I Schrödinger's Cat and radioactive atom

[entropy1]

Particles can be made to be in superposition of their states. Concerning Schrödingers cat, if the cat is in superposition of being dead and alive, does that mean that the atom that drives the narcotic is in superposition of having decayed and not having decayed? And does that mean that any radioactive atom is always in that superposition?

 

[anuttarasammyak]

Yes, they are in corresponding superposition states.

 

[phinds]

Concerning Schrödingers cat, if the cat is in superposition of being dead and alive,

The cat is never in superposition of being dead and alive. It's always either dead or alive.

Schrodinger constructed his imaginary experiment with the cat to demonstrate that simple misinterpretations of quantum theory can lead to absurd results which do not match the real world.

https://www.wtamu.edu/~cbaird/sq/2013/07/30/what-did-schrodingers-cat-experiment-prove/

And here on PF:
https://www.physicsforums.com/threads/quantum-cat-question.798022/

 

[StevieTNZ]

The cat is never in superposition of being dead or alive. It's always either dead or alive.

https://www.wtamu.edu/~cbaird/sq/2013/07/30/what-did-schrodingers-cat-experiment-prove/

And here on PF:
https://www.physicsforums.com/threads/quantum-cat-question.798022/

Please explain how this OR case comes about.

 

[phinds]

Please explain how this OR case comes about.

Do you mean you want me to explain why pop-science presentations, which are not actual physics but rather are entertainment, will always say that the cat is alive OR dead?

Get a textbook.

 

[StevieTNZ]

Do you mean you want me to explain why pop-science presentations, which are not actual physics but rather are entertainment, will always say that the cat is alive OR dead?

Get a textbook.

Which textbook do you suggest? And actually answer my question. Is that how you got your SA badge? By telling people to Google for answers or get "a textbook" without being specific, instead of being knowledgeable yourself about the subject and answering accordingly? What a joke PF has become.

EDIT: tagging @Greg Bernhardt into this post so something can be done once and for all.

 

[phinds]

Which textbook do you suggest?

It depends on your level and just how deep you want to dive in. There are a number of theads here discussing what books are best. Here's a good one to check out
https://www.physicsforums.com/threads/good-textbook-for-qm.238397/

 

[StevieTNZ]

Threads like this -- https://www.physicsforums.com/threads/macroscoptic-quantum-entanglement.1009354/ -- become rather pointless seeing you dismiss the idea of superposition states for macroscopic objects. Your reasoning, I don't know.

 

[phinds]

And actually answer my question.

Your question boils down to "why do people who do not know what they are talking about, not know what they are talking about". I don't have a good answer for that in general but in this particular case it's likely because they are getting THEIR information from pop-science presentations instead of from textbooks.

Unfortunately the cat thing is one of many items in physics that have been grossly misrepresented in the popular press, usually starting not long after the time the concept is first discovered and continuing all the way up to the present.

 

[StevieTNZ]

Your question boils down to "why do people who do not know what they are talking about, not know what they are talking about". I don't have a good answer for that in general but in this particular case it's likely because they are getting THEIR information from pop-science presentations instead of from textbooks.

Unfortunately the cat thing is one of many items in physics that have been grossly misrepresented in the popular press, usually starting not long after the time the concept is first discovered and continuing all the way up to the present.

You are simply misguided. At no point in the linear Schrodinger equation does 'collapse of the wave function' occur. How does a superposition state (AND) suddendly become OR? Answer me that.

 

[phinds]

Hm ... I thought I was addressing the OP. Sorry about that.

 

[phinds]

@StevieTNZ are you saying that you believe there IS a state in which the cat is both dead and alive?

 

[StevieTNZ]

@StevieTNZ are you saying that you believe there IS a state in which the cat is both dead and alive?

Indeed I am. It follows from the fundamental Schrodinger equation; decoherence does nothing to depart from that view as its simply entanglement with the environment. While interference is supressed, its present in principle. However, taking all systems together and doing a double slit will show interference.

 

[StevieTNZ]

I actually suggest you read "Sneaking a Look at God's Cards", the book most referenced here as a good source of information on quantum theory for the layman. It seems you don't have the appropriate qualifications in physics to deal with this subject. I'm not trying to sound harsh, but it might illuminate the quantum world for your information.

 

[phinds]

@StevieTNZ are you saying that you believe there IS a state in which the cat is both dead and alive?

Indeed I am.

Well, I have no answer to that other than to suggest you read the links I provided and a few hundred others here on PF which all show in various ways why the cat is not, and cannot be, both dead and alive at the same time.

