I What is the current perspective on quantum interpretation?

[Quanundrum]

Back in the day, there were a few Quantum Interpretation polls on here, as of late I have not seen any. I love that we now have a sub-forum for Foundations discussions. I figured it would be interesting to see how the participants of PhysicsForums feel about the different interpretations these days. Rather than making a list, which is never exhaustive, it would be great if people simply wrote their preferred interpretation, which also allows for nuance. E.G. "I believe X version of de-Broglie Bohm is promising" or "I am not a proponent of any of the current interpretations, rather I believe some deeper theory with character X, Y and Z is promising"

 

[StevieTNZ]

I trust in the consciousness causes 'collapse' (regardless if its physical or not) interpretation.

 

[Demystifier]

IBM (instrumental Bohmian mechanics)

For details see the link in my signature.

 

[anuttarasammyak]

I poll consciousness.

However I wonder whether interaction with environment, e.g. decoherence of quantum computer chips by heat noise from environment, is in category of observation or quantum leap and if it is so it seems to have little to do with consciousness.

 

[akvadrako]

I see a combination of the subjective aspects of QBism and the objective aspects of many worlds as the most likely. QBism looks at the world from a subjective viewpoint and says the most general theory an agent can have is QM, but it doesn't provide any ontology. If we take those views of agents as all objectively real and overlap them, agent A will see a superposition of agent B and visa-versa, so we get many worlds. Many worlds provides the clear ontology that QBism lacks, but translating it into subjective experience is tough. This is along the lines of multi-solipsism or many minds interpretations.

One more thing is that self-locating uncertainty seems important. Say you are Wigner's friend and you make a final measurement inside you box. An all-powerful Wigner can undo that measurement and your memory, and that's what he would see, but not what you would experience. This is because you aren't really Wigner in that box; you are all places in the infinite multiverse that overlap with your subjective wavefunction, which will continue to evolve as you would expect; you will usually find yourself in compatible and objectively common situation.

I hope in this thread it's okay to get a little more into my personal view of interpretations.

 

[vanhees71]

minimal statistical interpretation

 

[Morbert]

Instrumentalist Consistent Histories

CH because, according to CH, a quantum theory of a microscopic system offers a multiplicity of consistent logics a physicist can use to reason about that system independently from any experimental apparatus interfacing with the system or its membership in an ensemble. It does so with the ordinary observables, sample spaces, and event algebras of QM or more general QFTs etc.

Instrumentalism because the consistent logics mentioned above don't have to come with any realist baggage. A physicist can switch between incompatible consistent logics at their leisure if it aids in predicting measurement outcomes, without having to worry about any ontological* implications of this incompatibility.

*Though I don't yet rule out realist flavours of CH presented by Gell-Mann, Hartle, and Griffiths.

[edit] - For posterity, here are the rules of CH outlined by Roland Omnes

1: The theory of an individual isolated physical system is formulated in terms of a state space and operator algebra and the implied mathematical notions

2: Vectors in Hilbert space evolve in time according to the Schroedinger equation

3: A physical system $S$ can be said to consist of two noninteracting systems $S'$ and $S''$ when the Hilbert space of $S$ can be written as $$\mathcal{H}_S = \mathcal{H}_{S'}\otimes\mathcal{H}_{S''}$$ and the Hamiltonian of $S$ as $$H_S = H_{S'}\otimes I_{S''} + I_{S'}\otimes H_{S''}$$

4: Every description of a physical system should be expressed in terms of properties belonging to a consistent logic. A valid reasoning relating these properties should consist of implications holding in that logic.

5: Physical reality is unique, it evolves in time in such a way that when actual facts arise from identical antecedents, they do so randomly and their probabilities are the ones given by the theory

 

[bayakiv]

Random walk in $\mathbb{R}^3\times S^1$

 

[physika]

Intrisic Collapse.

 

[Sunil]

Caticha's entropic dynamics.

It describes reality as defined by a stochastic configuration space trajectory $q(t)\in Q$ and the wave function as epistemic, providing a nice counterexample to the PBR theorem and all subsequent similar impossibility theorems. Obviously none of them knows that the counterexample has been already proposed before they proved their theorems.

Ref:

Caticha, A. (2011). Entropic Dynamics, Time and Quantum Theory, J. Phys. A44:225303, arxiv:1005.2357

Pusey, M., Barrett, J., Rudolph, T. (2012). On the reality of the quantum state. Nature Phys. 8, 475-478

Leifer, M.S. (2014). Is the Quantum State Real? An Extended Review of $\psi$-ontology Theorems. Quanta 3(1), 67-155 arxiv:1409.1570

 

[EPR]

The Copenhagen.

