Yes, that was my confusion due to old fashioned Copenhagen. Thanks, I get it!PeterDonis said:
PeterDonis said:
haushofer said:
That is a misunderstanding of the relation between a mathematical theory and its interpretations. No interpretation can contradict the theory. But the theory itself stays silent on certain questions, and different interpretations can give different answers to such questions on whicht the theory itself stays silent. And giving a different answer to the same question means that they can contradict each other.Ken G said:
Well, it's only "incomplete", if you don't accept the possibility that Nature is indeed inherently random. There's no hint that this might not be the case. It's anyway pretty unimportant, whether a physical theory is "complete" or not. I think QT is incomplete a priori not because it's probabilistic since it may well be that Nature is "really" random in the sense of QT, but because we don't have a satisfactory quantum description of the gravitational interaction and/or of spacetime. Rather the specific form of the quantum dynamics is dictated in a large part by the symmetries of the underlying classical (!) spacetime model. In which sense this classical spacetime description is to be understood as an emergent phenomenon from a more fundamental (quantum?) theory seems to be likely to be part of the puzzle, how to describe the gravitational interaction within QT.bhobba said:
Can you give an example of something falsifiable that any of the interpretations disagree on?gentzen said:
You speak of contradictions, but I see only disagreements in interpretation, no contradictions at all. Why do people think QT is any different from classical mechanics on this score? One interpretation of CM says there are forces and particles, another says there are extremum principles and symmetries. They make all the same predictions, in a different language. No contradictions there, and no one thinks there are. Do classical particles move due to forces, or don't they? It's pure philosophy, nothing falsifiable, but no one seems to mind that in CM, as well they should not.gentzen said:
But what does it even mean to talk about nature being "inherently" anything? This is physics, nothing is "inherent." That's my whole point, the problem is a tendency to mix physics and philosophy, as if they were the same. But science is one thing, and philosophy is another. Both have their place, and are important, but they are clearly distinct in terms of objective falsifiability. How would one falsify that nature is "inherently" something, rather than just behaving in some way?vanhees71 said:
I agree, but would go even farther. No physics theory is even trying to be "complete," unless we are kidding ourselves, because that has never been the purpose of physics, nor the process of physics. The sketchy history of "completeness" of physics theories should be clear enough on that point, and we must steadfastly avoid the error of every era of physicists thinking that they have seen the final truth in a process that demonstrably simply progresses.vanhees71 said:
It would never make sense to claim that nature is really random, or really determinate. Randomness and determinism are both ways of thinking about nature, which have their various advantages. Nature could never actually be one or the other, that would be a category error. We must not confuse nature with how we think about nature, as the latter is tied up in the motivations we have for thinking that way. (The former is probably not a coherent notion anyway, since all we have is how we think, and how would a way we think be called "inherent" or "actually random" anyway?)vanhees71 said:
The puzzle is indeed the key, and these are deep insights into the theories and the puzzles they represent, without equating the philosophical puzzles with the physics they inspire. Don't get me wrong, philosophy is great for motivating our thinking, it's just something different from the physics. Hence "inherentness" is an attribute of a philosophy, that an individual might take value in, but it is not an attribute of a physics theory because it is merely waiting to be falsified, even when it takes a thousand years to do so.vanhees71 said:
Most mathematical theories can be interpreted both "literal" and "in their original context". For QT, MWI would be an attempt at a "literal" interpretation, and Copenhagen the practice of interpretation close to "the original context" of QT. My reaction to any "literal" interpretation is fear of the dangers of stupid mistakes. I have no illusions about my chances to convince proponents of "literal" interpretations of the existence of such dangers. Logical arguments, like for example those given in Decoherence is Dephasement, Not Disjointness, often have very little chance to reach them. So let me instead go with the existence of scalable quantum computers that are exponentially faster than classical computers for certain problems (like factoring) as a falsifiable predictions of MWI (because of David Deutsch). Notice that quantum computers are outside of "the original context" of QT. Additionally, for Copenhagen, quantum computers are related to limits like zero temperature or perfect isolation, and it remains unclear whether those limits stand in the way of scalability or not.Ken G said:
Where Euclidean geometry is quite untypical is that after the addition of THE missing axiom, it was indeed a complete theory from a purely mathematical point of view. Still, its applicability to the world around us and its range of validity remains open to interpretation. This again is quite typical for mathematical theories, as discussed again and again by Werner Heisenberg in his book Physics and Philosophy:gentzen said:
Werner Heisenberg said:
Paul Davies said:
Sadly, now this comment is already too long to also discuss Bohmian mechanics (and hope that anybody would read it). The examples would have been Bell type experiments where the decision by the experimenter can be modeled by a time dependent Hamiltonian in Bohmian mechanics, but not in MWI. And Quantum State Teleportation where the measurement result dependent unitary transformations applied to particle 3 work fine for Copenhagen, but not for Bohmian mechanics (this is the paragraph just above section 6 Conclusion in the link above):Werner Heisenberg said:
To see that this is incompatible with Bohmian mechanics, note that what is described here would be a backaction of the trajectories on the wavefunction, which is not possible (or at least not included) in Bohmian mechanics.
