Can QM interpretations be reconciled?

[entropy1]

Is there already consesus in the (mainstream) scientific world that the various interpretations of QM can't and won't be reconsiled? (unless, perhaps, they find a completely different mathematical framework?)

And what is the cause of that?

 

[jfizzix]

There is at least consensus as to the predictions of quantum mechanics, and we can use it to predict the results of many experiments to surprising accuracy.

However, since the interpretation of quantum formalism is tied up in philosophical debates in the foundations of science transcending quantum mechanics (such as, whether a Bayesian of a Frequentist interpretation of probability is more sound), I don't think we will ever have a broad consensus as to precisely what relation our mathematics has on elementary properties of the real world (beyond the minimalist stance that we can use it to calculate things we want to know).

Rarely, but occasionally, someone actually derives a testable consequence of a particular kind of interpretation, and in doing so, we can sometimes rule out certain interpretations. In particular, the experimental violation of Bell inequalities rules out local hidden variable interpretations of quantum mechanics.

 

[entropy1]

Suppose there would be found an entirely different interpretation covering all others, perhaps accompagnied by a slight change in math. Would such a thing be conceivable?

 

[jfizzix]

It is conceivable that we could find a better model for physics than quantum mechanics (i.e., give more accurate predictions), but interpreting this new trans-quantum formalism may well be at least as difficult as interpreting standard quantum mechanics because the interpretation has to make sense of the same experimental data (such as the diffraction and interference of atoms).

 

[Strilanc]

Suppose there would be found an entirely different interpretation covering all others, perhaps accompagnied by a slight change in math. Would such a thing be conceivable?

The extra details added by interpretations are mutually contradictory. Collapse says only one outcome survives, many-worlds says all the outcomes continue to be tracked.

The closest thing to an interpretation covering all of them is their intersection: the raw math of quantum mechanics. The "shut-up-and-calculate" interpretation.

 

[entropy1]

@ jfizzix

I am thinking of the probabilistic nature of the quantum world; is this probabilistic nature a fundamental property of of the quantum world? With respect to this thread: does the nature of nature prevent us per se (in any case) of knowing certain things about it? And consequently, does this uncertainty dictate (limit) the theories we develop about it?

 

[entropy1]

The closest thing to an interpretation covering all of them is their intersection: the raw math of quantum mechanics. The "shut-up-and-calculate" interpretation.

I heard that expression come along very often. Does that mean that the math of QM is so good, that we don't want nor need a different one? Or: is it (almost) self-evident, logical, symmetrical, etc.? I am really wondering about this, for as a layman, it all seems so complicated! And your answer to that would really help me a lot!

 

[jfizzix]

Thus far, we have never seen an experimental measurement that contradicts the predictions of quantum theory. This doesn't mean the current formulation of quantum mechanics is manifestly "correct", but only that it fits the data we see from the situations we've been able to look at. In more extreme situations (ultra high energies, black holes, etc) it may be that our current quantum theory fails to give accurate predictions, and a better theory may lay yet undiscovered.

 

[entropy1]

So the fact that the interpretations contradict each other shouldn't worry us very much, right? It just occurs to me why the fact that they do shouldn't be a driving force behind developing a consistent and comprehensive theory? (I just don't seem to get it yet )

 

[jfizzix]

So the fact that the interpretations contradict each other shouldn't worry us very much, right?

Since the interpretations don't give different predictions of experimental results, there's no need to be worried.

Somehow, physicists have hit upon "something" that appears to work really well.
Perhaps someday we'll have a way of integrating quantum theory into a satisfying conceptual model of reality.
That will be a good day.
Until then, quantum mechanics will likely be as hard to learn as a foreign language where most of the words have no English translation, so you're stuck going over it again and again until you start getting the gist of what they mean.

 

[A. Neumaier]

Does that mean that the math of QM is so good, that we don't want nor need a different one? Or: is it (almost) self-evident, logical, symmetrical, etc.? I am really wondering about this, for as a layman, it all seems so complicated!

Yes, the math of quantum mechanics is in an excellent state (if one neglects some yet unsolved problems in quantum field theory that don't affect anything in the interpretation of quantum mechanics). While not self-evident, it is simple, logical, elegant, efficient, and comprehensive, hence has all the attributes something permanent should have.

Of course it is complicated for someone not well educated in math. But it is not more complicated than any other subject that requires a number of years of concentrated study to be mastered.