We should be mum about things unobservable.

 

[PeterDonis]

I have no preferred interpretation. I think the fact that there are so many interpretations, and no general agreement about which one is "right", indicates that we still don't know very much about this domain, and therefore we should not be trying to pick favorites at this point; I think it's highly likely that none of our current interpretations are "right", and that if we end up at some time in the future finding a QM interpretation that does achieve general acceptance as being "right", it will be something we can't even imagine today. We can use QM to make predictions without having to adopt any interpretation (beyond the minimal "interpretation" that tells us how to make the predictions and compare them with experimental results), and as a practical matter, that is enough for now.

 

[vanhees71]

With quantum interpretations it's like with religion. Anybody has one, and there's no right or wrong decision possible.

 

[Demystifier]

With quantum interpretations it's like with religion. Anybody has one, and there's no right or wrong decision possible.

If you want to point out that there is no right or wrong interpretation, it's much more fair and accurate to say that interpretation is philosophy, rather then religion. Philosophy and religion are very different.

 

[vanhees71]

Well, I think interpretation of QT is much more religion than philosophy, but it may depend what you understand under philosophy...

 

[martinbn]

I don't have a preferred interpretation, but recently I have developed some respect and liking for the statistical interpretation.

 

[Demystifier]

Well, I think interpretation of QT is much more religion than philosophy, but it may depend what you understand under philosophy...

What do you then understand by philosophy?

 

[Demystifier]

I don't have a preferred interpretation, but recently I have developed some respect and liking for the statistical interpretation.

Which version, Ballentine's or vanhees's?

 

[martinbn]

Which version, Ballentine's or vanhees's?

Not sure what vanhees' is, so I will say Ballentine's.

 

[Demystifier]

Not sure what vanhees' is, so I will say Ballentine's.

So you agree with Ballentine that QM is nonlocal (his book) and that particles have positions even they are not measured (his review paper, the sentence discovered by you)?

 

[martinbn]

So you agree with Ballentine that QM is nonlocal (his book) and that particles have positions even they are not measured (his review paper, the sentence discovered by you)?

QM is nonlocal in the Bell's sense, there is no arguing about that. I only argue against the use of the word nonlocal. As to the second, yes, you can take it that particles have positions or you can be agnostic about it, or say that it doesn't matter, or that it is meaningless. Whichever one you prefer.

 

[Demystifier]

As to the second, yes, you can take it that particles have positions or you can be agnostic about it, or say that it doesn't matter, or that it is meaningless. Whichever one you prefer.

What do you prefer?

 

[martinbn]

What do you prefer?

I don't subscribe to any interpretation, nor to any of the statistical interpretation versions. So each of them is fine as interpretations go.

 

[MathematicalPhysicist]

Well I am not advocating any interpretation over the others. (since anyway the empirical results are the same in every interpretation).

I did hear this year of the Montevideo Interpretation from some LQGists;
http://www.montevideointerpretation.org/
So it made me wonder, how many interpretations to QM are there?

 

[martinbn]

So it made me wonder, how many interpretations to QM are there?

More than the number of physicists that think about them.

 

[PeterDonis]

More than the number of physicists that think about them.

Sounds like an old joke about politics: when three humans get together, they promptly found four political parties.

 

[martinbn]

I don't remember for sure, but i think Mermin said that. Which makes sense given that he is responsible for Qbism and Ithaca interpretation, and shut up and calculate.

 

[DennisN]

I subscribe to the almost unknown and unorthodox MII (Many Interpretations Interpretation), which only have two postulates:

1. A quantum system exists in a superposition of interpretations until it is measured (in addition to the other QM superpositions).
2. When the system is measured, there is a state reduction to the interpretation favored by the one doing the measurement.

Why this state reduction happens is an open question left for future research.

Seriously:

it would be great if people simply wrote their preferred interpretation

I favor the ensemble (statistical) interpretation...

which also allows for nuance

...because it gives me the least headache.

