I Why don't we bury Schrodinger's Cat?

[gentzen]

I'm still not seeing any observable contradictions. You are explaining what an interpretation is. I'm explaining what a physics theory isn't.

Well, before trying to react to your other reactions, let's discuss your points about "what a physics theory isn't" and my points about "what an interpretation is".

So the main problem is, we tend to imagine that the purpose of a physics theory is to describe what is actually happening in a place we think of as "objective reality." But such a place is never part of any physics theory, since theories just manipulate abstract notions to create predictions that can be tested.

I have no problem with that. And as you already noticed, Heisenberg defended a very similar position.

What this also means is that we are untrue to the scientific method when we hold that any interpretation of quantum mechanics is a statement about objective reality, because if the theory was never demonstrably that, then so much less is an interpretation of a theory.

Except that again and again, interpretations did just that. But you are right that Laplace was "untrue to the scientific method" when he did that. So you seem to be talking about what "interpretations in an ideal world" should be, not about how proponents of different "interpretation" behave in our world.

Instead, an interpretation of a theory is a kind of lens through which to look at a theory, a way to understand or make sense of the theory, not objective reality itself (as the latter is only what we observe, and the predictions we test thereby).

But if you look at a sphere from the front, no lens will change the fact that you cannot see the back. And if somebody else looks at the back, he can see something unrelated and different from what you see. And just like in the parable of the blind men and an elephant, he might start to argue that your observations contradict his observations.

When I write that you misunderstand the relation between theory and interpretation, then I have something in mind like my own surprise, when I learned that a group representation is defined as

More formally, a "representation" means a homomorphism from the group to the automorphism group of an object. If the object is a vector space we have a linear representation. Some people use realization for the general notion and reserve the term representation for the special case of linear representations.

Intuitively, I would have expected that it should say monomorphism. But it says homomorphism. But the words realization and representation suggested to me that the "entire group" should be encoded, not just some arbitrary aspect of it. But then I learned that things just work better this way. And it is the same with interpretations. It is OK that Bohmian mechanics can only interpret non-relativistic QM, and has trouble interpreting QFT. It keeps its status as an interpretation, even if it cannot overcome its troubles with QFT. And other interpretations are allowed to miss aspects of QT too. For example, I would argue that MWI is currently blind when it comes to temperature.

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I'm still not seeing any observable contradictions. You are explaining what an interpretation is. I'm explaining what a physics theory isn't.

In total, I gave four examples. The MWI example and the Euclidean geometry example were carefully selected "especially for your request", and are in principle open to experimental falsification. However, your reaction was a sort of disbelief, or at least the feeling that I somehow lost you, or didn't get your point.

I initially thought about using the Eikonal equation and the inviscid Burgers' equation as examples. Here, the math stays silent about what happens in the singularities (i.e. the points where the characteristics intersect), and there are different "interpretational attitudes", from "singularities only occur in pathological artificially constructed mathematical examples," over "obviously the entropy solution is the correct one, and the math told you that from the start", and "the caustic with multivalued functions is the correct solution, because the equation comes from geometric optics", to "we are agnostic as to what happens in the singularities, because this is outside of the domain of application for which the equations have been derived".

But I was unsure whether you would accept those equations as physical or mathematical theories. They are not theories in my sense, because nobody calls them theories, they are called equations. (For me, the actual usage of the words in practice is what is most important.) The investigation of partial differential equations on the other hand is called a theory, but "today" those interpretational ambiguities are already part of that theory. But maybe it would have been clearer for you that there are different interpretations with observable consequences, which can be right or wrong, depending on context. (They can contradict each other, but not the equation.)

It sounds like you are saying the Copenhagen interpretation predicts something about quantum computers that other interpretations do not predict.

Why are you so sure that the prediction from MWI is correct? I just say that Copenhagen is agnostic, especially with respect to scalability, as long as the context of temperature and non-isolation is missing. Maybe MWI should stay agnostic too, but some of its proponents claim that scalable quantum computers would be one of its predictions.

