What makes schrodinger cat quantum?

[Count Iblis]

But you should never provide the misinformation that (i) this is an accepted idea in standard physics and, more importantly, (ii) you have experimental evidence to back it up.

Ok, but it is more accurate to say that the converse, i.e. that the wavefunction does "really" collapse, is not accepted either.

In my original post, I did write that:

Then the state of the box is not a classical state but we cannot do any experment in practice to demonstrate the non classical features of the superposition. That inability cannot be used to argue that the state is classical.

Apart form the last sentence, this is what the quoted article says.

 

[ZapperZ]

Ok, but it is more accurate to say that the converse, i.e. that the wavefunction does "really" collapse, is not accepted either.

Would you like to re-read what I posted originally that you responded to, and tell me where the issue of "really collapse" was brought up by me?

What you did instead is to construct a non-verified, highly speculative "wavefunction" for coin tossing. When I asked for experimental justification for that (meaning, experimental evidence), for some ODD reason, you gave me more theoretical arguments. Can't you see how silly this has been so far? This has been a very dumb game, and I'm the last person you want to try this on.

I have zero clue on what you are arguing, or what that has anything to do with what I said. I'm guessing that you are battling a different battle and somehow hooked your baggage onto this one. Don't do that!

Classical state is classical state! Until we know more about how quantum states evolve into classical world, you, the authors of that "preprint", and anyone else can't simply make such outlandish claim. It only reveals how little you pay attention to how we consider something to be valid in physics.

Zz.

 

[Count Iblis]

Well, the subject under discussion in this thread is "what makes Schrodinger's cat quantum", and I was just arguing along the lines of my first reply, i.e. from the point of view that there doesn't exist such a thing as a "classical world" to begin with. The world is fundamentally quantum mechanical and any classical physics can only arise as an approximation in the "classical limit".


This view may be controversial, but the whole point of the "Schrödinger cat" thought experiment, is to expose potential difficulties with this view, namely that the superpositions do not really go away for macroscopic systems.


I.m.o., the way to make experimental progress is to do experiments designed to measure violations of unitary time evolution like in the proposed expriment using a small mirror in a superposition.


How can it be reasonable to demand that to verify a theory one has to demostrate the non classical features of a system for which the theory itself implies that such effects cannot be detected for all practical purposes?

You were the one who brought up coin throws and stated that they were classical. You presumably meant that to be in an effective instrumental sense, but that's surely not in the spirit of this threat, because the same could be said of Schrödinger's cat itself.

 

[ZapperZ]

Well, the subject under discussion in this thread is "what makes Schrodinger's cat quantum", and I was just arguing along the lines of my first reply, i.e. from the point of view that there doesn't exist such a thing as a "classical world" to begin with. The world is fundamentally quantum mechanical and any classical physics can only arise as an approximation in the "classical limit".This view may be controversial, but the whole point of the "Schrödinger cat" thought experiment, is to expose potential difficulties with this view, namely that the superpositions do not really go away for macroscopic systems.I.m.o., the way to make experimental progress is to do experiments designed to measure violations of unitary time evolution like in the proposed expriment using a small mirror in a superposition.How can it be reasonable to demand that to verify a theory one has to demostrate the non classical features of a system for which the theory itself implies that such effects cannot be detected for all practical purposes?

You were the one who brought up coin throws and stated that they were classical. You presumably meant that to be in an effective instrumental sense, but that's surely not in the spirit of this threat, because the same could be said of Schrödinger's cat itself.

But you have never provided any means of showing that such "unitary evolution" can be carried out up to the classical limit AND actually derive such classical state!

In fact, one doesn't HAVE to go even that far to start "seeing" a destruction of the QM description. It has been shown that even ONE single interaction can do that!

https://www.physicsforums.com/showpost.php?p=1498616&postcount=55

I definitely brought up the coin throw AND insisted that they are classical. I have plenty of experimental evidence to back it up. Your assertion was the one without a single shred of experimental evidence. When I asked for it, you gave something else. That is what I find highly annoying and deceptive.