 

[bhobba]

I formally believe in the minimal statistical interpretation. But I caution those interested in such that all of physics is really a mathematical model. But QM is special - it is a model of something we have no direct experience with. The connection with what we do have direct experience with is via this thing called a observation. What we really need is a way of explaining the world we do have experience with and observations via the model. We haven't quite done that yet - some issues do remain - but much progress has been made and research is ongoing. Current investigations center around 'coarse graining':
https://www.williamjames.com/transcripts/gell1.htm

There is even a particular interpretation that has these ideas central to it (decoherent/consistent histories), but really the existence of quasiclassical realms does not depend on interpretation. It could be viewed as a 'sharpening' of the minimal statistical interpretation or even of the Copenhagen interpretation depending on your 'taste'. I personally am not a 'fan' of Copenhagen for reasons Einstein himself gave. He subscribed to the minimal statistical interpretation - although some will argue his take is different to its modern version as detailed by Ballentine in a classical 1970 paper:
https://www.informationphilosopher.com/solutions/scientists/ballentine/PR70.pdf

Here is Einstein's Version:
https://www.informationphilosopher.com/solutions/scientists/ballentine/AJP72.pdf

You can make up your own mind about differences - if any.

Thanks
Bill

 

[Demystifier]

I don't subscribe to any interpretation, nor to any of the statistical interpretation versions. So each of them is fine as interpretations go.

My problem is that I don't get you. It looks as if you don't care about interpretations, yet you like to discuss interpretations and ask questions about them. Is it just a chit-chat for you, or what?

 

[vanhees71]

QM is nonlocal in the Bell's sense, there is no arguing about that. I only argue against the use of the word nonlocal. As to the second, yes, you can take it that particles have positions or you can be agnostic about it, or say that it doesn't matter, or that it is meaningless. Whichever one you prefer.

Of course, particles have positions (provided they have a position observable). As with any other observable positions can be more or less well determined (they are never precisely determined because the specctrum of the corresponding self-adjoint operator is continuous). I also don't think that "my interpretation" of QT is much different from Ballentine's.

I also don't like to say, "quantum theory is nonlocal", because it can be misleading. The most successful physical theory is local (sic) relativistic quantum field theory and the Standard Model based on it, and there the interactions are by construction local (microcausality principle, upon which the unitarity and Poincare invariance of the S-matrix is guaranteed). I'd rather call the phenomena related to entanglement between far-distant parts of a quantum system "long-ranged correlations", because that's what's described, namely a statistical property which goes beyond the statistical properties deterministic "classical" systems can have.

 

[Demystifier]

I also don't like to say, "quantum theory is nonlocal"

I would say that quantum theory is local, but nature isn't.
Quantum theory in its minimal form is incomplete, in the sense that it cannot answer all questions one can ask about nature. To answer such questions one must talk about quantities (e.g. the Bell's $\lambda$) which are not a part of the minimal quantum theory. Quantum theory in its minimal form retains locality by refusing to talk about such quantities. Experimental violation of Bell inequalities shows that nature is nonlocal, irrespective of the theory.

 

[martinbn]

My problem is that I don't get you. It looks as if you don't care about interpretations, yet you like to discuss interpretations and ask questions about them. Is it just a chit-chat for you, or what?

No, I said that I don't have a favorite interpretation. I am not emotionally invested in any of them. That doesn't mean that I am not interested.

 

[martinbn]

Experimental violation of Bell inequalities shows that nature is nonlocal, irrespective of the theory.

NO! It shows that nature is nonlocal in the Bell's sense. It doesn't show that nature is nonlocal period.

 

[martinbn]

Of course, particles have positions (provided they have a position observable). As with any other observable positions can be more or less well determined (they are never precisely determined because the specctrum of the corresponding self-adjoint operator is continuous). I also don't think that "my interpretation" of QT is much different from Ballentine's.

That is not what I am saying. I am saying that you can think of individual particles having exact positions at any time i.e. they are there at a specific point at any given time.

 

[Demystifier]

No, I said that I don't have a favorite interpretation. I am not emotionally invested in any of them. That doesn't mean that I am not interested.

Does it mean that you are "emotionally invested" in the whole field of quantum interpretations?

 

[vanhees71]

That is not what I am saying. I am saying that you can think of individual particles having exact positions at any time i.e. they are there at a specific point at any given time.

You can determine (prepare) the position of a particle with as small an uncertainty as you want but not precisely due to Heisenberg and Robertson's uncertainty relation, $\Delta x \Delta p \geq \hbar/2$. All what's observable on an ensemble of equally prepared particle positions are the probability distributions, $|\psi(t,x)|^2$.

 

[MathematicalPhysicist]

Sounds like an old joke about politics: when three humans get together, they promptly found four political parties.

I heard some variant of it about jews.
Where there are two jews there are three opinions.

I guess this is common for every human...

 

[Buzz Bloom]

I heard some variant of it about jews.
Where there are two jews there are three opinions.

There is a deeper interpretation of this than the joke. Each individually has a different and disagreeing opinion, but together they can find an agreeable mddle ground.

