What makes schrodinger cat quantum?

[SW VandeCarr]

the cat isn't 'quantum'. it's the radioactive decay that is governed by quantum mechanics. if the geiger-counter detects radiation, then the vial is broken, killing the cat. since the radioactive decay is probabilistic, we cannot say whether the geiger-counter detects it until we open the box and observe what has happened.

this is one of the problems i have with this experiment. the cat is not physically, both alive and dead. however, we cannot deduce whether it is dead or alive until we look in the box. it's a silly thought experiment to be honest.

If you're talking about the cat being in two states until someone looks, I agree there's a certain "silliness" to that. However there is another aspect of the Schrodinger thought experiment that is overlooked: it undermines strict determinism. While QM is deterministic, our knowledge of quantum states is probabilistic. Through human agency we can introduce this indeterminism into the large scale world. The Many Worlds interpretation was introduced to counter this, but MW is by its very nature is metaphysical. Given this, the Schrodinger thought experiment (which could easily be an actual experiment) is profound.

 

[canoe]

Wasn't the basic premise behind Shrodinger's cat to highlight the measurement problem? And hasn't the measurement problem been adequately addressed by decoherence (i.e. that measurments are essentially taking place continually and everywhere)? Is not the principal argument against decoherence primarily thermodynamical (i.e. that it does not adequate address the possibility of a Poincare recurrence)?

I do not mean to be rhetorical in posting the above questions. I was intending to post something similar as this is my current understanding...and to see if this explanation dovetailed with the understanding of others.

 

[SW VandeCarr]

Wasn't the basic premise behind Shrodinger's cat to highlight the measurement problem? And hasn't the measurement problem been adequately addressed by decoherence (i.e. that measurments are essentially taking place continually and everywhere)? Is not the principal argument against decoherence primarily thermodynamical (i.e. that it does not adequate address the possibility of a Poincare recurrence)?

I do not mean to be rhetorical in posting the above questions. I was intending to post something similar as this is my current understanding...and to see if this explanation dovetailed with the understanding of others.

I don't know if you are responding to my post or not. If you are, you're missing the point I'm trying to make. The fate of the cat is not determined by an extended causal chain of events. It's determined by the random outcome of an experiment. The human actor is the agency by which the direction of contingent processes are dependent, not on extended (deterministic) causal chains, but rather dependent on random outcomes.

This probably was not the intent of Schrodinger's thought experiment. I'm not arguing against decoherence. Nevertheless there's a sense that we are bypassing decoherence by a construction that makes macroscopic processes directly contingent on random outcomes. MW offers a way out, but I don't think science should depend on metaphysical arguments to maintain the idea that the world is, in principle, deterministic.

 

[ZapperZ]

I don't know if you are responding to my post or not. If you are, you're missing the point I'm trying to make. The fate of the cat is not determined by an extended causal chain of events. It's determined by the random outcome of an experiment. The human actor is the agency by which the direction of contingent processes are dependent, not on extended (deterministic) causal chains, but rather dependent on random outcomes.

This was probably was not the intent of Schrodinger's thought experiment. Nevertheless there's a sense that we are bypassing decoherence by a construction that makes macroscopic processes directly contingent on random outcomes. MW offers a way out, but I don't think science should depend on metaphysical arguments to maintain the idea that the world is, in principle, deterministic.

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. Even if I toss the coin in such a way that no one could see it, and I perform or not perform the dreaded act, what is the state of the cat? It will be EITHER dead OR alive, but not a combination of both!

The latter is what is so puzzling about the quantum scenario that is being illustrated by the cat, the superposition principle. Opening the box and determining the state of the cat, actually, is nothing unusual. A classical probability has that ability as well. It is what happens BEFORE. In a classical probability, there never was any case where all the possible states exists simultaneously. In QM case, they do! It is what makes the Schrodinger Cat scenario so puzzling, and it is what makes the Bell-type experiments more than just a conservation of momentum, angular momentum, spin, etc.

The existence of superposition of states is what has been verified in chemistry, in the Delft/Stony Brook experiments, etc.. etc. These are actual, real observation of the effects of superposition, i.e. it is not merely a reflection of our "ignorance" of the system as implied in classical statistics. These observed effects are absent in classical statistics.

Zz.

 

[Count Iblis]

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. Even if I toss the coin in such a way that no one could see it, and I perform or not perform the dreaded act, what is the state of the cat? It will be EITHER dead OR alive, but not a combination of both!