 

[PeterDonis]

At no point in the linear Schrodinger equation does 'collapse of the wave function' occur.

You don't need "collapse of the wave function" to have a cat state that is not a coherent superposition of dead and alive. All you need is decoherence. Even the many worlds interpretation recognizes that, once the "dead" and "alive" branches of the wave function have decohered, they will never interfere with each other, and without interference, there is no way to ever demonstrate experimentally that both branches of the superposition are there.

 

[entropy1]

You don't need "collapse of the wave function" to have a cat state that is not a coherent superposition of dead and alive. All you need is decoherence. Even the many worlds interpretation recognizes that, once the "dead" and "alive" branches of the wave function have decohered, they will never interfere with each other, and without interference, there is no way to ever demonstrate experimentally that both branches of the superposition are there.

So, don't we then have superposition without interference, in this case the cat dead AND alive but in different world threads? (Although that might be semantics)

 

[entropy1]

If a radioactive atom is continuously in superposition of not having decayed and having decayed, has it or hasn't it? What can actually be said about it?

 

[Lord Jestocost]

Statements about the past cannot in general be made in quantum-mechanical (QM) language. The wave-function doesn’t allow statements like “The radioactive atom has decayed or not at this or that time.” Quantum mechanics differs from classical physics because the assumption that one of the answers is "objectively" realized in between observations or measurement is simply impossible.

Quantum probabilities are not the probabilities that the radioactive atom has decayed or not at a certain instant of time. It’s the probabilities that an observer will find it decayed or not at a certain instant of time. That’s all what QM has to say.

 

[Lord Jestocost]

The cat is never in superposition of being dead and alive. It's always either dead or alive.

The formalism of quantum mechanics doesn’t change one whit between the microscopic and macroscopic levels. Thus, with all due respect, statements like “the cat is always either dead or alive” make physically no sense and merely reflect classical wishful thinking. One cannot make a quantum theoretical thought experiment using a living organism as its integrating part, but on the other hand do not take seriously the application of quantum theory, i.e., here simply the concept of probability, to the organism.

 

[entropy1]

Quantum probabilities are not the probabilities that the radioactive atom has decayed or not at a certain instant of time. It’s the probabilities that an observer will find it decayed or not at a certain instant of time. That’s all what QM has to say.

, there is no way to ever demonstrate experimentally that both branches of the superposition are there.

So does that mean that we can only find ourselves observing the cat being either alive or dead, and that it means the cat is either alive or dead? Or is the reality that QM actually does not make a choice between those? (And that might mean the observed world is pretty much "an illusion", and that some other notion, like the wave function is ontic)

 

[AlexCaledin]

If I remember correct, James Hartle says that the actual decoherent history (which we find ourselves observing) is ontic, it's determined by the "quantum dice throwing" - so we can use the QM to calculate the probabilities, so the cat superposition is merely a thinking tool...

 

[WernerQH]

If a radioactive atom is continuously in superposition of not having decayed and having decayed, has it or hasn't it? What can actually be said about it?

There's a widely held conviction that "every physical system is described by a wave function". Yet through reductio ad absurdum Schrödinger himself disproved this idea with his (in)famous cat experiment. Surprisingly many physicists think that it ought to be meaningful to speak of a cat that is both dead and alive at the same time, but Schrödinger chose the example of a cat precisely because he considered it too absurd even to contemplate.

The superposition of photon states is well defined and meaningful, e.g. $\frac 1 {\sqrt 2} |\text{horizontal}\rangle + i × \frac 1 {\sqrt 2} |\text{vertical}\rangle$ represents circular polarization. But the "analogous" state $\frac 1 {\sqrt 2} |\text{undecayed}\rangle + i × \frac 1 {\sqrt 2} |\text{decayed}\rangle$ for an unstable nucleus is without any operational meaning. An undecayed atom that has been sitting there for one half-life is experimentally indistinguishable from another atom that is "freshly prepared". The decay happens in a fraction of a second, on a timescale much shorter than the half-life. To insist that the first atom is in a different state that should be represented by $\frac 1 {\sqrt 2} |\text{decayed}\rangle + \frac 1 {\sqrt 2} |\text{undecayed}\rangle$ is to attribute more reality to the wave function than to reality itself.

 

[PeterDonis]

don't we then have superposition without interference, in this case the cat dead AND alive but in different world threads?

In the many worlds interpretation, yes. But there are other interpretations in which that is not the case.