 

[MathematicalPhysicist]

There is a deeper interpretation of this than the joke. Each individually has a different and disagreeing opinion, but together they can find an agreeable mddle ground.

if you have someone arguing for A and the other one for ~A (negation of A), how can you have a third option?!

 

[Buzz Bloom]

if you have someone arguing for A and the other one for ~A (negation of A), how can you have a third option?!

A disagreement between two opinions does not necessarily imply the two opinions are completely opposite. Opinions generally involve a combination of concepts, including opinions about QM interpretatons.

 

[MathematicalPhysicist]

A disagreement between two opinions does not necessarily imply the two opinions are completely opposite. Opinions generally involve a combination of concepts, including opinions about QM interpretatons.

Obviously if it's possible, but not always it's possible...

 

[ObjectivelyRational]

Of course, particles have positions (provided they have a position observable). As with any other observable positions can be more or less well determined (they are never precisely determined because the specctrum of the corresponding self-adjoint operator is continuous). I also don't think that "my interpretation" of QT is much different from Ballentine's.

I also don't like to say, "quantum theory is nonlocal", because it can be misleading. The most successful physical theory is local (sic) relativistic quantum field theory and the Standard Model based on it, and there the interactions are by construction local (microcausality principle, upon which the unitarity and Poincare invariance of the S-matrix is guaranteed). I'd rather call the phenomena related to entanglement between far-distant parts of a quantum system "long-ranged correlations", because that's what's described, namely a statistical property which goes beyond the statistical properties deterministic "classical" systems can have.

Given the arbitrariness of time ordering of space-like separated events, how does one interpret "correlated" microcausality (as a principle)? I'm not proposing "nonlocal" is any better than "correlated" (all things considered), but does thinking "locally" have its own problems?

 

[vanhees71]

It is utmost important to keep in mind that correlations and causal connections are two different things (not only in the context of quantum theory).

The microcausality principle says that any local observable $\mathcal{O}(x)$ ($x$: space-time four-vector) commutes with the Hamilton density, $[\mathcal{O}(x),\mathcal{H}(y)]=0$ for $(x-y)^2<0$, i.e., if $x$ and $y$ are space-like separated. That implies that there are no faster-than light interactions and thus no faster-than light causal effects.

The correlations described by entanglement can refer to space-like separated measurements on an extended quantum object like the usual polarization-entangled two-photon Bell states used to demonstrate the violation of Bell's inequality, teleportation, quantum erasure by postselection, entanglement swapping, ... Here you clearly predict and also empirically confirm stronger correlations between observables (here the polarization of both photons measured at far distance) than is possible within a local deterministic hidden-variable model (according to Bell).

The cause for the correlations in all such cases is not due to the one or the other measurements, which can be space-like separated and thus cannot causally influence one another within local relativistic QFT, and still you observe the correlations due to entanglement predicted by this theory. Within this theory it's also clear that the correlations between the measured polarization observables is due to the preperation of the photon pairs, i.e., they are made as an entangled pair (usually by parametric downconversion using a BBO crystal and a laser). Such Bell states have the amazing property to describe a very "non-classical" situation: Though the single-photon polarizations are completely indetermined when the photon pair is prepared in such a state, there are strong correlations in the outcome of measurements which can be revealed by comparing the measurement protocols of the two space-like separated measurements (which of course can only be done via sending the information of the outcomes between the observers, which cannot be done in any way via faster-than light signals).

With this "minimal interpretation" of the quantum state, i.e., that it describes (and only describes!) the statistical properties of measurement outcomes when measuring observables of the prepared the system, there is no contradiction between microcausality and the observed correlations between space-like separated measurements on an entangled system.

 

[gentzen]

One question raised in the answers to this Quantum Interpretation poll is what it means that "quantum theory is nonlocal". It could mean that Bohmian mechanics is nonlocal, and that any similar deterministic interpretation has to be nonlocal too. But is this really true, and does it help?

For example, Arnold Neumaier's thermal interpretation is deterministic, but less obviously nonlocal than Bohmian mechanics. Its state variables (i.e. properties of the state) includes nonlocal correlations, and the (state of the) one universe may also be seen as nonlocal in character. But being nonlocal is probably more a statement about the evolution of the state than a statement about the state itself. In conclusion, it is unclear to me whether the thermal interpretation is nonlocal or not, or how I could nail down what it means that an interpretation is nonlocal.