 

[vanhees71]

In short: Interpretations are a philosophical or metaphysical issue of pretty little relevance to physics. The physical part is pretty convincingly solved with the just achieved loophole-free Bell experiments. All unanimously lead to the conclusion that local realism is ruled out and quantum theory is correct. The physical part of the quantum theoretical interpretation, i.e., the minimal interpretation is unique and thus the various ideas of metaphysics behind the formalism are very interesting but irrelevant for physics as a science. It's of course of high relevance for philosophy.

 

[vanhees71]

Of course it is complicated for someone not well educated in math. But it is not more complicated than any other subject that requires a number of years of concentrated study to be mastered.

I'd say the math of the QM 1 lecture is even simpler than the math for classical electrodynamics. You just deal with a scalar field and with differential operators like the Laplacian. For E&M you need pretty all of the vector calculus and a bit of tensor calculus either. At least when I think back to my undergrad studies the electromagnetics lecture was mathematically more challenging than the QM1 lecture. What's of course way more difficult is the physics, because QT is much more abstract than all of classical physics (point mechanics as well as field theory), where you have a pretty simple one-to-one mapping of the mathematical objects to the physical ones, i.e., the quantities in the formalism are linked to the observables directly via operational definitions ("Meßvorschriften"). This is different in QT: Here you have an abstract level of description in terms of a (rigged) Hilbert space and (unbound) linear operators on it, and the link between these abstract objects and observables in the real world is pretty complicated via the probabilistic meaning of the quantum state.

 

[entropy1]

I have a layman idea (not a theory!) that I've never seen discussed elsewhere yet. In short it proposes having global realism rather than local realism. Now my question is: is there an interpretation that endorses global realism? (by that I mean that decoherence into macro objects (the universe itself) has to be taken into account considering outcomes of measurements) I hope I am clear enough. I can't share the details, but I'd really like to know is such kind of interpretation exists!

 

[Nugatory]

I have a layman idea (not a theory!) that I've never seen discussed elsewhere yet. In short it proposes having global realism rather than local realism. Now my question is: is there an interpretation that endorses global realism? (by that I mean that decoherence into macro objects (the universe itself) has to be taken into account considering outcomes of measurements) I hope I am clear enough. I can't share the details, but I'd really like to know is such kind of interpretation exists!

If by "global" you mean "non-local", dBB is non-local and realistic.
Decoherence is a prediction of the mathematical formalism of QM, so it happens with all interpretations.

 

[entropy1]

If by "global" you mean "non-local", dBB is non-local and realistic.

Thanks!

Decoherence is a prediction of the mathematical formalism of QM, so it happens with all interpretations.

But does there exist a model involving spacetime-stamps-labeling (information) when decoherence in the form of entanglement takes place?

 

[bcrowell]

The extra details added by interpretations are mutually contradictory. Collapse says only one outcome survives, many-worlds says all the outcomes continue to be tracked.

The closest thing to an interpretation covering all of them is their intersection: the raw math of quantum mechanics. The "shut-up-and-calculate" interpretation.

It sounds like you're visualizing the wrong Venn diagram here. The intersection of CI and MWI is MWI; that is, the postulates we use for MWI are a proper subset of the postulates we use for CI. Here's a good discussion of that: http://www.preposterousuniverse.com...hanics-is-given-by-the-wave-function-squared/ . Because of this structure, no experiment can disprove MWI without disproving CI as well. In fact, the postulates of MWI are so generic to quantum mechanics that any violation of them (e.g., a violation of unitarity) would probably disprove quantum mechanics in general, including all its interpretations.

 

[A. Neumaier]

the postulates of MWI are so generic to quantum mechanics that any violation of them (e.g., a violation of unitarity) would probably disprove quantum mechanics in general, including all its interpretations.

This is not only specific to the MWI but to all interpretations of quantum mechanics that deserve this name. If there are predictions that differ from canonical quantum mechanics one speaks of a modification of QM (as, e.g., in the Ghirardi-Rimini-Weber objective collapse theory), not of an interpretation in the strict sense.

 

[stevendaryl]

I don't actually see a huge difference between the various interpretations. If you start with ManyWorlds, then you can, at any time, make the pragmatic decision to throw away any "possible" world that is not reachable from yours (by "yours", I mean the one that is consistent with your memories). That pragmatic decision looks like a collapse. If you do this purging of unreachable possible worlds every time you make a measurement, what you're doing will be indistinguishable, for practical purposes, from Copenhagen.

I've heard an argument that Bohmian mechanics can be interpreted via ManyWorlds, as well, where you split up the ManyWorlds according to positions of particles, and then just declare one of those to be "the real world", and the others are only relevant through the quantum potential.