When you can show either (i) superposition in coin tossing OR (ii) a detailed derivation from QM principles that produce the classical phoenomena of coin tossing, then you will have a leg to stand on to make such argument. But not a single single second before. What you have done is to make an unsubstantiated, speculative assertion, which violates our Guidelines.

Zz.

 

[SW VandeCarr]

I'm not so sure this is the case.

For example, if I, instead toss a coin, and heads means I kill the cat, and tails means the cat lives, would this be equivalent to the Schrodinger Cat scenario? It isn't.

Zz.

I don't think a coin toss is equivalent of the Schrodinger Cat experiment (SCE) either, but for an additional reason: the result of a coin toss is, in principle, not a random outcome. It may be the case that the only true example of a random outcome is the realization of a quantum state.

We can argue forever about the mixed state of the cat in a box. It hasn't gotten us anywhere since the 1920's. I'm arguing that the real significance of the SCE is that it makes the case that strict determinism is not a true description of the world insofar as the world can be modeled by science. I've stated my reasoning in my two previous posts here (#36 and 38).

 

[Count Iblis]

But you have never provided any means of showing that such "unitary evolution" can be carried out up to the classical limit AND actually derive such classical state!

That is covered in numerous papers, e.g.:

http://arxiv.org/abs/quant-ph/0006117

http://arxiv.org/abs/quant-ph/0204129

http://arxiv.org/abs/quant-ph/0205108

http://arxiv.org/abs/quant-ph/0612118

 

[ZapperZ]

That is covered in numerous papers, e.g.:

http://arxiv.org/abs/quant-ph/0006117

http://arxiv.org/abs/quant-ph/0204129

http://arxiv.org/abs/quant-ph/0205108

http://arxiv.org/abs/quant-ph/0612118

Wonderful! Now derive the coin-tossing scenario! That shouldn't bee too difficult now, should it? After all, you have all the "tools" here that you need!

{For the final time, references to ArXiv article are NOT sufficient or recommended as a valid citation. Please find out where these preprints were published and use those as references}

Zz.

 

[ZapperZ]

I don't think a coin toss is equivalent of the Schrodinger Cat experiment (SCE) either, but for an additional reason: the result of a coin toss is, in principle, not a random outcome. It may be the case that the only true example of a random outcome is the realization of a quantum state.

We can argue forever about the mixed state of the cat in a box. It hasn't gotten us anywhere since the 1920's. I'm arguing that the real significance of the SCE is that it makes the case that strict determinism is not a true description of the world insofar as the world can be modeled by science. I've stated my reasoning in my two previous posts here (#36 and 38).

The problem here is that you're making an argument based on a matter of TASTES. I made an argument based on available empirical observations. The FACT that we have no "weird quantum-like phenomena" at the classical scale is my evidence. Despite what has been claimed about coin tossing, we have never, ever detected the effects of superposition of states of [head,tail]. We have, however, detected effects of superposition when 10^11 supercurrent flows simultaneously in two opposite directions.

Experimental evidence trumps any and all theoretical assertions.

Zz.

 

[SW VandeCarr]

The problem here is that you're making an argument based on a matter of TASTES. I made an argument based on available empirical observations. The FACT that we have no "weird quantum-like phenomena" at the classical scale is my evidence. Despite what has been claimed about coin tossing, we have never, ever detected the effects of superposition of states of [head,tail]. We have, however, detected effects of superposition when 10^11 supercurrent flows simultaneously in two opposite directions.

Experimental evidence trumps any and all theoretical assertions.

Zz.

I'm not sure I'm disagreeing with you. Perhaps you misunderstood my post. I'm simply saying that regardless of how decoherence actually operates, we obtain random outcomes in observing quantum states. Other outcomes in the classical world can be made contingent (by us) to these random outcomes such that they too cannot be predicted in principle (such as SCE). This is an argument against strict determinism even though QM is a deterministic model. I don't see how this is a matter of taste. I said the outcome of a coin toss is not, in principle, a random event (unless it could be demonstrated to be dependent on the realization of a quantum state).

 

[canoe]

Maybe, maybe not.

