Understanding the Cat in a Box Paradox

[mn4j]

Couldn't the cat in the box paradox probably be boiled down to the following question:

Is there any specific property of a quantum mechanical process that indicates whether or not it triggers the collapse of the wave function ?

I think the basic question is more fundamental than that.

"What are you trying to measure?" or rather
"Does it make sense to try to measure something that does not exist until it is measured?"

There exists a physical reality which underlies all scientific inquiry, without which science is meaningless.

Many physicists, as long as they consider nature to made up of wavefunctions according to the copenhagen interpretation, have misunderstood Bohr. Bohr's idea of physics was never to describe what nature is. He was not interested in ontological questions issues like "what was really happening." He said following:

"There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature." [J. C. Polkinghorne (1989, pp. 78-79)]

I disagree with him. Physics is about finding out how nature is, and what the laws of nature are. However, Bohr's statements must be understood through his perspective which unlike Einstein's, is epistemological rather than ontological.

 

[OOO]

Kinda.. the problem with the cat is that we haven't defined the observables involved in the system. We certainly can't tell what would be commuting and non-commuting.

Note that the effects of superposition is extremely common in chemistry. The existence of bonding and antibonding states are clear evidence of such a thing. So it isn't just restricted to SQUIDs. The reason that Leggett suggested the SQUID experiment in the first place is that it would involve the superposition of not just one or two or three particles, but a gazillion particles, thus testing the the "size" effect for detecting quantum behavior. Superconductors are idea for that because the supercurrent behaves as a single, coherent "entity", causing Carver Mead to proclaim that nowhere in nature is there a better demonstration of quantum mechanics[1].

Zz.

[1] C. Mead, PNAS v.94, p.6013 (1997).

As you mention it: a real superconductor is finite, so the cooper-pair wave function must be a "heavy" superposition of plane waves. Doesn't this give rise to the same question as the cat in the box, namely whether there is some mesoscopic level of description where quantum goes to classical ? Or am I mixing things up ?

I'm not so familiar with superconductivity so I don't know if BCS theory is able to explain a bounded superconductor consistently or if it's done with a little hand-waving.

 

[f95toli]

The superconducting wavefunction is not directly revelant in this case. There are a number of "technical" reasons why superconductors are very useful for fabricating qubits (the main one being that the presence of a gap gives it some protection from excitations) but in all types of "ring qubits" (RF-SQUIDs, Mooij-type qubits etc), only flux quantization and "stiffness" of the phase (to preserve coherence aorund the whole loop) is relevant.
Hence, you do NOT need a microscopic description of superconductivity to model a superconducting qubiot. The only thing you need is a double well potential.

Also, note that the first type of superconducting qubit that was ever realized was the charge qubit (by Nakamura in 1999) which uses a superposition of charge ;in ring type qubits it is the conjugate, flux, which is the relevant variable (most qubits can be classified as being either charge- or flux qubits depending on the value of the charging and Josephson energies; there are "hybrids" such as at the quantronium and the transmon; there are also single junction phase qubits, used by e.g, Martinis and co-workers)

 

[OOO]

I think the basic question is more fundamental than that.

"What are you trying to measure?" or rather
"Does it make sense to try to measure something that does not exist until it is measured?"

What makes you sure that these questions are more fundamental ? I would say that in order to find more fundamental questions we have to investigate more fundamental physics. For me this cat-in-the-box experiment seems to add unnecessary complications to the matter.

Specifically, discussing about measurements and reality seems to be useless to me as long as I cannot answer the question why such a fundamental observation like the position of an electron on the screen behind the single-slit experiment is basically unpredictable.

Both of your questions appear to be answerable to me in the case of the single-slit experiment. 1) we measure position, ideally of a single silver atom, 2) yes it does make sense to repeat the experiment many times finding out that there is an amplitude of the wave the square of which happens to give the probability of measuring position of the reduction of a silver atom.

On the other hand it is not possible for me to answer the question that I've mentioned. And finally with the cat-in-the-box I can answer none of these questions. So I'd consider my question more fundamental (don't be pissed off by this formulation, I don't want to do contest about who generates the more fundamental questions)

 

[f95toli]

I disagree with him. Physics is about finding out how nature is, and what the laws of nature are.

That is what most scientists belived 100 years ago. However, we have gradually come to the conclusion that this is a rather meaningless goal from a scientific point of view.
Whether or not or theories describe the "real world" is strictly speaking irrelevant, a good theory must be able to predict the outcome of experiments; that is all.
In principle someone could up with theory that explained the world in terms of invisible pink unicorns; as long as it was falsifiable; agreed with all existing data and was better at predicting the outcome of new experiments than existing theories we would have to conclude that it was a good theory. Whether or not the unicorns were real or not is irrelevant.