If a radioactive atom is continuously in superposition of not having decayed and having decayed, has it or hasn't it?

Neither.

What can actually be said about it?

That it is in a superposition of not having decayed and having decayed.

So does that mean that we can only find ourselves observing the cat being either alive or dead, and that it means the cat is either alive or dead?

Remember, the comment of mine you responded to here was specifically about the MWI. In the MWI, the cat has no definite state because it is entangled. Only the joint system of radioactive atom plus cat has a definite state. That is a general property of entangled systems: neither system alone has a definite state, only the joint total system does.

Or is the reality that QM actually does not make a choice between those?

This is too vague to be meaningful.

(And that might mean the observed world is pretty much "an illusion", and that some other notion, like the wave function is ontic)

In the MWI, yes, the wave function is ontic, and it is the only thing that is ontic.

 

[PeterDonis]

There's a widely held conviction that "every physical system is described by a wave function". Yet through reductio ad absurdum Schrödinger himself disproved this idea with his (in)famous cat experiment.

Whether or not that is true depends on which interpretation of QM you adopt. This is the interpretations subforum, so it's ok to discuss interpretations, but if you review the ground rules for this forum, you will see that it is not permitted to assert that any particular interpretation is the "correct" one. You have to state which interpretation you are using and make clear that what you are saying is only true for that interpretation. In your case, you are adopting an interpretation in which "collapse of the wave function" is some kind of real process (there are multiple interpretations along these lines, according to what kind of real process "collapse" is claimed to be). But on an interpretation like the MWI, "every physical system is described by a wave function" is indeed correct, and the Schrodinger's cat thought experiment is not a reductio ad absurdum of anything, but just a description of how the MWI describes a scenario in which a cat is involved.

 

[PeterDonis]

If a radioactive atom is continuously in superposition of not having decayed and having decayed

I probably should have commented on this when responding before to this post of yours, but this is not a good description of a radioactive atom in QM. See below.

the "analogous" state $\frac 1 {\sqrt 2} |\text{undecayed}\rangle + i × \frac 1 {\sqrt 2} |\text{decayed}\rangle$ for an unstable nucleus is without any operational meaning.

Which is fine since this state is not a state that figures in any actual analysis of radioactive decay. The actual state of a radioactive atom is one in which the amplitude for decay in some finite period of time is constant in time. Viewing it as being in a superposition like the one you write down (and which is the kind of state @entropy1 appeared to be imagining) is not correct.

 

[entropy1]

Remember, the comment of mine you responded to here was specifically about the MWI. In the MWI, the cat has no definite state because it is entangled. Only the joint system of radioactive atom plus cat has a definite state. That is a general property of entangled systems: neither system alone has a definite state, only the joint total system does.

Do you mean that the superposition of the two world threads (eg $|E_0, v_0 \rangle + |E_1, v_1 \rangle$) is the definite value of the joint system?

 

[PeterDonis]

Do you mean that the superposition of the two world threads (eg $|E_0, v_0 \rangle + |E_1, v_1 \rangle$) is the definite value of the joint system?

I don't know where you're getting the specific superposition you wrote down from, but if we idealize the whole experiment as being isolated from the rest of the universe, then yes, the joint system has a definite state which is entangled, and if you write that state down it will be a sum of two terms (in the case where whatever measurement is being made has two possible outcomes).

 

[Dr Wu]

Interesting discussion! 'Sneaking a Look at God's Cards' has certainly garnered a lot of praise in Amazon's review section, particularly from self-confessed laymen. . . whose ranks include this poster! Unhappily its current price tag is a bit fierce for this pocket, even secondhand (£28) which may be due to it being possibly out of print? I did find 'In Search of Schrodinger's Cat' by John Cribbin an informative and engaging read, likewise the far more recent 'Beyond Weird' by Philip Ball, who I trust is no 'pop' scientist.

 

[akhmeteli]

Well, I have no answer to that other than to suggest you read the links I provided and a few hundred others here on PF which all show in various ways why the cat is not, and cannot be, both dead and alive at the same time.

You provided a link to what looks like a private web-site and referred us, your readers, to PF threads. Those are not reliable sources. If you insist that a cat cannot be both dead and alive, apparently you don't believe that quantum mechanics is correct for macroscopic objects. Nobody performed sufficiently precise experiments with cats, so it is possible that you are right, but it is also possible that you are wrong. Mind you, I am not saying that observer's consciousness plays any role in quantum experiments.