While decoherence may play a significant role in the quantum-to-classical transition, there are several other possible explanations. Some of them are not necessarily in competition with decoherence. For example, a recent paper has shown that our "coarse-grained" measurement of such a system can already cause an onset of classical measurements. Read my entry on this:

https://www.physicsforums.com/showthread.php?p=1520644

This issue of classical-quantum separation isn't a done deal, not by a long shot. We simply cannot make any definitive statement about how one evolve into another because, in physics, as it should be, we need to be able to empirically verify them, and verify them to our satisfaction. There's a lot still to be done before we get even close to that.

Zz.

An interesting read...

And of course you are entirely correct on the transitory nature of theories. Yet at any moment in time a particular principle or theory prevails. At this point in time I believe that the principle of decoherence has gained favor. I will initiate a new post based on my particular question.

 

[Adrian59]

The problem here is that you're making an argument based on a matter of TASTES. I made an argument based on available empirical observations. The FACT that we have no "weird quantum-like phenomena" at the classical scale is my evidence. Despite what has been claimed about coin tossing, we have never, ever detected the effects of superposition of states of [head,tail]. We have, however, detected effects of superposition when 10^11 supercurrent flows simultaneously in two opposite directions.

Experimental evidence trumps any and all theoretical assertions.

Zz.

It appears you are moving towards a consensus despite a mild disagreement about the experimental evidence. I couldn't agree more with the last line of the above quote however one shouldn't lose sight of the fact that this thread is all about a thought experiment so that other thought experimental points should be valid - even the great Steven Weinberg used a head / tails analogy in his Dirac memorial lecture (Cambridge University Press). Also as has been commented, QM is deterministic at the wave function level. It is when we essentially ask binary questions like is the cat dead or alive that QM appears probablistic. I suspect that the cat is classical and though not actually shielded from decoherance as commented by another contributor we are mentally delaying the discovery of its fate so that in effect we're delaying the point where we discover decoherance has taken place.

 

[Count Iblis]

Decoherence is a red herring here, as all it will do is make a superposition that is confined to the box to transform into a global superposition.

I strongly disgree with ZapperZ's denialism by hiding behind what can be directly measured in experiments. It should be clear that you won't be able to detect superpositions involving heads and tails, just like you won't be able to verify that Nature is really time symmetric at the macro level.

I.e. I take a gram of salt and poor that into a litre of water. This process is reversible at the micro-level given all we know about physics. Why then does ZapperZ not dispute this also by demanding a direct experimental verification?

 

[ZapperZ]

Decoherence is a red herring here, as all it will do is make a superposition that is confined to the box to transform into a global superposition.

I strongly disgree with ZapperZ's denialism by hiding behind what can be directly measured in experiments. It should be clear that you won't be able to detect superpositions involving heads and tails, just like you won't be able to verify that Nature is really time symmetric at the macro level.

I.e. I take a gram of salt and poor that into a litre of water. This process is reversible at the micro-level given all we know about physics. Why then does ZapperZ not dispute this also by demanding a direct experimental verification?

But you CAN detect such superposition! That's the whole point of Leggett's paper, the existence of the coherence gap in the Delft/Stony Brook experiments. Did you even READ those papers? It is NOT just MY interpretation of it.

If you disagree, then write a rebuttal to Leggett's paper and to those two experimental paper and tell them that the coherence gap has NOTHING to do with the superposition of the supercurrent flow! Till then, your opinion on here really has zero value.

Zz.

 

[Count Iblis]

But you CAN detect such superposition! That's the whole point of Leggett's paper, the existence of the coherence gap in the Delft/Stony Brook experiments. Did you even READ those papers? It is NOT just MY interpretation of it.

If you disagree, then write a rebuttal to Leggett's paper and to those two experimental paper and tell them that the coherence gap has NOTHING to do with the superposition of the supercurrent flow! Till then, your opinion on here really has zero value.

Zz.

The superposition of the current flowing in the two direction is protected from decoherence. I think the typical decoherence time here is of the order of 10^(-7) seconds or so. This is what makes it detectable. One can use this to make qubits and implement the necessary quantum logic gates to implement quantum computations.