Why not? Let's assume that you are inside a perfectly isolated box with the cat, so that the initial state of the box comprising of you, the coin and cat can be taken to be a pure state (which makes the following argument easier to formulate but it isn't strictly necessary to assume this). The many particle wavefunction of the box then evolves according to the Schrödinger equation to something of the form:

|psi> = sum_i a_i |head_i>|dead_i> + sum_j b_j |tail_j>|alive_j>


It is entirely reasonable to assume that it is not the case that either all of the a_i are zero or that all of the b_j are zero, because the outcome of the coin throw depends on how you will throw the coin, which is regulated by very complex processes in the brain. If you don't do the experiment immediately, but wait a few days after entering the box, then the state of the box will suely have evolved into superposition of classical states that would each yield a different outcome of a coin throw (so, we are still assuming that coin throws are deterministic under normal circumstances).

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.


It would be different if a real physical mechanism that explains how a pure state can evolve into a mixed state is proposed. So, it should be explained how, even if you keep track of all the degrees of freedom and don't trace out any environmental degrees of freedom, the density matrix loses its off diagonal terms.

 

[canoe]

I don't know if you are responding to my post or not. If you are, you're missing the point I'm trying to make. The fate of the cat is not determined by an extended causal chain of events. It's determined by the random outcome of an experiment. The human actor is the agency by which the direction of contingent processes are dependent, not on extended (deterministic) causal chains, but rather dependent on random outcomes.

This probably was not the intent of Schrodinger's thought experiment. I'm not arguing against decoherence. Nevertheless there's a sense that we are bypassing decoherence by a construction that makes macroscopic processes directly contingent on random outcomes. MW offers a way out, but I don't think science should depend on metaphysical arguments to maintain the idea that the world is, in principle, deterministic.

Thank you for replying. I was not directly responding to your post, but rather was extending the discussion into a realm where I had a few questions. Perhaps that was not approriate. Nonetheless, it would seem to me that decoherence favorably augments the Copenhagan interpretation and that some of the more bizarre (bordering on metaphysical) interpretations of QT (i.e. many worlds (Everett) and consciousness causing the collapse of the wave function (Wigner)) might fall by the wayside. Perhaps the moderator might advise if I should start a new thread?

 

[ZapperZ]

Why not? Let's assume that you are inside a perfectly isolated box with the cat, so that the initial state of the box comprising of you, the coin and cat can be taken to be a pure state (which makes the following argument easier to formulate but it isn't strictly necessary to assume this). The many particle wavefunction of the box then evolves according to the Schrödinger equation to something of the form:

|psi> = sum_i a_i |head_i>|dead_i> + sum_j b_j |tail_j>|alive_j>

You need to justify why you are able to do this for a purely classical system. Just because you can write the 'wavefunction' of something, doesn't mean that it has any physical justification for it. For example, I can measure the bonding-antibonding bonds in chemistry, which is a direct consequence of the superposition of electrons being in many places. Can you show me what physical observation that I can measure from your wavefunction for coin-tossing that indicates that this description is valid?

If this is valid, Schrodinger would have used a simple coin-tossing as the source that triggers the poison, rather than a radioactive decay.

Zz.

 

[canoe]

Incidentally, doesn't decoherence provide an answer to the Schrodinger Cat scenario? Basically, that the wave functions of constituent particles become "all mixed up" (i.e. strong mixing). Thus the probalistic nature of quantum objects remains intact while mixed (i.e. non-quantum objects) decohere. Therefore, the cat is either alive or dead when the box is opened and not in some superposition of half-alive/half-dead states.

 

[Count Iblis]

You need to justify why you are able to do this for a purely classical system. Just because you can write the 'wavefunction' of something, doesn't mean that it has any physical justification for it. For example, I can measure the bonding-antibonding bonds in chemistry, which is a direct consequence of the superposition of electrons being in many places. Can you show me what physical observation that I can measure from your wavefunction for coin-tossing that indicates that this description is valid?

If this is valid, Schrodinger would have used a simple coin-tossing as the source that triggers the poison, rather than a radioactive decay.

Zz.

Well, we can read here that

Schrödinger and Einstein had exchanged letters about Einstein's EPR article, in the course of which Einstein had pointed out that the quantum superposition of an unstable keg of gunpowder will, after a while, contain both exploded and unexploded components.

Then why Schrödinger replaced the gunpowder by a radioactive atom has presumably to do with the fact that this makes the argument easier to argue.