Schrödinger discussed his gedanken experiment in 1935 and argued that a cat being both dead and alive is clearly absurd. However, that was 85 years ago, and since then there has been a lot of progress in experimental physics. You may wish to look at Haroche's Nobel lecture . He mentions Schrödinger's cats and "kittens" without any sarcasm maybe 50 times. He says: "We have realized the non-destructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of “Schrödinger cat” states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world." He says: "The Schrödinger cat experiments in Cavity QED illustrate the fragility of quantum coherences in systems made of increasing number of particles." Fragility, not impossibility! I heard his lecture at 2013 APS March meeting, where he said during the discussion: "There is no fundamental decoherence". (There is a possibility though that I mixed up something and it was not Haroche but Wineland, who presented on the same day). So I would say quantum mechanics predicts superpositions of dead and alive cats and experiments do not disprove that (yet).

 

[phinds]

You provided a link to what looks like a private web-site

No, I did not. You ignored the link I provided and instead clicked on the link in my signature.

 

[akhmeteli]

No, I did not. You ignored the link I provided and instead clicked on the link in my signature.

No, I did not click on the link in your signature, I clicked on the link to Dr. Baird's page. The page is at a university's site, not at a private site, as I thought, and I apologize for this mistake, but still it is not peer-reviewed, so still unreliable.

Baird says: "Schrodinger constructed his imaginary experiment with the cat to demonstrate that simple misinterpretations of quantum theory can lead to absurd results which do not match the real world." So what does Baird mean? That Schrödinger believed in 1935 that this prediction of quantum mechanics is wrong? Then I agree with Baird. Or did Baird mean that this prediction is indeed wrong? It follows from the following text on the page that this is indeed what Baird believes, and I disagree with that, as it is just Baird's opinion, not a fact. In the real world, experiments with cat-sized objects have never been performed with due effort to avoid decoherence. We don't know what happens in the real world with such objects if extreme measures are taken to prevent decoherence, but quantum mechanics predicts a superposition. Again, I am not saying that observer's consciousness causes wavefunction's collapse. Haroche's work suggests that what looks like collapse is caused by decoherence, and there is no fundamental decoherence, so it is not obvious that macroscopic superpositions are not possible even if measures are taken to prevent decoherence.

 

[phinds]

No, I did not click on the link in your signature, I clicked on the link to Dr. Baird's page. The page is at a university's site, not at a private site, as I thought, and I apologize for this mistake, but still it is not peer-reviewed, so still unreliable.

OK. Have you done a forum search and verified that other members here (who, unlike me, actually know what they are talking about), agree that the cat is NOT both alive and dead?

I do not have the in-depth background to make an independent informed decision on this and readily admit that what I have stated is my understanding, backed by what I consider to be reliable sources. I understand that that does not make me right, but it's enough for me. I'm a simple-minded engineer and I just find it hard to believe that, even theoretically, you could ever sufficiently isolate something as massively complicated as a cat.

 

[akhmeteli]

OK. Have you done a forum search and verified that other members here (who, unlike me, actually know what they are talking about), agree that the cat is NOT both alive and dead?

I do not have the in-depth background to make an independent informed decision on this and readily admit that what I have stated is my understanding, backed by what I consider to be reliable sources. I understand that that does not make me right, but it's enough for me. I'm a simple-minded engineer and I just find it hard to believe that, even theoretically, you could ever sufficiently isolate something as massively complicated as a cat.

I am not going to do such forum search. With all due respect, whatever is written in the forum is not peer-reviewed, unless there are appropriate citations. I am under no obligation to write only what other forum members agree with. According to the forum rules, I just cannot promote theories that are not properly published. I cite Haroche's Nobel lecture to (partially?) support my views, which are (the quoted parts are from M. Schlosshauer, Annals of Physics 321, 112 (2006)):

quantum mechanics predicts macroscopic superpositions;

"the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far;"

"all observed restrictions can be correctly and completely accounted for by taking into account environmental decoherence effects;"

experiments with cat-sized objects have never been performed with proper measures to prevent decoherence.

Which of these statements do you dispute?

Again, maybe you are right, and quantum mechanics is wrong for large macroscopic systems, but there has been no proof of that yet. I agree, it is possible that one will never be able to experimentally "isolate something as massively complicated as a cat" (although I don't understand why this is impossible "even theoretically"), but this only means that this will be an open question forever. However, if experiments will continue to demonstrate quantum superpositions for larger and larger systems, there will be more reasons to believe that macroscopic superpositions can exist in principle.