Clearly, there is no way you could do all of this using coins. But that then doesn't mean that the formalism of quantum mechanics, which generically lead to superpositions, is invalid. It is simply that in case of cpoins, the environment will "see" the difference between the two terms in the superposition, i.e. it will evolve differently and thus you'll have a superposition that involves the coin plus environment.

To experimentally prove that this picture is not correct and wavefunctions do really collapse as opposed to the system getting more and more entangled with the rest of the universe, one has to find evidence for non-unitary time evolution. That means that we need experimental results that fail to show effects of superpositions where theory predicts they should be detectable.

The proposed experiment with the mirror in a superposition is exactly such an experiment. If it can be carried out, one can test if Penrose is right and that you have gravitational state reduction or perhaps state reduction due to some other unknown mechanism.

But, lacking any such experimental evidence, I don't see why we have to a priori assume that a non-unitary state reduction really happens.

 

[ZapperZ]

This is getting silly. Since you really want to examine this carefully, let's look at exactly what you said, OK?

I strongly disgree with ZapperZ's denialism by hiding behind what can be directly measured in experiments. It should be clear that you won't be able to detect superpositions involving heads and tails, just like you won't be able to verify that Nature is really time symmetric at the macro level.

Let's tackle the last part. We can't verify that Nature is really time symmetric at the "macro level"? Really? And Nature can be verified to be "symmetric" at the "micro level" instead? You wouldn't go nuts if I show broken time symmetry processes at the micro level then?

Next, "... It should be clear that you won't be able to detect superpositions involving heads and tails..."

That actually, should be the END of the discussion. You are saying you can't detect it, which is something I had ASKED repeatedly. However, you continue with some theoretical argument that such lack of detection somehow is irrelevant. Really? Since when? And this is in addition of you admitting that "... there is no way you could do all of this using coins..." So you can't show it, but you BELIEVE in it! Just listen to yourself!

Lacking any such experimental evidence somehow gives you the license to make the speculation, which is rather strange considering that typically, the lack of experimental evidence means that you CANNOT make such speculation. Lacking the ability to show how such unitary time evolution can be done for coin flipping, AND observing that no kind of superposition effect has EVER been detected in such a system, you chose to believe that it is still valid until it is proven to be false! In other words, there are a gazillion angels on a pinhead until someone shows you that it is false!

I am so done with this weird sense of logic.

Zz.

 

[SW VandeCarr]

Decoherence is a red herring here, as all it will do is make a superposition that is confined to the box to transform into a global superposition.

you won't be able to detect superpositions involving heads and tails, just like you won't be able to verify that Nature is really time symmetric at the macro level.

I don't want to get into the middle of this discussion except to say I don't think a heads/tails superposition is a strong argument. Coin tossing appears to be a chaotic (deterministic) process in that it's extremely sensitive to initial conditions. Snedecor and Cochran give a good example in their textbook "Statistical Methods" (I think they still have it in their latest addition. Mine is fairly old).

Essentially, plot H or T results of trials along the x-axis of an x,y plot starting with the 0th trial at the origin. Add 1 on the y-axis for each sequential toss if heads and subtract 1 if tails. (So for three tosses: 1+1-1: y= 1 etc.) Each set of n trials will have, with increasingly high probability as n grows large, a unique plot. Moreover, the plot will be predominantly above or below the x-axis for a given set. Overall the plots of individual sets will tend to diverge as you would expect in a chaotic process.

 

[Count Iblis]

See my reply here:


https://www.physicsforums.com/showpost.php?p=2241666&postcount=64


You need to have at least some theoretical framework to interpret experimental results, otherwise the experiments are pointless. Then, if you have different theories you can do experiments to decide which one is right. Or you could have a single theory and try to falsify that.


Suppose we have a theory that is not falsified which tells us something about the universe that is difficult to directly verify and in some cases impossible to verify. That difficulty is then consistent with the theory itself. Where it can be verified, it has been verified (otherwise the theory would already been falsified).