Now, I realize that many people think this is very unsatisfactory from a philosophical point of view and I suspect even most scientist assume that the theories they work with somehow describes an "objective reality"; but that is a separate issue which has nothing to do with physics as such. When working with QM it is extremely important to keep the scientific and philosophical discussions apart.

 

[f95toli]

Can you cite some examples?

There are plenty. Just google "solid-state qubit".
Note that whereas most experiements use a "statisical" readout (single-shot readouts are hard; but not impossible) there are some that do not.

You can find some of my "usual" references for superconducting qubits at the end of this paper.

http://www.arxiv.org/abs/0704.0727

See e.g. refs 1-4, 10,15, 19

 

[Anonym]

The problem with a lot of these discussions is a confusion…

There exists a physical reality which underlies all scientific inquiry, without which science is meaningless.

Many physicists, as long as they consider nature to made up of wavefunctions according to the copenhagen interpretation, have misunderstood Bohr. Bohr's idea of physics was never to describe what nature is. He was not interested in ontological questions issues like "what was really happening."

This is fortunate in the sense that most quantum phenomena to date have been studied using ensembles of large numbers of individual entities. And this is the only reason the faulty copenhagen interpretation has appeared to work to date. It is unfortunate because to date, QM continues to be paradoxical and unclear when explaining phenomena involving individual particles.

I would say that mainly it was and it is the communication problem. To maintain the communication you need the following (consider end-to-end):

1) the available information (knowledge) at each end user;
2) the common code;
3) the overlapping bandwidth;
4) the matched receivers;
5) the proper communication media.

You have in mind the condensed matter physicists and not HEP. They (in average):

1) don’t know math, QM, QFT and statmech;
2) use fuZzy logic and not usual as rest of normal people;
3) consider the statistical ensembles only;
4) don’t know to read;
5) immediately run any organized discussion into chaos.

The outcome of a lot of these discussions is confusion…

Nothing is happening in the system, but a lot is happening in your mind.

Now you describe the inverse process: the communication of the information as input to the quantum computer.

Regards, Dany.

 

[OOO]

You have in mind the condensed matter physicists and not HEP. They (in average):

1) don’t know math, QM, QFT and statmech;
2) use fuZzy logic and not usual as rest of normal people;
3) consider the statistical ensembles only;
4) don’t know to read;
5) immediately run any organized discussion into chaos.

The outcome of a lot of these discussions is confusion…

You seem to spend more time on sociology than on physics. At least from what I have seen here, you prefer to annoy people by throwing insults around. I haven't seen you saying something substantial besides citing authorities.

 

[shaun_o_kane]

mn4j: Just a point of interest. Copenhagen was all about destroying Schrodingers semi-classical view of a real waveform propogating though space according to Schrodinger's equations.

Copenhagen regards the waveform as a construction of the observer (human mind, machine) etc. Therefore Schrodinger's Cat is not a paradox according to Copenhagen; the waveform doesn't actually effect the cat. Copenhagen works just fine with "ensembles of large numbers of individual entities". You need to be more specific if you think this is not the case.

 

[rewebster]

You seem to spend more time on sociology than on physics. At least from what I have seen here, you prefer to annoy people by throwing insults around. I haven't seen you saying something substantial besides citing authorities.

well, I'll be, 00, I live in a glass house, too


(that reminds me, I've got to get to the glass store today)

 

[AbedeuS]

So the cat paradox is that the cat can be both dead and alive until an external observer confirms that the cat is in one state or the other.

Is it ok if you could explain how this ties into MO theory of bonding and antibonding, my eyebrow raised a little when this was mentioned, and I'm always keen too see somones outlook on stuff like this.

 

[mn4j]

mn4j: Just a point of interest. Copenhagen was all about destroying Schrodingers semi-classical view of a real waveform propogating though space according to Schrodinger's equations.

Copenhagen regards the waveform as a construction of the observer (human mind, machine) etc. Therefore Schrodinger's Cat is not a paradox according to Copenhagen; the waveform doesn't actually effect the cat. Copenhagen works just fine with "ensembles of large numbers of individual entities". You need to be more specific if you think this is not the case.