That would be impossible, because then the CT would not be an interpretation of a theory, it would be different theory that makes different predictions using different equations.

Maybe MWI simply tempted some of its proponents to make a stupid mistake? Maybe "stupid" was an unfortunate word from my side. What I mean is "unnecessary" or "avoidable", in the sense that making that mistake is not necessary for defending the interpretation.

It could not use all the same equations, because how can you use all the same equations and make a different prediction? I think you are not talking about interpretations of QT, you are guessing at whatever the next theory might be.

But this is exactly the point, at issue is not "how it can be modeled", but rather, the testable predictions it makes. From the perspective of a scientific theory, everything else is counting angels on a pin. From the perspective of personal philosophy, and inspirations for the next theory, that's where we have interpretations.

Guessing at the next theory is not what I do. I admit that my writing may be hard to follow, or that I didn't get your point. But this "next theory" thing was nowhere contained in my text. You have put it in there yourself. Also for Euclidean geometry, it is not that general relativity is a better theory where it is no longer "literally" valid. It might simply not be "literally" valid in the world out there (without any next theory having anything to do with that).

 

[kered rettop]

Well, Schroedinger's Cat started, as Schroedinger himself said, as a "quite ridiculous case". It was intended to highlight a serious problem with the Copenhagen Interpretation. However the essential problem has not gone away.

 

[vanhees71]

Which problem, do you think, "has not gone away"? Of course, the Copenhagen Interpretation is indeed a problem, particularly with those variants which postulate a collapse.

 

[kered rettop]

Which problem, do you think, "has not gone away"? Of course, the Copenhagen Interpretation is indeed a problem, particularly with those variants which postulate a collapse.

 

[kered rettop]

Yeah that was careless of me. I just meant that we still need to understand the superposition, not that there is an unsolved problem in "ordinary" QM. Of course, my preferred understanding is MWI, which many people would say is "quite ridiculous". But that's another problem.

 

[vanhees71]

What do you think is the problem with superposition? I have never understood what the point of the MWI is. For me, QT predicts probabilities for the outcome of measurements, and in everyday lab experience there's a clear outcome of each measurement (modulo if there are technical problems with the equipment, but this has of course nothing to do with QT as a theory). The outcome is random with probabilities as predicted by QT, and QT tells me that this randomness is an objective property of Nature. That leaves just the statistical (ensemble) interpretation of QT, and with this "minimal interpretation" there are never contradictions or logical problems, and until today, even under very stringent tests, no example for a failure of QT has become known.

If there is some problem with QT, it's not a physical one, as far as the realm of applicability is concerned. The only real problem is the apparent incompatibility between QT and general relativity, i.e., a QT of the gravitational interaction. The problem here seems to be, however, not a mere interpretational/philosophical problem but the lack of empirical input for figuring out, how a QT of gravitation should look like.

 

[kered rettop]

What do you think is the problem with superposition? I have never understood what the point of the MWI is. For me, QT predicts probabilities for the outcome of measurements, and in everyday lab experience there's a clear outcome of each measurement (modulo if there are technical problems with the equipment, but this has of course nothing to do with QT as a theory). The outcome is random with probabilities as predicted by QT, and QT tells me that this randomness is an objective property of Nature. That leaves just the statistical (ensemble) interpretation of QT, and with this "minimal interpretation" there are never contradictions or logical problems, and until today, even under very stringent tests, no example for a failure of QT has become known.

If there is some problem with QT, it's not a physical one, as far as the realm of applicability is concerned. The only real problem is the apparent incompatibility between QT and general relativity, i.e., a QT of the gravitational interaction. The problem here seems to be, however, not a mere interpretational/philosophical problem but the lack of empirical input for figuring out, how a QT of gravitation should look like.

I didn't say there was a problem with superposition, in fact I said there wasn't a problem in ordinary QT.
Thanks for explaining your preferred interpretation.

 

[kered rettop]

One has to start the experiment by putting the cat, poison and trigger in the box so the state is completely specified when the box is closed. What then causes the state to become entangled?