 

[CoolMint]

I am not going to do such forum search. With all due respect, whatever is written in the forum is not peer-reviewed, unless there are appropriate citations. I am under no obligation to write only what other forum members agree with. According to the forum rules, I just cannot promote theories that are not properly published. I cite Haroche's Nobel lecture to (partially?) support my views, which are (the quoted parts are from M. Schlosshauer, Annals of Physics 321, 112 (2006)):

quantum mechanics predicts macroscopic superpositions;

"the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far;"

"all observed restrictions can be correctly and completely accounted for by taking into account environmental decoherence effects;"

experiments with cat-sized objects have never been performed with proper measures to prevent decoherence.

Which of these statements do you dispute?

Again, maybe you are right, and quantum mechanics is wrong for large macroscopic systems, but there has been no proof of that yet. I agree, it is possible that one will never be able to experimentally "isolate something as massively complicated as a cat" (although I don't understand why this is impossible "even theoretically"), but this only means that this will be an open question forever. However, if experiments will continue to demonstrate quantum superpositions for larger and larger systems, there will be more reasons to believe that macroscopic superpositions can exist in principle.

What is the biggest object that has been put into a state of superpoition? Wasn't it just barely visiible to the naked eye? Is it sufficiently different from a cat to be an exception to quantum mechanical rules?

 

[akhmeteli]

What is the biggest object that has been put into a state of superpoition? Wasn't it just barely visiible to the naked eye? Is it sufficiently different from a cat to be an exception to quantum mechanical rules?

I don't know if the following is what you need:

U. Delić et al., Science
10.1126/science.aba3993 (2020).
Cooling of a levitated nanoparticle to the motional quantum ground state
Abstract:
"Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allows us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises of $10^8$ atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses."

Can conclusions obtained in experiments with such systems be extended to cat-size objects? Opinions on the results of unperformed experiments may differ.

 

[gentzen]

Baird says: "Schrodinger constructed his imaginary experiment with the cat to demonstrate that simple misinterpretations of quantum theory can lead to absurd results which do not match the real world." So what does Baird mean? That Schrödinger believed in 1935 that this prediction of quantum mechanics is wrong? Then I agree with Baird.

In the quoted sentence, Baird is pretty clear that this prediction results from a simple misinterpretation of quantum theory, and that Schrödinger believed in 1935 that this prediction is wrong. Baird later makes it clear that the "simple misinterpretation" in question here is "that quantum particles only collapse to a single state when viewed by a conscious observer".

quantum mechanics predicts macroscopic superpositions;

"the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far;"

"all observed restrictions can be correctly and completely accounted for by taking into account environmental decoherence effects;"

experiments with cat-sized objects have never been performed with proper measures to prevent decoherence.

Which of these statements do you dispute?

I dispute the first statement, namely that QM would predict superpositions for systems that are not sufficiently small such that one can produce and manipulate them in more or less arbitrary quantities. Of course, some interpretations like MWI do "believe" that, but many other interpretations are either agnostic about that point, or even explicitly reject it, like for example A. Neumaier's thermal interpretation:

One cannot superpose whole universes. In any case I do not know how one could prepare such a situation. ... The superposition principle only applies to systems that are sufficiently small that one can produce and manipulate them in more or less arbitrary quantities. Macroscopic systems are definitely not in this category!

For the second statement, the claim about "macroscopic realm" would need some qualifications:

Steven Weinberg in "Lectures on Quantum Mechanics" in section "8.3 Broken Symmetry" seems to "suggest" that often even molecules won't be in strange superpositions of states (even if that superposition would constitute the minimal energy eigenstate), if some related timeinterval far exceeds the lifetime of the universe.

I certainly agree with the third statement in the sense that no observations so far have hinted at any need to modify quantum theory (like collapse theories a la Penrose or Ghirardi). The fourth statement is pointless, and I disagree with the implicit suggestion that "proper measures to prevent decoherence" exist even in principle for "experiments with cat-sized objects".

 

[CoolMint]

I don't know if the following is what you need:

U. Delić et al., Science
10.1126/science.aba3993 (2020).
Cooling of a levitated nanoparticle to the motional quantum ground state
Abstract:
"Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allows us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises of $10^8$ atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses."

Can conclusions obtained in experiments with such systems be extended to cat-size objects? Opinions on the results of unperformed experiments may differ.

Maybe people have trouble with the notion of a quantum cat. If one accepts the notion that the cat is entirely quantum and all the quantum rules apply to the cat as well, maybe they can come to different conclusions?
If the cat is not quantum, then what is it made of?

