What makes schrodinger cat quantum?

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

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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.
 
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.
 

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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.
 
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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).
 
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ZapperZ

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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.
 

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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.
 
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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).
 
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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.
 
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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.
 
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

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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.
 
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.
 

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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.
 
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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.
 
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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.
 

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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.
 
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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.
 
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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.
 
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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.
 
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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.)
 
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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 accross 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. :rofl:


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.
 

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That is still not the complete story.

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

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

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

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

and what are those observational consequences might be?
 

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