The triggering atom evolves into a superposition of |decayed> and |not decayed> The |decayed> state includes the emitted particle, which triggers the killing mechanism and kills the cat. The |not-decayed> state essentially leaves the cat unaffected. So the complete state is a superposition of |dead-cat stuff> and |living-cat stuff>
I'm not sure where the term entanglement came into the discussion, but you can define an entanglement as a superposition of correlated states. So just divide the "stuff" into the atom and the cat etc. You then have a superposition of |atom intact>|cat alive> with |atom decayed>|cat dead> which is, by definition, an entanglement between the cat etc and the atom.

 

[vanhees71]

Indeed, in the same article Schrödinger introduced his famous cat, he also coined the notion of entanglement as THE key aspect of the interpretational issues of QM in his time. For me it's incomprehensible, why this very clearly written article is so much less famous than the cloudy EPR article with the opaque response by Bohr. For the English translation, see

https://www.jstor.org/stable/986572

 

[PeterDonis]

I'm not sure where the term entanglement came into the discussion

Because there is an interaction between the radioactive atom and the cat (mediated by the poison that the decay of the atom releases) that entangles their respective degrees of freedom.

you can define an entanglement as a superposition of correlated states.

This is not the correct definition of entanglement. The correct definition is that an entangled state of a system cannot be expressed as a product state of its subsystems.

 

[kered rettop]

I'm not sure where the term entanglement came into the discussion

Because there is an interaction between the radioactive atom and the cat (mediated by the poison that the decay of the atom releases) that entangles their respective degrees of freedom.

You misunderstand me. I was not asking why entanglement come into it. I was querying why a discussion that had been using the term superposition exclusively, suddenly became a question about entanglement. To answer it was necessary to bridge between the concept of superposition and that of entanglement. Which I attempted to do.

you can define an entanglement as a superposition of correlated states.

This is not the correct definition of entanglement. The correct definition is that an entangled state of a system cannot be expressed as a product state of its subsystems.

True. I should probably have said something like "a superposition of correlated states is a sufficient condition for the state to be an entanglement according to the correct definition". But the rest of what I said follows anyway, and I really don't think anyone is likely to have been confused.

Have a Happy New Year!

 

[kered rettop]

There is never any problem if the components "stay in their lanes": observations establish objective reality, theory is a prescription for making testable predictions that show up in objective reality when we test them. All we have to do is take greater pains to track what science actually does, and the "Schroedinger cat problem" never appears at all.

That's a fairly common stance, but it only crops up in connection with QM, all other sciences expect their theories to be ontic. But since 1957 we've had ontic QM in the shape of MWI. You don't need to exclude onticity in order to get rid of the SC problem.

 

[gentzen]

That's a fairly common stance, but it only crops up in connection with QM, all other sciences expect their theories to be ontic. But since 1957 we've had ontic QM in the shape of MWI. You don't need to exclude onticity in order to get rid of the SC problem.

We don‘t need to wait for MWI, de Broglie Bohm was there much earlier, and is ontic too. Don‘t get me wrong, MWI has its place and motivations too, and it would be good for PF to have some true MWI proponent(s) again.

 

[vanhees71]

The trouble with de Broglie-Bohm is that there's no satisfactory version for relativistic QFT. Also, I don't get the obsession about a theory being "ontic" or "epistemic". I think all physical theories are about a quantative description of observable facts from as little assumptions as possible.

 

[PeterDonis]

I was querying why a discussion that had been using the term superposition exclusively, suddenly became a question about entanglement.

I would say, because entanglement is the correct term for this discussion, whereas superposition is not.

 

[kered rettop]

Ha ha! Yes, quite so.

 

[tomj]

Do you have a reference for superconducting qubits? AFAIK quantum computing experiments are done using polarizations of single photon states as qubits.

A cat made up of super conducting qubits would be one cool cat

 

[PeroK]

A cat made up of super conducting qubits would be one cool cat

 

[Ken G]

That's a fairly common stance, but it only crops up in connection with QM, all other sciences expect their theories to be ontic. But since 1957 we've had ontic QM in the shape of MWI. You don't need to exclude onticity in order to get rid of the SC problem.