 

[PeterDonis]

quantum mechanics predicts macroscopic superpositions

Since this claim, according to the quote you give a little bit later in your post, depends on the universal validity of unitary dynamics, the claim is only valid under an interpretation of QM such as the MWI, in which the universal validity of unitary dynamics is accepted. Other interpretations, such as collapse interpretations, do not accept that.

Note that experimentally we do not know that unitary dynamics is universally valid. We only know it's valid when quantum coherence is maintained. Once decoherence comes into play, we can no longer verify that the dynamics is unitary.

Note also that the restriction I just stated is not a restriction on the size of the system for which we can verify unitary dynamics. If we could somehow figure out how to keep a cat from decohering, we could in principle test to see if the cat's dynamics were unitary. The problem is that there are so many internal interactions within the cat that it decoheres itself.

 

[PeterDonis]

experiments with cat-sized objects have never been performed with proper measures to prevent decoherence.

How would you propose performing such an experiment?

 

[akhmeteli]

In the quoted sentence, Baird is pretty clear that this prediction results from a simple misinterpretation of quantum theory, and that Schrödinger believed in 1935 that this prediction is wrong. Baird later makes it clear that the "simple misinterpretation" in question here is "that quantum particles only collapse to a single state when viewed by a conscious observer".I dispute the first statement, namely that QM would predict superpositions for systems that are not sufficiently small such that one can produce and manipulate them in more or less arbitrary quantities. Of course, some interpretations like MWI do "believe" that, but many other interpretations are either agnostic about that point, or even explicitly reject it, like for example A. Neumaier's thermal interpretation:For the second statement, the claim about "macroscopic realm" would need some qualifications:I certainly agree with the third statement in the sense that no observations so far have hinted at any need to modify quantum theory (like collapse theories a la Penrose or Ghirardi). The fourth statement is pointless, and I disagree with the implicit suggestion that "proper measures to prevent decoherence" exist even in principle for "experiments with cat-sized objects".

As for Baird, he also writes: "Careful analysis reveals that the Schrodinger Cat "experiment" would play out in the real world as follows: as soon as the radioactive atom interacts with the Geiger counter, it collapses from its non-decayed/decayed state into one definite state. The Geiger counter gets definitely triggered and the Cat gets definitely killed. Or the Geiger counter gets definitely not triggered and the cat is definitely alive. But both don't happen." This statement is controversial in the best case. It is not clear why the system containing the atom, the cat, and the Geiger counter cannot be in a superposition.

So you dispute my first statement, but you seem to agree that the statement is within main stream. I agree that there are interpretations disputing such statement, such as the objective collapse interpretation, but predictions of such interpretations are not supported by experimental results (yet). As for Neumaier's thermal interpretation, I have not seen his peer-reviewed articles on his interpretation; if you know such articles, please advise. I know he issued a book, but I am not sure about the status of the book.

As for the second statement, it contains the words "in all experiments conducted thus far", so Weinberg's words do not seem to "qualify" it as his words self-qualify with "if some related time interval far exceeds the lifetime of the universe".

I don't understand why the fourth statement is pointless, and I don't feel comfortable with discussing "implicit suggestions".

 

[akhmeteli]

Since this claim, according to the quote you give a little bit later in your post, depends on the universal validity of unitary dynamics, the claim is only valid under an interpretation of QM such as the MWI, in which the universal validity of unitary dynamics is accepted. Other interpretations, such as collapse interpretations, do not accept that.

Note that experimentally we do not know that unitary dynamics is universally valid. We only know it's valid when quantum coherence is maintained. Once decoherence comes into play, we can no longer verify that the dynamics is unitary.

Note also that the restriction I just stated is not a restriction on the size of the system for which we can verify unitary dynamics. If we could somehow figure out how to keep a cat from decohering, we could in principle test to see if the cat's dynamics were unitary. The problem is that there are so many internal interactions within the cat that it decoheres itself.

You seem to agree that the claim of universal validity of unitary dynamics is at least main stream. If so, I don't have to accept phinds' categorical statements on impossibility of Schrödinger's cat. I keep repeating: maybe the cat can be both dead and alive, maybe not, but at this stage we just cannot be sure.

As for "many internal interactions", we discussed this earlier. I mentioned that molecules containing thousands of atoms and numerous degrees of freedom were experimentally shown to interfere.

 

[akhmeteli]

How would you propose performing such an experiment?

Experimentalists have hard time trying to prevent decoherence even with much smaller systems, so I cannot give a recipe to eliminate decoherence for cat-size objects. However, experimentalists keep making progress in this direction, showing what kind of measures should be taken to prevent decoherence.