I'm still not seeing any observable contradictions. You are explaining what an interpretation is. I'm explaining what a physics theory isn't.gentzen said:
And why should you try? What keeps you from making "stupid mistakes" is personal, like all philosophy. It would be valid science to test a thousand people of equal experience and aptitude who are trying to use QT to make predictions within the various interpretations, and score the number of stupid mistakes they make using each interpretation, but you can see how that is not the way debates over interpretations usually go.gentzen said:
The issue is not the "context" of QT, but the predictions of it. If what you are saying is outside the predictions of QT, then it's simply not QT. That's the point, a physics theory makes predictions, and if the predictions are different, it's a different theory, not a different interpretation. I realize that different interpretations of one theory can suggest different ways to alter that theory to create a new one, just as different interpretations of Newtonian physics led to general relativity. But that's because general relativity is a different theory, not because it's a different interpretation of Newton's theory. It is perfectly valid to debate how different interpretations of QT could lead to new different theories, indeed that's precisely what interpretations are for. But you see the pointlessness of arguing the interpretations themselves.gentzen said:
It sounds like you are saying the Copenhagen interpretation predicts something about quantum computers that other interpretations do not predict. 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. 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. To be clear, I don't say that interpretations have no value, they have value in understanding a given theory, and a value in inspiring new theories that are different from the given theory. That's exactly what interpretations are supposed to do, but the interpretations don't make different predictions because only a theory can make a prediction.gentzen said:
If there is such a contradiction, it must not come from "mathematical examples," it must come from different predictions. Experiment A must come out X for one and not X for the other, where X is a quantitative prediction. Theories produce such things, interpretations do not.gentzen said:
You are now talking about mathematical theories. I would have thought it would be very clear in that context that interpretations of a mathematical theory are not a different theory, or they would be called a different theory with different axioms and postulates, not a different interpretation of the same axioms and postulates.gentzen said:
Any mathematical theory is defined by its axioms, so it is logically impossible for an axiom to be missing from one. I believe what you mean is that a better version of the theory that Euclid was trying to create can be made by adding an additional axiom that he did not think to add. That would make it a different mathematical theory. Whether a mathematical theory is "better" by using a different set of axioms is a question completely outside mathematics, but it does relate to the philosophical reasons we do mathematics. That is also like interpretations that are outside of science, but relate to the personal and philosophical reasons we do science.gentzen said:
A new meaning for "interpretation." Now you are not talking about interpretations of a mathematical theory, you are talking about interpretations of real world applicability. That sounds like philosophy for sure, and certainly not mathematics.gentzen said:
Thank you for quoting the insights of Heisenberg, but they just sound to me like a careful distinction between physics and philosophy, very much along the same lines as what I am saying. Heisenberg is saying that the physics is the theory that makes predictions, but how well those predictions correspond to the real world is something outside the physics theory. It is in the realm of testing the theory, and deciding if it meets whatever needs were set out for it. The interpretations of a theory are among the latter of those two, and can be very personal, like all philosophy. Paul Davies quote seems to agree with this as well, it's just what I'm saying. All I'm adding is that I find it curious people think QT is somehow different from all other physics theories, when in fact they all possess these exact same properties. You could take the quotes by Heisenberg and Davies and replace all references to QT with references to Newtonian mechanics and it's all just the same.gentzen said:
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. But just as Heisenberg implied, the interpretations don't tell you anything, you still need to interview reality to obtain knowledge.gentzen said:
Again that all sounds, at best, like confusing an interpretation with a theory, and at worst, a category error about what an interpretation of a theory is. It is not the purpose of a theory to say what is really happening, the purpose is to make testable predictions, period. Some hold that the job of an interpretation is to say what is really happening, but that just sounds like kidding oneself to me. If one stays within the realm of science, the job of an interpretation is to create a personal sense of understanding of a theory, not to make sense of reality (as the latter can be done by no other means than experimentation). It is pure personal philosophy to use interpretations for the latter purpose, which is fine as a personal philosophy, but its only projection onto the scientific process appears in how it can be used to inspire new theories. Whether or not it is ever "what is actually happening" is entirely unscientific, because it is not testable beyond simply testing the theory itself, which never requires any interpretation or philosophy.gentzen said:
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".Ken G said:
I have no problem with that. And as you already noticed, Heisenberg defended a very similar position.Ken G said:
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.Ken G said:
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.Ken G said:
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.
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.Ken G said:
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.Ken G said:
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.Ken G said:
Ken G said:
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).Ken G said:
vanhees71 said:
I didn't say there was a problem with superposition, in fact I said there wasn't a problem in ordinary QT.vanhees71 said:
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>bob012345 said:
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.kered rettop said:
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 said:
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.PeterDonis said:
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.PeterDonis said:
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.Ken G said:
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.kered rettop said:
I would say, because entanglement is the correct term for this discussion, whereas superposition is not.kered rettop said:
A cat made up of super conducting qubits would be one cool catPeterDonis said:
tomj said:A cat made up of super conducting qubits would be one cool cat
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.kered rettop said:
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.)vanhees71 said: