.
Quantum effects at the macro scale would shed light on decoherence mechanisms, it would potentially have a lot of applications like in quantum communications, quantum computing, quantum metrology, etc.

You would expect that size does matter as far as the decoherence rate is concerned. Numerous articles point that out. That doesn't mean that you could not somehow be able to make a device that is able to circumvent decoherence effects to some extent. I think that most people who work in this field will agree that macroscopic quantum superpostions are very fragile in the sense that they'll decohere very fast if not kept isolated from the envirmonment.

In case of the Higgs, there are alternative Higgsless theories. In case of quantum mechanics, all we have is a suggestion by Penrose that gravity may cause a real state reduction.
 

ZapperZ

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Quantum effects at the macro scale would shed light on decoherence mechanisms, it would potentially have a lot of applications like in quantum communications, quantum computing, quantum metrology, etc.

You would expect that size does matter as far as the decoherence rate is concerned. Numerous articles point that out. That doesn't mean that you could not somehow be able to make a device that is able to circumvent decoherence effects to some extent. I think that most people who work in this field will agree that macroscopic quantum superpostions are very fragile in the sense that they'll decohere very fast if not kept isolated from the envirmonment.

In case of the Higgs, there are alternative Higgsless theories. In case of quantum mechanics, all we have is a suggestion by Penrose that gravity may cause a real state reduction.
Read again! I said that there are indications that SIZE DOESN'T MATTER.

And the alternative to decoherence has been mentioned several times already beyond Penrose's model. Read about the coarse-grained effect of our measurement that I've cited several times already!

Zz.
 
I see, double slit experiment for one particle at a time shows interference for a probability wavelet. I understand this experiment quiet well but still I wish for something simpler.

One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result? But I could also argue that doing double slit interference in non vacuum will decohere the wavelet too?
 
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One last question, I am always wondering, what if I do a double slit interference in a cloud chamber, will we get different result? But I could also argue that doing double slit interference in non vacuum will decohere the wavelet too?
A good question which deserves an answer. Of course the get out of jail answer would be that since the cloud chamber reveals the paths taken by sub-atomic by interaction between the particle and the cloud chamber molecules one suspects as you yourself conclude that the pure quantum situation will decohere. The more challenging problem would be what would you see if you could accurately view the paths taken through the apparatus without decoherence. My hunch is that the particle would go through one slit but interference would still occur. As mentioned in one of my previous posts there is some experimental evidence that you can get both interference and particle behaviour together but alas I've lost the reference.
 
It's been proven that hidden-variable theories can't properly describe reality - Bell's theorem, I think, and associated experiments. I don't remember the details right now...
Studies have shown that LOCAL hidden variable theories can't properly describe the full reality (well, there are actually some assumptions made which may not turn out as expected). It has NOT been shown that NON-LOCAL hidden variable theories cannot fully account for it. (This is the Bohm interpretation.)

The Schrödinger's cat thought experiment was actually devised as a sort of reduction ad absurdum to demonstrate problems with the Copenhagen interpretation of quantum mechanics, and was not intended as a true description of reality.

http://en.wikipedia.org/wiki/Bell's_theorem
http://en.wikipedia.org/wiki/Loopholes_in_Bell_test_experiments
http://en.wikipedia.org/wiki/Bohm_interpretation
http://en.wikipedia.org/wiki/Schrodinger's_cat
 

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