Then, lacking well motivated reasons based on physics to doubt the validity of the theory in the regime where we can't verify that aspect of the theory, why would we doubt it?

E.g., why not doubt whether in the interior of Zeta Riticuli, the laws of physics operate as we think it does? Unless there are good theoretical reasons to believe that, the mere lack of direct experimental verification is not a good reason for such doubt.

 

[ZapperZ]

See my reply here:


https://www.physicsforums.com/showpost.php?p=2241666&postcount=64


You need to have at least some theoretical framework to interpret experimental results, otherwise the experiments are pointless. Then, if you have different theories you can do experiments to decide which one is right. Or you could have a single theory and try to falsify that.

Then use a theoretical framework to interpret the result of a coin toss and SHOW ME the effect of superposition!

You will NOTE that I used such theoretical framework as developed by Leggett to interpret the existence of the coherent gap in the SQUID experiment as signifying the presence of superposition of the supercurrent. Now do the SAME for the coin toss.

If you can't, then you've made an empty statement with zero justification.

Zz.

 

[Adrian59]

My last comment was an attempt to get back to the starting question in this thread. However since there still is some debate on the validity of the coin toss, can I suggest a compromise. We all know that a coin is essentially a classical object but since it is extremely difficult to measure all its initial conditions; it behaves like a random event and as such we could accept it as a surrogate marker for one.

The converse is illuminating, since we are accepting as read that QM is inherently random, though it may only be random because we haven’t yet discovered a more fundamental theory. So in effect both events (coins & QM) may be considered random in the absence of a more complete understanding of the underlying mechanics.

Finally, those who don’t like tossing coins can always think of measuring spin whenever coins are mentioned – that’s a pure QM binary event & exhibits superposition. Of course a classical object like a coin won’t exhibit superposition which brings me back to the cat which I, also, do not think will exhibit superposition. The cat is in a multiple decohered state as any large multi-cellular being must be and Schrodinger’s thought experiment is just a very clever way of challenging our belief that QM is probabilistic.

 

[SW VandeCarr]

We all know that a coin is essentially a classical object but since it is extremely difficult to measure all its initial conditions; it behaves like a random event and as such we could accept it as a surrogate marker for one.

I agree with one caveat. Chaos Theory is a classical deterministic theory (with no allowance for superpositions). QM is also deterministic in terms of predicting probabilities. Given our current state of knowledge, QM is the only source of "true" randomness (under strictly calculated probability distributions). No doubt there are a host of classical phenomena which are effectively random for most practical purposes. I don't see why tossing coins should be singled out as having some strange kind of superposition. Coins are, after all, manufactured objects and no two coins are exactly alike.

 

[ArielGenesis]

I am sorry but I think it helps me if I ask the question this way:

Assume that coin tossing is quantum and super position applies.

What should we expected as the result of that assumption?

in other words: beside the absence of evidence, what is the proof that coin tossing is not quantum, while Schrodinger cat is.

 

[SW VandeCarr]

I am sorry but I think it helps me if I ask the question this way:

Assume that coin tossing is quantum and super position applies.

What should we expected as the result of that assumption?

in other words: beside the absence of evidence, what is the proof that coin tossing is not quantum, while Schrodinger cat is.

I would simply say that coins are not quantum "objects". Every electron is exactly like every other electron. Every uranium atom is exactly like every other uranium atom. That's why it's impossible to detect which atom will be observed to decay next and when it will do so. Every U238/U235 atom is assumed in QM to be in a superposition of the lighter and heavier isotope at least until it decays (and actually afterward as well since time is symmetric at the quantum level). That's the weirdness of QM. I've just been converted to the statistical (ensemble) interpretation of QM so I don't worry about this anymore. Suffice to say that coins, while they can in theory be described by a wave function, are not quantum 'objects'. Every coin is unique. (And never ask anyone to prove a negative.)

 

[Count Iblis]

I am sorry but I think it helps me if I ask the question this way:

Assume that coin tossing is quantum and super position applies.