Schrödinger proposed the cat paradox, precisely to illustrate that the Copenhagen interpretation was not a description of reality.

see:
Jaynes, E. T., 1990, `Probability in Quantum Theory,' in Complexity, Entropy, and the Physics of Information, W. H. Zurek (ed.), Addison-Wesley, Redwood City, CA, p. 381

http://bayes.wustl.edu/etj/articles/prob.in.qm.pdf

 

[shaun_o_kane]

I really don't want to wade through another publication looking for the bit that you feel supports your view. Please be specific. Jaynes is not a physicist or philosopher, he is Bayesian. Having read other publications by Jaynes I'm not at all sure that he understands QM interpretations. He, like Bohr and Schrodinger, is dead now so can't elaborate or clarifiy on his writings.

A phrase like "illustrate that the Copenhagen interpretation was not a description of reality" is so imprecise that it is impossible to comment.

It is some time since I looked at Schrodinger's paper but if I remember correctly the Cat paradox was contained in a report of the progress of his own research group. Since Schrodinger believed in a physically real waveform, it is hardly surprising that he complains about "the present state" of QM. The paper did not provoke the same response from Bohr that Einstein's crtiticisms did.

If you think that Schrodinger's Cat is a problem for Copenhagne, please explain why?

 

[mn4j]

Jaynes is not a physicist or philosopher, he is Bayesian.

This tells me you have not read any of his works. Jaynes is indeed phyciscist! He did a lot of work in statistics but you'd be sorely mistaken to write him off. If you are really interested in the topic, please read the article then come back. It discusses particularly the issue you raised. His biography is here: http://bayes.wustl.edu/etj/etj.html

Having read other publications by Jaynes I'm not at all sure that he understands QM interpretations. He, like Bohr and Schrodinger, is dead now so can't elaborate or clarifiy on his writings.

Having read his publications, I'm not sure you understand him. If you can claim that a person (ET Jaynes) with PhD in physics, who studied under people Oppenheimer and Eugene Wigner is not a physicist, I wonder what indeed you are talking about.

It is some time since I looked at Schrodinger's paper but if I remember correctly the Cat paradox was contained in a report of the progress of his own research group. Since Schrodinger believed in a physically real waveform, it is hardly surprising that he complains about "the present state" of QM. The paper did not provoke the same response from Bohr that Einstein's crtiticisms did.If you think that Schrodinger's Cat is a problem for Copenhagne, please explain why?

Here is the quote from the translated version of Shroedinger's paper that explains it (bolded sentence):

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.
6.​

If you want it more elaborate, the article I quoted to you discusses the issue in depth.

To summarize:
The copenhagen interpretation wrongly treats an epistemological theory as an ontological one. The error is called "The Mind Projection Fallacy".

elsewhere Jaynes describes it as follows:

The error occurs in two complementary forms, which we might indicate thus:
(A) (My own imagination) ==> ! (Real property of Nature)
(B) (My own ignorance) => ! (Nature is indeterminate)
Form (B) arose out of quantum theory; instead of covering up our ignorance with fanciful assump-
tions about reality, one accepts that ignorance but attributes it to Nature. Thus in the Copenhagen
interpretation of quantum theory, whatever is left undetermined in a pure state \psi is held to be
unknown not only to us, but also to Nature herself. That is, one claims that \psi represents a phys-
ically real "propensity" to cause events in a statistical sense (a certain proportion of times on the
average over many repetitions of an experiment) but denies the existence of physical causes for the
individual events below the level of \psi .​

Nothing illustrates this as clearly as the cat in box paradox, which is interpreted by copenhagans to mean until we look (ignorance), the Cat is both dead and alive (nature is indeterminate).

 

[ShadowWorks]

So the cat paradox is that the cat can be both dead and alive until an external observer confirms that the cat is in one state or the other.

To me that puts to much stock in the human mind hologram perspective by saying something is only real if we can process it, the God complex, as if our brains are the best processors in the universe.

Lots of thing are real that we can't see hear or touch or even imagine right now and they have been here long before us and will out live us, perception does not making something real or not IMHO.

Probably 99.999999999 of the universe is unknown to science as we know it because we just don't know how to measure it or even know that its there, so when somebody discovers something new does that mean it only began when it was discovered?

I don't think it matters if we observe a phenomenon or not, the cat in the box is just a trick question, a head fook as they say in Scotland.

 

[Strafe]

To me that puts to much stock in the human mind hologram perspective by saying something is only real if we can process it, the God complex, as if our brains are the best processors in the universe.

Lots of thing are real that we can't see hear or touch or even imagine right now and they have been here long before us and will out live us, perception does not making something real or not IMHO.