 

[maline]

If you do this purging of unreachable possible worlds every time you make a measurement, what you're doing will be indistinguishable, for practical purposes, from Copenhagen

One difference is that in Copenhagen you can postulate the Born Rule of probability as an additional law beyond the unitary evolution of the state. In deterministic conceptions the probabilities must somehow be derived from "first principles", leading to much debate whether this is possible in MWI, and some debate in dBB as well.

 

[haushofer]

In short: Interpretations are a philosophical or metaphysical issue of pretty little relevance to physics. The physical part is pretty convincingly solved with the just achieved loophole-free Bell experiments. All unanimously lead to the conclusion that local realism is ruled out and quantum theory is correct. The physical part of the quantum theoretical interpretation, i.e., the minimal interpretation is unique and thus the various ideas of metaphysics behind the formalism are very interesting but irrelevant for physics as a science. It's of course of high relevance for philosophy.

Can one make such a clear distinction between philosophy and physics? I'd say an interpretation is part of the physics, not 'mere philosophy'.

 

[haushofer]

@ jfizzix

I am thinking of the probabilistic nature of the quantum world; is this probabilistic nature a fundamental property of of the quantum world? With respect to this thread: does the nature of nature prevent us per se (in any case) of knowing certain things about it? And consequently, does this uncertainty dictate (limit) the theories we develop about it?

No. A counterexample is the Bohmian interpretation.

 

[bcrowell]

This is not only specific to the MWI but to all interpretations of quantum mechanics that deserve this name. If there are predictions that differ from canonical quantum mechanics one speaks of a modification of QM (as, e.g., in the Ghirardi-Rimini-Weber objective collapse theory), not of an interpretation in the strict sense.

It sounds like you've misunderstood my post. Have you looked at the Carroll article that I linked to? His axioms 3-5 do not "differ from canonical quantum mechanics," but they do distinguish CI from MWI.

 

[A. Neumaier]

It sounds like you've misunderstood my post. Have you looked at the Carroll article that I linked to? His axioms 3-5 do not "differ from canonical quantum mechanics," but they do distinguish CI from MWI.

They distinguish it only in the talk. The predictions are not different, since both collapse and many worlds are just different ways of talking about stuff that doesn't figure in the actual calculations. Both are interpretations of orthodox quantum mechanics.

My qualification about differing theories was aimed at other theories, with an objective collapse, not at MWI and CI.

 

[bcrowell]

They distinguish it only in the talk.

I don't know what you mean by "in the talk."

the postulates of MWI are so generic to quantum mechanics that any violation of them (e.g., a violation of unitarity) would probably disprove quantum mechanics in general, including all its interpretations.

This is not only specific to the MWI but to all interpretations of quantum mechanics that deserve this name. If there are predictions that differ from canonical quantum mechanics one speaks of a modification of QM (as, e.g., in the Ghirardi-Rimini-Weber objective collapse theory), not of an interpretation in the strict sense.

It sounds like you've misunderstood my post. Have you looked at the Carroll article that I linked to? His axioms 3-5 do not "differ from canonical quantum mechanics," but they do distinguish CI from MWI.

The predictions are not different, since both collapse and many worlds are just different ways of talking about stuff that doesn't figure in the actual calculations. Both are interpretations of orthodox quantum mechanics.

I don't think you're reading carefully enough. Your "This is not only specific to the MWI..." is incorrect. A counterexample would be Carroll's axiom 5. Your "The predictions are not different..." is also incorrect. A counterexample is Carroll's axiom 4 (the Born rule); it is logically possible for an experiment to disprove the Born rule without disproving Carroll's axioms 1-2 (which constitute MWI). However, Gleason's theorem says that this has to have other undesirable consequences.

I agree with your general point of view that interpretations are more philosophy than science, but it's important to get the logic right and not oversimplify.

 

[ogg]

I wonder why nobody who knows enough to add substantitively to this post (I don't, really) didn't jump all over the claim that quantum mechanics is correct. We KNOW it is NOT correct - is NOT a full theory of sub-atomic processes! That is clear. Also, an interpretation is a mental construct which depends on our brains' (and minds') structure and capabilities. Einstein said something like the most incomprehensible thing about the Universe is its comprehensibility - but he was no fan of QM. It could just be that we simply aren't capable of correctly interpreting. There is nothing "wrong" with simply applying the rules and math to solve a problem without an intuitive understanding of the "why it works" of that process. It is unsatisfying, but it is what it is. We also can give example after example where our interpretation of the Laws of Physics has led to deeper Laws and further insight, so I'm not arguing that we should throw up our hands and just accept the status quo, but I am saying that we do not need to pick the correct interpretation in order to apply it. Anyway, two things we know QM fails at: 1. Gravity (as is well publicized) and 2. The arrow of time. QM may or may not be consistent with with Dark Matter and/or Dark Energy, we just don't know about them...yet.