What should we expected as the result of that assumption?

in other words: beside the absence of evidence, what is the proof that coin tossing is not quantum, while Schrodinger cat is.

I think you can't just assume things on an ad hoc basis without precisely defining what it means in a mathematical consistent way. So, you should simply formulate two complete and consistent theories and then see what difference it would make.

That is basically the point I made to ZapperZ, but that didn't come across to him well. So, you can assume that quantum mechanics is always valid, which then leads to macro superpositions that will typically be completely decohered and thus unobservable, except for the very special cases cited by ZapperZ involving SQUIDS.

An alternative model would be some hybrid quantum/classical theory where superpositions, even in a completely decohered form do not exist.

I think that it is difficult to formulate such a consitent theory. But what you could do is do experiments designed to falsify the validity of quantum mechanics in the macro domain, e.g. by doing experioments with SQUIDS and show that the macro superpositions that can be observed in that case, decohere faster than predicted by theory, or try to find some other discrepancy with theory.


Okun explains here:

http://arxiv.org/abs/hep-ph/0505250

what the problem is if you do experiments or try to interpret the results of experiments without a firm and consistent theoretical basis. He examins the case of the charge of the photon and discusses experimental bounds on the charge. His point is that because there is no consistent theory that allows the photon to have a charge, at least no such theory is known as of yet, all claims of the form: "observation X had constrained the charge of the photon to be below Y" are nonsensical statements.


Another case is that of the photon mass, see here:

http://arxiv.org/abs/hep-ph/0306245

People have made claims about observations or experiments ruling out the mass of the photon to be higher that some value. However, as the article points out, such statements are conditional on the theory. The authors come up with a more realistic theory about the photon mass and what do we see: All of the extremely sharp experimental/observational bounds are null and void; we are left with the bound implied with a test of Coulomb's law done somewhere in the early 1970s.


It is for these reasons that I insist on discussing things within a framework of a consistent theory. Since a hybrid quantum/classical theory is inconsistent, at least if you formate it in a naive way by assuming that macro superpositions do not exist, then there is little point in trying to do experiments or think of what experiments you could do rule out such hybrid quantum/classical theories in favor of quantum mechanics being valid on all scales.

This does not mean that there is no point in trying to create macro superpositions in the lab. How exactly quantum theory effectively becomes classical mechanics is not exactly known, so such experiments can probe this. But I don't think it makes sense to question if coin tosses can be in superpositions (that are completely decohered and thus unobservable), unless one can formulate some consistent theory in which superpositions cease to exist.

 

[ZapperZ]

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.

But we are also forgetting that our measurement can, in fact, cause the classical properties to arise out of the system. It has been shown that https://www.physicsforums.com/showpost.php?p=1520644&postcount=58" actually can easily be responsible for our classical world!

So there are at least now, two different factors that can easily cause a lot of destruction to any quantum effects, and none of these have ever been carried out theoretically all the way to the coin-tossing phase. Now I have no idea if we can eventually do a quantum description of a coin-tossing. However, my original objection was the naive idea that one can actually write a wavefunction for coin-tossing! That wavefunction was MY objection! To me, that contradicts your latter claim and admission that we cannot carry through such unitary evolution up to the coin-tossing scale. Yet, you had zero qualm in writing such nonsensical wavefunction. To me, when you CAN write such wavefunction then there are observational consequences that can be checked. That was what I ASKED!

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.

Zz.

 

[ArielGenesis]

"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?

 

[Adrian59]

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.

 

[Count Iblis]

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]

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]

"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]

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.

 

[ArielGenesis]

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]

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]

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]

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.

 

[Count Iblis]

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.

 

[Count Iblis]

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.

 

[ArielGenesis]

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.

 

[SW VandeCarr]

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.

 

[ZapperZ]

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.

 

[Count Iblis]

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]

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]

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.

 

[Adrian59]

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

 

[ArielGenesis]

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.

 

[Adrian59]

The simplest example of probabilistic interference is the double slit experiment.

 

[Count Iblis]

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]

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.

 

[ArielGenesis]

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?

 

[Adrian59]

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.

 

[ibcnunabit]

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