Probably 99.999999999 of the universe is unknown to science as we know it because we just don't know how to measure it or even know that its there, so when somebody discovers something new does that mean it only began when it was discovered?

I don't think it matters if we observe a phenomenon or not, the cat in the box is just a trick question, a head fook as they say in Scotland.

I agree 100%.

I believe that in ACTUALITY the cat is either in 1 state OR in the other (It is either dead or alive but NOT BOTH). Each outcome has the same equal chance to exist. There is no inbetween state of alive or dead, assuming we all have the same interpretation of living or not living (dead), (which we should, otherwise we have something else to discuss). Just because there is not a known fact to US if the cat is alive or dead doesn't mean that it is neither.

Do you guys disagree?

 

[Ableman]

Wavefunction Collapse

I think that the cat in a box paradox is very exciting, and gets told a lot, and thus gets misinterpreted. You see, in this case, it is easy to believe that it makes no difference whether you can observe it or not. But there are cases where it does. Two-slit interference patterns with electrons is a case that is similar, but where it is more obvious. If you look through which hole the electrons go through, the pattern disappears, if you don't there is one. The cat in a box involves the same principles, except there's no obvious difference between looking and not looking.
Of course to me, it also seems ridiculous to say that looking and not looking can make a difference. Suppose you get a spreadsheet to tell you which holes the electrons went through, but you don't look at the spreadsheet. Now, you look to see whether there was an interference pattern, and you see one since you haven't observed the spreadsheet. But later you look at the spreadsheet, and find that something incredibly improbable happened (or, alternatively, history gets altered so that you saw that there was no interference pattern, in which case however you could set up a huge paradox by deciding to yourself: if I don't see an interference pattern, I won't look at the spreadsheet, if I do, I will). Therefore the instrument you use to observe must be the observer.
That is, when you open the box, the cat is either dead or alive, because the Geiger counter is the observer. There should be no superposition of states.
Now, personally I think I've got to be wrong, because I respect Schrodinger greatly, and it seems silly that he would've missed something like this, so someone please tell me why I'm wrong.
Additionally, the point of using a cat (rather than say, an exploding keg of powder, is that the cat could potentially be an observer).

 

[cesiumfrog]

I like that this topic has drawn so many to make their very first comments here.

 

[Strafe]

I like that this topic has drawn so many to make their very first comments here.

I registered just so I could reply to this post..I am very interested :-)

What lead me here is a whole new topic though, and I am still searching for the answer.

 

[Anonym]

you saw that there was no interference pattern, in which case however you could set up a huge paradox by deciding to yourself: if I don't see an interference pattern, I won't look at the spreadsheet, if I do, I will). Therefore the instrument you use to observe must be the observer.
That is, when you open the box, the cat is either dead or alive, because the Geiger counter is the observer. There should be no superposition of states.
Now, personally I think I've got to be wrong, because I respect Schrodinger greatly, and it seems silly that he would've missed something like this, so someone please tell me why I'm wrong. Additionally, the point of using a cat (rather than say, an exploding keg of powder, is that the cat could potentially be an observer).

In order to tell you whether you are right or wrong, one should understand what you want to say. I was not able to do that. Therefore, let me ask you the introductory question: How do you know that the cat is the macroscopic system and not microscopic?

Regards, Dany.

 

[lightarrow]

I
That is, when you open the box, the cat is either dead or alive, because the Geiger counter is the observer. There should be no superposition of states.

Agree with you.

 

[Anonym]

Agree with you.

You are welcome also to explain the connection between the fact that the Geiger counter is the detector and the state of the cat.

Regards, Dany.

 

[Ableman]

I apologize that I was unclear, I'm new to this forum. Now, I'm an undergraduate physics student, so maybe I'm missing something. But it shouldn't make a difference whether it's a macroscopic or microscopic system.
What I mean is that as soon as the Geiger counter detects an atomic decay the wave function collapses. The cat is not even necessary. The atom itself is no longer in a superposition of decayed and undecayed as soon as the Geiger counter registers the decay, no more than a coin is in a superposition of heads or tails just because you haven't looked at the outcome. That is, I am saying as soon as it is possible to tell whether the atom decays (or the electron has passed through a particular slit) the wavefunction collapses, and the atom is no longer in a superposition of decayed and undecayed and the cat is not in a superposition of alive and dead. The need for a conscious observer is eliminated, and any kind of an observer (like a Geiger counter) will do.

 

[Anonym]

I apologize that I was unclear. I'm an undergraduate physics student, so maybe I'm missing something.