This is the context that made me read Nicolas Gisin’s short book Quantum Chance. Gisin somehow goes beyond the notion of true randomness or “bit strings with proven randomness” and tries to capture the experimentally observable nature of quantum chance itself, independent of any interpretation of quantum mechanics (or even the validity of quantum mechanics). Something like that it is a nonlocal randomness, and because the nonlocal correlations of quantum physics are nonsignalling, it has to be random, because otherwise it would allow faster than light communication.

 

[Demystifier]

It could mean that Bohmian mechanics is nonlocal, and that any similar deterministic interpretation has to be nonlocal too.

Similar non-deterministic interpretations also have to be nonlocal. Examples are the stochastic Nelson interpretation and Caticha interpretation.

 

[vanhees71]

The question is what you mean by "local". Relativistic quantum field theory is local by construction. As any quantum theory it allows for strong non-local correlations, i.e., for correlations of outcomes of space-like separated measurements at far distant places on quantum systems where the far-distantly measured observables on parts of this system are entangled (e.g., the single-photon-polarization measurements on polarization entangled photon pairs created by parametric downconversion).

As usual Einstein was much more careful to formulate his quibbles. He didn't like the confusing EPR paper and wrote another one, where he introduced the much better notion of "inseparability" to describe these long-ranged correlations through entanglement, and his true critizism was more against the Copenhagen doctrine (particularly the collapse hypothesis, which is however only part of some flavors of Copenhagen interpretations) to interpret more into the quantum state than the statistical predictions it makes. He considered QT incomplete, because it only makes statistical predictions on the outcome of measurements. For Einstein only a deterministic description, where all values of observables have, at least in principle, a determined value and can be measured. For him the need of statistics (as in classical kinetic theory) should only be due to limited knowledge about the state of the system but not a property of the state. E.g., in classical mechanics the state is a point in phase space, and if you know this state exactly at one time all the observables of the system have a determined value and can be predicted by solving (Hamilton's) equation of motion, while within QT even complete determination of the state (e.g., by determining the values a complete set of compatible observables, defining a unique pure quantum state of the system) does not imply the determination of all observables' values of this system, and the randomness of the outcome of measurements within QT is not due to incomplete knowledge about the state of the system but "irreducible", i.e., within QT there is no state such that all observables take determined values (that's the content of the Heisenberg-Robertson as well as the Schrödinger-Robertson uncertainty relation).

 

[gentzen]

The question is what you mean by "local". Relativistic quantum field theory is local by construction.

I guess not allowing faster than light signaling will be part of it. I could also try to be more specific. For example, in section "8. Locality and Special Relativity" of the SEP article "The Consistent Histories Approach to Quantum Mechanics", Robert Griffiths writes:

In addition, the histories approach makes it possible to establish on the basis of quantum mechanics itself a principle of Einstein locality (Griffiths 2011b):

Objectively real internal properties of an isolated individual system do not change when something is done to another non-interacting system.

However, being too specific risks limiting the meaning of "local" more than necessary. On the other hand, I do believe that it is possible to define exactly in which sense quantum mechanics (or rather relativistic quantum field theory) should be "local", and that it has actually been defined (in a less trivial way than my quote from Griffiths).

However, I do believe that many quantum interpretations (including the thermal interpretation) will be "local" in this sense (or at least compatible with it), even so I am currently unable to define it. But that won't stop those interpretations from being nonlocal, simply because quantum physics itself is nonlocal. Therefore, clarifying what is meant by "quantum theory is nonlocal" remains important. Describing it as nonlocal randomness like in Gisin's book might be wrong, but at least it has some intuitive appeal to me.

 

[vanhees71]

The problem with the debates about interpretation, which is usually a rather philosophical topic without much meaning for physics as a natural science anyway, is that philosophers tend to use fuzzy definitions of their words, particularly the ones we discuss here (locality and reality among them).

Local in the sense of relativistic QFT means that the Hamiltonian density is a function of the field operators and their derivatives at one spacetime point and that local observables commute with the Hamiltonian density at space-like separated arguments, from which unitarity of the S-matrix and the linked-cluster principle follows, including the impossibility for causal effects propagating faster than light (excluding thus faster-than-light signalling).

 

[gentzen]

As usual Einstein was much more careful to formulate his quibbles. He didn't like the confusing EPR paper and wrote another one, where he introduced the much better notion of "inseparability" to describe these long-ranged correlations through entanglement

Can you be more specific about which paper exactly? I have now checked
Physics and Reality (1936)
Considerations concerning the fundamentals of theoretical physics (1940)
Quantum Mechanics and Reality (1948)
but didn't find the word "inseparability" in them (or any suitable substitute).