Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

by Davide Castelvecchi

Quantum theory has a long history of thought experiments, and in most cases these are used to point to weaknesses in various interpretations of quantum mechanics. But the latest version, which involves multiple players, is unusual: it shows that if the standard interpretation of quantum mechanics is correct, then different experimenters can reach opposite conclusions about what the physicist in the box has measured. This means that quantum theory contradicts itself.

The conceptual experiment has been debated with gusto in physics circles for more than two years — and has left most researchers stumped, even in a field accustomed to weird concepts. “I think this is a whole new level of weirdness,” says Matthew Leifer, a theoretical physicist at Chapman University in Orange, California.

Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

by Davide Castelvecchi

Thanks for this. The technical details are beyond me, but it seems this should be quite a big deal. The Nature news article you linked to discusses this mainly in terms of the Copenhagen Interpretation of quantum mechanics, for which the implication is inconsistency between the conclusions of different observers. In reaction to that, it gives the opinion of a physicist named Matthew Leifer:

"For Leifer, producing inconsistent results should not necessarily be a deal breaker. Some interpretations of quantum mechanics already allow for views of reality that depend on perspective. That could be less unsavoury than having to admit that quantum theory does not apply to complex things such as people, he says."

Admitting that quantum theory doesn't apply to complex things such as people might not be at all unsavoury to some. It sounds as though it could be the perfect antidote to the arguments of Sean Carroll, Brian Cox and others, that souls/ghosts can't exist because physics doesn't allow any mechanism for them to interact with the material world.

However, that's not quite what the original paper is saying. The thought experiment implies that at least one of three conditions (one of which is consistency) must be violated. The Copenhagen Interpretation would lead to inconsistency because it satisfies the other two conditions. But some other interpretations wouldn't have to be inconsistent, because they already violate one of the other conditions. According to the authors, these interpretations include both hidden-variable interpretations such as Bohmian mechanics and many-worlds interpretations:
https://www.nature.com/articles/s41467-0...8#ref-CR26

However, that's not quite what the original paper is saying. The thought experiment implies that at least one of three conditions (one of which is consistency) must be violated. The Copenhagen Interpretation would lead to inconsistency because it satisfies the other two conditions. But some other interpretations wouldn't have to be inconsistent, because they already violate one of the other conditions. According to the authors, these interpretations include both hidden-variable interpretations such as Bohmian mechanics and many-worlds interpretations:
https://www.nature.com/articles/s41467-0...8#ref-CR26

Confusingly, an earlier version of the paper published online in 2016 argued that Bohmian mechanics did satisfy the other two conditions (albeit with a different mathematical definition), and was therefore also doomed to inconsistency. The title of that version reflected a more sweeping conclusion - "Single-world interpretations of quantum theory cannot be self-consistent".

The updated version just published argues that Bohmian mechanics violates the "universal validity of quantum mechanics" condition, "although in a less obvious manner" than other approaches.

That seems quite a big modification, presumably reflecting the discussion of the preprint in the intervening two years. It suggests the concepts involved really are quite slippery.

I don't think I have fully understood the famous thought experiment.  My understanding is this:

A cat is shut in a container with a vial of cyanide gas connected to a quantum device, connected to a particle in such a way that if the particle is one thing, the vial breaks and the cat dies but if it is another, the vial remains intact and the cat lives.  The idea of course is that the experimenter has to make a prediction without opening the box but the particle is in a superposition where it is both things simultaneously.  The classic interpretation is that the cat must be both alive and dead simultaneously, but surely the position that doesn't break the vial doesn't prevent the vial from being broken, so the vial breaks and the cat dies?

This bugs me.  Can somebody please clear up my misunderstanding. 

I don't think I have fully understood the famous thought experiment.  My understanding is this:

A cat is shut in a container with a vial of cyanide gas connected to a quantum device, connected to a particle in such a way that if the particle is one thing, the vial breaks and the cat dies but if it is another, the vial remains intact and the cat lives.  The idea of course is that the experimenter has to make a prediction without opening the box but the particle is in a superposition where it is both things simultaneously.  The classic interpretation is that the cat must be both alive and dead simultaneously, but surely the position that doesn't break the vial doesn't prevent the vial from being broken, so the vial breaks and the cat dies?

Sorry, I didn't understand the very last bit. Why does the vial break anyway?

(Obviously this new paper is based on a much more complicated thought experiment in which there are two experimenters, each with a large box containing a lab with another experimenter in it.)

I don't think I have fully understood the famous thought experiment.  My understanding is this:

A cat is shut in a container with a vial of cyanide gas connected to a quantum device, connected to a particle in such a way that if the particle is one thing, the vial breaks and the cat dies but if it is another, the vial remains intact and the cat lives.  The idea of course is that the experimenter has to make a prediction without opening the box but the particle is in a superposition where it is both things simultaneously.  The classic interpretation is that the cat must be both alive and dead simultaneously, but surely the position that doesn't break the vial doesn't prevent the vial from being broken, so the vial breaks and the cat dies?

This bugs me.  Can somebody please clear up my misunderstanding. 

Well, the particle is radioactive. It may or may not decay. If it decays, the poison is released. The superposition supposedly includes the state of the entire system: particle, vial, cat, box.

I don't think I have fully understood the famous thought experiment.  My understanding is this:

A cat is shut in a container with a vial of cyanide gas connected to a quantum device, connected to a particle in such a way that if the particle is one thing, the vial breaks and the cat dies but if it is another, the vial remains intact and the cat lives.  The idea of course is that the experimenter has to make a prediction without opening the box but the particle is in a superposition where it is both things simultaneously.  The classic interpretation is that the cat must be both alive and dead simultaneously, but surely the position that doesn't break the vial doesn't prevent the vial from being broken, so the vial breaks and the cat dies?

This bugs me.  Can somebody please clear up my misunderstanding. 

The point Schrödinger was making was to show how counter intuitive QM is. That's the quandary presented in this new thought experiment only more so since it seemingly can give contradictory results. In real life the cat is either dead or alive.

The point Schrödinger was making was to show how counter intuitive QM is. That's the quandary presented in this new thought experiment only more so since it seemingly can give contradictory results. In real life the cat is either dead or alive.

But in order for an illustration to be useful, it has to make sense as if it were a real event.  If it were real, the position that doesn't break the vial doesn't prevent the vial from being broken so the cat dies.  Also, I believe you are wrong about Schrödinger's motive.  My understanding is that he was trying to show how untenable the Copenhagen Interpretation was, something that Einstein later congratulated him for, after all, a cat cannot be both dead and alive!