 

[Nugatory]

We KNOW it is NOT correct - is NOT a full theory of sub-atomic processes!

"Not correct" and "not a full theory" are different things, so it is quite possible for a theory to be correct yet not to be a "full theory". For example, the ideal gas law, $PV=nRT$, is about as correct as any theory can be (it's called the ideal gas law because two centuries ago it was fashionable to call the really successful theories "laws", and the name stuck), but it's not the whole story because it emerges from statistical mechanics.

It's possible that there is a more full theory that underlies quantum mechanics in the same way that statistical methods applied to classical single-particle physics underlie the theory of ideal gases - but so far no one has found such a thing. Even if such a thing is eventually discovered, it wouldn't make quantum mechanics incorrect, it would explain why quantum mechanics is correct.

 

[Jeff Rosenbury]

@ jfizzix

I am thinking of the probabilistic nature of the quantum world; is this probabilistic nature a fundamental property of of the quantum world? With respect to this thread: does the nature of nature prevent us per se (in any case) of knowing certain things about it? And consequently, does this uncertainty dictate (limit) the theories we develop about it?

I believe there is a limit on what can be known. But this isn't anything mystical. I like an example from music:

Suppose a really good piano player plays a whole note middle c (261.626 Hz). We can measure the frequency and thus know the note. Now suppose she plays a half note? We have less of a sample, so the frequency is less precise (but still fairly easily measured). But as the note gets shorter and shorter it gets harder and harder to measure precisely. In fact, when the note is less than 1/2 a wavelength long, it becomes impossible to tell what the note is. There simply isn't enough sound to measure. We can take a statistical sample of many notes and hope to reconstruct something like a middle c, but we can never be sure just by listening.

The nature of nature is that the information simply isn't there.

We could also guess (interpret) that the sound came from the middle c of a piano. But perhaps it came from a horn? Or perhaps the piano was a little out of tune? Or perhaps a thousand other things. We simply can't tell from the sound.

Of course some day we may develop a way of "seeing" the piano with other senses (assuming there is one), but we seem to far from that point now in QED.

But then I'm an electrical engineer and may be way off base here.

 

[vanhees71]

Can one make such a clear distinction between philosophy and physics? I'd say an interpretation is part of the physics, not 'mere philosophy'.

Yes, one can. As Einstein said (talking about theoretical physicists), look at their deeds rather than listening to their words. This means, you should look at how the theory is applied to describe the outcome of real-world experiments. Then you know what's the physical core of a theory. Everything else is metaphysics and philosophy. I don't deny that these are important from a cultural point of view and should be addressed, but it's not of much relevance for physics itself. Of course, natural science is only a small subset of knowledge we can achieve, and thus the philosophical aspects of physics are interesting and important, but one must strictly keep apart the different levels of knowledge. E.g., the difference between the interpretations is not subject to empirical tests and thus not part of physics, because by definition all interpretations lead to the same testable predictions concerning the outcome of measurements.

If, on the other hand, an "interpretation" makes different predictions from standard QT, you created a new theory. E.g., the idea of local realistic hidden-variable theory is distinguishable from QT, because it predicts Bell's inequality, which is violated according to QT. All experiments known so far rule out local realistic hidden-variable theories, even with all loopholes claimed to be closed. I'm not sure about the latter claim, but it is nevertheless very unlikely that local realistic hidden-variable theories are correct. That's the great achievement of Bell's work on the foundations of QT: It ruled out the EPR definition of "reality" showing that it is in fact not "real" ;-)).

 

[ProfChuck]

It is conceivable that we could find a better model for physics than quantum mechanics (i.e., give more accurate predictions), but interpreting this new trans-quantum formalism may well be at least as difficult as interpreting standard quantum mechanics because the interpretation has to make sense of the same experimental data (such as the diffraction and interference of atoms).

Predictability is a classical physics concept. Determinism is assumed in classical physics while probabilism is at the core of quantum physics. I see three fundamental models of reality; classical determinism, quantum probablisim, and chaotic existentialism.

Classical determinism assumes the ability to predict the future state of any system based on the properties of the current state. The ability to predict the location of celestial bodies at some arbitrary point in time, future or past, is an example of classical determinism.

Quantum probabilism assumes that the future state is unpredictable and indeterminate. The inability to predict when a radio isotope will decay is an example of quantum probabalism.

Chaotic existentialism is deterministic but unpredictable. The inability to predict the future value of a Mandelbrot set solution is an example of chaotic existentialism.