Quantum mechanics itself tells you that, far all practical matters, the superposition cannot be distinguished from an unknown classical state. See e.g. this article:

http://arxiv.org/abs/0903.2258

 

[ZapperZ]

Well, we can read here that




Then why Schrödinger replaced the gunpowder by a radioactive atom has presumably to do with the fact that this makes the argument easier to argue.

Quantum mechanics itself tells you that, far all practical matters, the superposition cannot be distinguished from an unknown classical state. See e.g. this article:

http://arxiv.org/abs/0903.2258

You have not answered my question. Can you point out to me the effect of superposition on coin tossing?

If quantum effects are present in such a system, then QM phenomena should not be THAT strange. None of what you are citing has any relevance to what I stated, or asked. Why did the group at Delft and Stony Brook went to ALL that trouble to illustrate the Schrodinger Cat state using a laborious setup? They could have just tossed coins and be done with.

If you noticed in my first post in this thread, I cited several experimental observations that points to the effect of quantum superposition. I request you show me the same experimental observation of superposition from coin tossing. If you cannot show me this, then (i) your understanding of quantum superposition is faulty and (ii) all your effort in that post is moot, because you were working based on a faulty assumption.

I also strongly suggest, rather than reading Wikipedia (you should know me by now not to give me Wikipedia references - I do not consider those as valid reference source), maybe you should read Anthony Leggett's classic treatment on this very subject that lead to the SQUID experiments testing the Schrodinger Cat states:

A.J. Leggett "Testing the limits of quantum mechanics: motivation, state of play, prospects", J. Phys. Condens. Matt., v.14, p.415 (2002).

Zz.

 

[ZapperZ]

Incidentally, doesn't decoherence provide an answer to the Schrodinger Cat scenario? Basically, that the wave functions of constituent particles become "all mixed up" (i.e. strong mixing). Thus the probalistic nature of quantum objects remains intact while mixed (i.e. non-quantum objects) decohere. Therefore, the cat is either alive or dead when the box is opened and not in some superposition of half-alive/half-dead states.

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.

 

[Count Iblis]

It is all explained here:

http://arxiv.org/abs/0903.2258

If you cannot show me this, then (i) your understanding of quantum superposition is faulty and (ii) all your effort in that post is moot, because you were working based on a faulty assumption.

As pointed out in the article ( http://arxiv.org/abs/0903.2258 ), this is absolutely not the case. The fact that you can demonstrate non classical effects in simple systems (even some macro systems, although the macroscopic nature of the squid experiments has been disputed), does not mean that this is demonstratable for any quantum superposition in practice.

 

[ZapperZ]

I'm sorry, but you continue to point to this PREPRINT as if it is an EXPERIMENTAL EVIDENCE! Investigating an experimental capability is NOT the same as an experimental evidence! I hate to think, and you should be worried as well, that you're putting ALL of your eggs into this basket. And I haven't even touched the fact that you are using a non-published work, which is something we require in the QM forum.

None of what I've cited are "investigating an experimental capability". They were ALL done, published, and verified!

Try again! I want to see the effect of superposition of coin tossing.

Zz.

 

[Count Iblis]

I'm sorry, but you continue to point to this PREPRINT as if it is an EXPERIMENTAL EVIDENCE!

It is not experimental evidence, it merely explains (giving refs to published articles) why, assuming unitary quantum mechanics, you cannot for all practical matters observe such superpositions.

This means that it is not a valid argument to point to coin tosses and then say that: "See, there is nothing quantum mechanical about this", because even if it were in some completely decohered macroscopic superposition, you could not detect any quantum effects anymore (See Ref. 14 of the article).

The question should be: "Where is the experimental evidence that Nature is not unitary"?

 

[ZapperZ]

It is not experimental evidence, it merely explains (giving refs to published articles) why, assuming unitary quantum mechanics, you cannot for all practical matters observe such superpositions.

This means that it is not a valid argument to point to coin tosses and then say that: "See, there is nothing quantum mechanical about this", because even if it were in some completely decohered macroscopic superposition, you could not detect any quantum effects anymore (See Ref. 14 of the article).

So in other words, you have no valid experimental evidence to point to me on the superposition effects in coin-tossing, something that I've asked in the very beginning. It also shows that you have no justification (unless you plan on tossing a coin near a black hole anytime soon) to show the validity of that wavefunction that you created out of thin air.

That renders the rest of your argument completely moot.

Just for future reference, I'm an experimentalist. Unless you can show empirical validity of what you are claiming, there's a very good chance that I won't buy it no matter how well you dress it.

Furthermore, if you truly believe that your coin tossing can be accurately described via your "wavefunction", then more power to you. 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.

Zz.

 

[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.