 

[PeterDonis]

You seem to agree that the claim of universal validity of unitary dynamics is at least main stream.

No, I don't. As I said, the claim is interpretation dependent, so since there is no single QM interpretation that is "mainstream", the claim that unitary dynamics is universally valid is not "mainstream".

I keep repeating: maybe the cat can be both dead and alive, maybe not, but at this stage we just cannot be sure.

In the sense that we do not have a single QM interpretation that is "mainstream", this is correct, since the cat would only be "both dead and alive" on certain interpretations, such as the MWI. (And one then has to be careful in interpreting what "both dead and alive" actually means.)

As for "many internal interactions", we discussed this earlier. I mentioned that molecules containing thousands of atoms and numerous degrees of freedom were experimentally shown to interfere.

"Numerous" is a relative term. Molecules containing thousands of atoms still have some $10^{25}$ or more fewer degrees of freedom than a cat.

Also, the molecules in question have very simple, homogeneous structures, and are cooled down during experiments to the point where only a few degrees of freedom with energy levels sufficiently close to the ground state are excited. You cannot do that with a cat, at least not if you want it to remain a cat. An object with the same atoms as a cat in which only a few degrees of freedom with energy levels sufficiently close to the ground state were excited would not even be a dead cat; it would be a mixture of various simple elements and compounds with no significant structure. What makes a cat a cat, even a dead one, is that it has an extremely complex structure in which many, many degrees of freedom are constantly excited, so many that it is impossible to keep track of them all. That is why a cat decoheres itself, and cannot be prevented from doing so without completely destroying its structure and making it not even a dead cat any more.

 

[akhmeteli]

No, I don't. As I said, the claim is interpretation dependent, so since there is no single QM interpretation that is "mainstream", the claim that unitary dynamics is universally valid is not "mainstream".In the sense that we do not have a single QM interpretation that is "mainstream", this is correct, since the cat would only be "both dead and alive" on certain interpretations, such as the MWI. (And one then has to be careful in interpreting what "both dead and alive" actually means.)"Numerous" is a relative term. Molecules containing thousands of atoms still have some $10^{25}$ or more fewer degrees of freedom than a cat.

Also, the molecules in question have very simple, homogeneous structures, and are cooled down during experiments to the point where only a few degrees of freedom with energy levels sufficiently close to the ground state are excited. You cannot do that with a cat, at least not if you want it to remain a cat. An object with the same atoms as a cat in which only a few degrees of freedom with energy levels sufficiently close to the ground state were excited would not even be a dead cat; it would be a mixture of various simple elements and compounds with no significant structure. What makes a cat a cat, even a dead one, is that it has an extremely complex structure in which many, many degrees of freedom are constantly excited, so many that it is impossible to keep track of them all. That is why a cat decoheres itself, and cannot be prevented from doing so without completely destroying its structure and making it not even a dead cat any more.

So we at least seem to agree that "maybe the cat can be both dead and alive, maybe not, but at this stage we just cannot be sure", that is good enough for me.

As for molecules being "cooled down during experiments to the point where only a few degrees of freedom with energy levels sufficiently close to the ground state are excited", I wrote previously:

"In Nature Physics volume 15, pages 1242–1245(2019), they don't seem to cool anything at all. They say: "The delocalized molecules in our experiment are each roughly the mass of the green fluorescent protein25 (27 kDa) or a small BEC, while exceeding the temperature of a BEC by more than nine orders of magnitude. High-contrast quantum interference persists despite the thousands of excited vibrational levels and billions of structural and conformational isomers present in the molecular beam."

 

[Morbert]

Statements about the past cannot in general be made in quantum-mechanical (QM) language. The wave-function doesn’t allow statements like “The radioactive atom has decayed or not at this or that time.” Quantum mechanics differs from classical physics because the assumption that one of the answers is "objectively" realized in between observations or measurement is simply impossible.

"The wavefunction doesn't allow statements..." is too strong imo. Say we have a wavefunction $\Psi$ and we take a measurement in the present. If we can identify an observable $O = \sum \lambda_i E_i$ whose possible measurement results correlate with possible (mutually exclusive) past events $\{C_i\}$ , such that [1] $$\mathrm{Re}\langle\Psi|E_i C_j |\Psi\rangle \approx 0, i\neq j$$ then a measurement of $O$ would, under the appropriate interpretation, let us infer time-information about events in the past.