Each of these provides a testable model of some observable aspect of reality but none of them can be considered complete. I suggest that classical determinism is an illusion brought about by the aggregate behavior of vast numbers of quantum states but It is not clear how one might go about proving that.

 

[Ghost117]

Although I haven't yet started QM, according to the layman's history I've read on it, the 'Copenhagen Interpretation' only became orthodox due to the influence of Bohr, Heisenberg and Pauli. It kinda came down to the politics between physicists apparently...

I've also read there were experiments using the Pilot Wave/de-Broglie-Bohm model recently which shows that it works really well to explain certain phenomenon. The problem is that no one uses this model and it is not taught at all. I also remember something about how dB-B can be shown to survive the Bell's Inequality issue somehow... although I don't have the expertise to discuss this further at this stage...

 

[vanhees71]

dBB is just another interpretation of quantum theory. All physically relevant outcomes are the same as with the minimal interpretation. It doesn't add anything to quantum theory. It "survives" the Bell inequality issues, i.e., the incompatibility of socalled local realistic hidden-variable models with quantum mechanics, because it's non-local by construction. I don't see any need for teaching it. You can just listen to a standard-quantum theory 1 lecture and then read about it in textbooks and papers for yourself.

 

[stevendaryl]

dBB is just another interpretation of quantum theory. All physically relevant outcomes are the same as with the minimal interpretation. It doesn't add anything to quantum theory. It "survives" the Bell inequality issues, i.e., the incompatibility of socalled local realistic hidden-variable models with quantum mechanics, because it's non-local by construction. I don't see any need for teaching it. You can just listen to a standard-quantum theory 1 lecture and then read about it in textbooks and papers for yourself.

I don't think it's that simple that dBB is the same as the Copenhagen interpretation. Specifically for position measurements, you can show that, under the assumption that the initial position of a particle is randomly chosen according to the $|\psi|^2$ distribution at one time, the probability of finding the particle at a particular location at a future time is again $|\psi|^2$ (evolved forward in time according to the Schrodinger equation). But, beyond that, there are questions about the equivalence (or at least, I have questions--the answers might be well-known to someone else):

1. If you do two measurements in sequence, what wave function $\psi$ do you use after the first measurement? The original, or the "collapsed" one? If you use the original one, then for the second measurement, your assumption about the relationship between $\psi$ and the probability of the particle being in some position is no longer true---you know exactly where it as after the first measurement.
2. What about other sorts of measurements that are not about position---for example, energy measurements or spin measurements, or momentum measurements? It's been claimed that in practice, all we ever measure is position, and that we infer other dynamic quantities from this. We estimate velocities (and thus momenta) by positions at two different times. We compute spin by noting which way a particle is deflected by a magnetic field. Etc. So it might be the case that dBB is for all practical purposes equivalent to Copenhagen, but it's not as trivial a conclusion as it first appears.

 

[rkastner]

The extra details added by interpretations are mutually contradictory. Collapse says only one outcome survives, many-worlds says all the outcomes continue to be tracked.

The closest thing to an interpretation covering all of them is their intersection: the raw math of quantum mechanics. The "shut-up-and-calculate" interpretation.

And of course that's not an interpretation. It's a statement that one should not try to understand the physical referents of quantum theory. In that sense, it's anti-science.

 

[rkastner]

I wonder why nobody who knows enough to add substantitively to this post (I don't, really) didn't jump all over the claim that quantum mechanics is correct. We KNOW it is NOT correct - is NOT a full theory of sub-atomic processes! That is clear. Also, an interpretation is a mental construct which depends on our brains' (and minds') structure and capabilities. Einstein said something like the most incomprehensible thing about the Universe is its comprehensibility - but he was no fan of QM. It could just be that we simply aren't capable of correctly interpreting. There is nothing "wrong" with simply applying the rules and math to solve a problem without an intuitive understanding of the "why it works" of that process. It is unsatisfying, but it is what it is. We also can give example after example where our interpretation of the Laws of Physics has led to deeper Laws and further insight, so I'm not arguing that we should throw up our hands and just accept the status quo, but I am saying that we do not need to pick the correct interpretation in order to apply it. Anyway, two things we know QM fails at: 1. Gravity (as is well publicized) and 2. The arrow of time. QM may or may not be consistent with with Dark Matter and/or Dark Energy, we just don't know about them...yet.

We are not capable of interpreting QM if we think classically, that's for sure! That was why Bohr ended up saying anti-realist things. See my recent paper on this:
http://arxiv.org/abs/1601.07545
Also, QM can be a complete theory of atomic processes if one recognizes the important role of bound states. See: http://arxiv.org/abs/1601.07169