Indeed I'm arguing something like the converse of this. It's true that if our goal is to regard ontology as the primary goal of science, then even QM can be shoehorned into that picture. However, I'm saying that QM is the place where we encounter what we should have known all along: ontology is a convenience of science not the goal of science. The goal of science is purely epistemological, as all science must be, by its very definition. QM is trying to remind us of that, but if we are not listening, we can ignore it there too.

 

[Ken G]

The trouble with de Broglie-Bohm is that there's no satisfactory version for relativistic QFT. Also, I don't get the obsession about a theory being "ontic" or "epistemic". I think all physical theories are about a quantative description of observable facts from as little assumptions as possible.

That means you are arguing for epistemics. The distinction between ontics and epistemics is the former is about what exists, and the latter is about our knowledge. A quantitative description that checks with observed facts is all clearly about knowledge. Knowledge of the quantities, knowledge of the observations, knowledge about how to compare the two in some kind of fair or useful way, all decided inside our brains. None of it has anything to do with "what actually is", it's all about "what we can know, and how we can use that knowledge to achieve our goals." That's literally exactly what epistemology is all about, or so it seems to me-- perhaps you think of something else associated with that term. (What meanings we associate with terms is also super important, so we should probably say that science combines epistemology with semantics, and uses ontology as a convenience along the way, but we should use that convenience carefully for fear that we begin to think that science is fundamentally ontological. Regarded as ontology, the history of science is a history of dismal failure. Regarded as epistemology, the history of science is a history of spectacular success.)

 

[kered rettop]

Indeed I'm arguing something like the converse of this. It's true that if our goal is to regard ontology as the primary goal of science, then even QM can be shoehorned into that picture. However, I'm saying that QM is the place where we encounter what we should have known all along: ontology is a convenience of science not the goal of science. The goal of science is purely epistemological, as all science must be, by its very definition. QM is trying to remind us of that, but if we are not listening, we can ignore it there too.

Well thanks for that, Ken, but I really have no palate for teleological or prescriptive theories of science. So "I'm out".

 

[vanhees71]

That means you are arguing for epistemics. The distinction between ontics and epistemics is the former is about what exists, and the latter is about our knowledge. A quantitative description that checks with observed facts is all clearly about knowledge. Knowledge of the quantities, knowledge of the observations, knowledge about how to compare the two in some kind of fair or useful way, all decided inside our brains. None of it has anything to do with "what actually is", it's all about "what we can know, and how we can use that knowledge to achieve our goals." That's literally exactly what epistemology is all about, or so it seems to me-- perhaps you think of something else associated with that term. (What meanings we associate with terms is also super important, so we should probably say that science combines epistemology with semantics, and uses ontology as a convenience along the way, but we should use that convenience carefully for fear that we begin to think that science is fundamentally ontological. Regarded as ontology, the history of science is a history of dismal failure. Regarded as epistemology, the history of science is a history of spectacular success.)

We can only know "what actually is" by observations. So what should ontology add to science, which is about what we can objectively observe in Nature and finding descriptions in terms of a few fundamental "natural laws" that describe a lot of different phenomena. Asking for more in the sense of "what it really is" is not a question answered by the Natural Sciences, and this restriction makes it much more successful and applicable than such metaphysical speculations. That's why, if I'm sick, I'll rather take some medication based on science rather than speculations like homeopathy ;-)).

 

[kered rettop]

We can only know "what actually is" by observations. So what should ontology add to science, which is about what we can objectively observe in Nature and finding descriptions in terms of a few fundamental "natural laws" that describe a lot of different phenomena. Asking for more in the sense of "what it really is" is not a question answered by the Natural Sciences, and this restriction makes it much more successful and applicable than such metaphysical speculations. That's why, if I'm sick, I'll rather take some medication based on science rather than speculations like homeopathy ;-)).

Disproven speculations!

 

[PeterDonis]

This thread has run its course and is now closed. Thanks to all who participated.