Quantum probabilities are not the probabilities that the radioactive atom has decayed or not at a certain instant of time. It’s the probabilities that an observer will find it decayed or not at a certain instant of time. That’s all what QM has to say.

The projectors for decayed and not decayed are $\Pi_D$ and $\Pi_N$ respectively. Say the time interval between the preparation of the experiment at $t_0$ and now is divided into $N$ smaller intervals $T_i = \left[t_{i-1},t_i\right)$. Let's also say the decay is registered by a detector that can register time of decay with pointer states $0,1,2,3,\dots,N$ represented by projectors $E_0,E_1,E_2,E_3,\dots,E_N$, with the state $0$ denoting the "ready" or "no detection" state. Maybe the detector is a time-of-death reading on the cat's collar. These pointer states record time of decay if $$\mathrm{Re}\langle \Psi|E_i \Pi_D(t_{j})\Pi_N(t_{j-1})|\Psi\rangle\approx 0, i\neq j$$In which case an observer can infer a past event from a present clock reading.

This description is of course interpretation-laden, and for the 2nd part of this post I am assuming a Von Neumann style partition of measured system (particle) + measurement device (detector in the box) + observer (physicist who opens the box)

[1] https://arxiv.org/pdf/1608.04145.pdf

 

[Halc]

The cat is never in superposition of being dead and alive. It's always either dead or alive.
...
And here on PF:
https://www.physicsforums.com/threads/quantum-cat-question.798022/

I want to throw in my $0.02. I don't want to post to the topic linked above due to its age and its presence in quantum physics instead of interpretations like this one, but it is heavy with talk of this quantum-classical 'cut' (by various names). This cut seems to be a feature of (and only of) Copenhagen interpretation. Much of the discussion in that topic seems overly interpretation dependent in my opinion.

I lay no claim to being an expert, so just let me paraphrase what I've been reading in this thread. There seems to be two different meanings of 'superposition' being used, resulting in people talking past each other.

Those that say that the cat cannot be in superposition of dead and alive are using an empirical definition, saying that even given a hypothetically (and unrealistic) perfect box, there is no way to empirically demonstrate interference between the two states in the box. With this I must agree. I cannot conceive of such a hypothetical box, blocking even undetectable gravitational waves and such. They're enough to cause decoherence with something like a cat. The cat is indeed one or the other, and simply in a unitary unknown state. Also, as PeterDonis points out, the conditions required by the containment would kill the cat before the poison bottle ever came into play. Yes, they've done it with 'macrosopic' objects, but even then the superposition lasted I think under a microsecond. We're needing minutes at least for our cat to be poisoned.

The other meaning of superposition is from the QM theory itself where a wave function of a closed system, unmeasured, which is what the box represents. This meaning says that given the locally impossible perfect box, the cat is indeed in superposition of being both dead and alive. To assert otherwise is to assert a counterfactual, which most interpretations do not support.

While I would not go so far as to qualify my opinion as 'belief', I prefer to frame things in relational terms, per Rovelli. This is a collapse interpretation, and given a simple metal box, relative to the guy that put it in there a short time ago, the cat within is indeed dead or alive and its indeterminate state is purely epistemological. But relative to somebody on Jupiter (a box based on isolation by distance instead of true closed system), the cat state is a temporary (15 minutes?) true counterfactual and is thus in superposition. There's an absolute impossibility to measure that superposition from that distance since any measurement will yield results slower than the decoherence occurs relative to him, so there is no pragmatic meaning to asserting the superposition state except as a descriptive demonstration of the interpretation, which seems to be along the lines of Schrodinger's purposes in bring up the cat, even if different conclusions are reached. Multiple classical states (worlds) were still not an option in those days. Biases take time to fall.

 

[PeterDonis]

I cannot conceive of such a hypothetical box, blocking even undetectable gravitational waves and such. They're enough to cause decoherence with something like a cat.

You don't even need that. As I said in an earlier post, the cat decoheres itself; it has such a huge number of heterogeneous degrees of freedom, most of which cannot be kept track of, that its own interactions with itself are sufficient to decohere it.

 

[CoolMint]

You don't even need that. As I said in an earlier post, the cat decoheres itself; it has such a huge number of heterogeneous degrees of freedom, most of which cannot be kept track of, that its own interactions with itself are sufficient to decohere it.

Is there a peer-reviewed paper that makes this claim?

 

[PeterDonis]

Is there a peer-reviewed paper that makes this claim?

Not directly about a cat, but the literature on decoherence is clear about what the requirements are.