You should not apologize. But you did not answer my question: How do you know that the cat is the macroscopic system and not microscopic? Try to focus on it only. I am not interesting to know what you mean; I am interesting to let you know what the problem (the measurement problem) is.

Regards, Dany.

P.S. Hint: the Geiger counter registers the decay but the hammer is broken; so?

 

[lightarrow]

You are welcome also to explain the connection between the fact that the Geiger counter is the detector and the state of the cat.

Regards, Dany.

The Geiger counter's "click" is an irreversible process. It's that which fixes one of the two possible values of the result. In that moment wavefunction has collapsed and there are no more superpositions.

 

[Ableman]

I don't understand what the significant difference would be between microscopic and macroscopic. If the hammer is broken, then the wavefunction of the cat would be in the alive state no matter what the Geiger counter registers.

 

[mn4j]

Two-slit interference patterns with electrons is a case that is similar, but where it is more obvious. If you look through which hole the electrons go through, the pattern disappears, if you don't there is one.

Do you know of anyone that has ever ACTUALLY done this experiment as you describe? I would like to read it. I've been looking everywhere for such an experiment but came up short.

In fact I found real experiments disproving the above claim:

Time-Resolved Diffraction and Interference: Young's Interference with Photons of Different Energy as Revealed by Time Resolution
N. Garcia; I. G. Saveliev; M. Sharonov
Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 360, No. 1794, Interference: 200 Years after Thomas Young's Discoveries. (May 15, 2002), pp. 1039-1059. http://www.jstor.org/view/1364503x/sp020018/02x0298l/0

The authors say:

In #3 we present interference experiments with photons of different energies going
through different slits. We know at which slit the blue and red, high- and low-energy,
photons arrived, although we cannot distinguish these photons when they reach the
screen. This observation may indicate that some modification is needed in the books
that state that interference is produced only if we do not know through which slit each
photon goes. In our experiments, the indeterminacy is after the slits.

I also hear claims about how wavefunction collapse has been experimentally proven. How can that be. The wavefunction by definition can NEVER be measured experimentally because it supposedly collapses as soon as you measure it. How then can a person claim with a straight face that wavefunction collapse has been demonstrated experimentally, I will really like to see the article in which a wavefunction was measured before it collapsed, and after it collapsed. There is no other way to experimentally prove that there is such a physical process as wavefunction collapse.

As you can see wavefunction collapse is a non-falsifiable theory which tells you a lot about it's validity.

 

[Anonym]

The Geiger counter's "click" is an irreversible process. It's that which fixes one of the two possible values of the result. In that moment wavefunction has collapsed and there are no more superpositions.

You make me crazy. I put the cat in the box alive and then take him out alive (the hammer is broken). Where you see an irreversible process?

Regards, Dany.

 

[Anonym]

I don't understand what the significant difference would be between microscopic and macroscopic.

Read E. Schrödinger, “THE PRESENT SITUATION IN QUANTUM MECHANICS”. When you will understand the significant difference between microscopic and macroscopic, we will continue the discussion.

Regards, Dany.

 

[OOO]

Do you know of anyone that has ever ACTUALLY done this experiment as you describe? I would like to read it. I've been looking everywhere for such an experiment but came up short.

In fact I found real experiments disproving the above claim:

Time-Resolved Diffraction and Interference: Young's Interference with Photons of Different Energy as Revealed by Time Resolution
N. Garcia; I. G. Saveliev; M. Sharonov
Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 360, No. 1794, Interference: 200 Years after Thomas Young's Discoveries. (May 15, 2002), pp. 1039-1059. http://www.jstor.org/view/1364503x/sp020018/02x0298l/0

The authors say:



I also hear claims about how wavefunction collapse has been experimentally proven. How can that be. The wavefunction by definition can NEVER be measured experimentally because it supposedly collapses as soon as you measure it. How then can a person claim with a straight face that wavefunction collapse has been demonstrated experimentally, I will really like to see the article in which a wavefunction was measured before it collapsed, and after it collapsed. There is no other way to experimentally prove that there is such a physical process as wavefunction collapse.

As you can see wavefunction collapse is a non-falsifiable theory which tells you a lot about it's validity.

There has never been a single experiment where the electromagnetic field has been measured in whole space or even in every point of a finite volume. Yet most of us believe somehow in the reality of the EM field. Likewise the wave function. It's clearly an abstraction but it works. If we are able to predict what comes out in the end, we may use any abstraction. If disproven we will finally end up with another abstraction.

I see no point in fighting against the confused religious ideas of various dead persons. So I repeat what I have said before: if you have something better, tell us about it.