Can QM interpretations be reconciled?

[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 Schrödinger 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

 

[vanhees71]

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 Schrödinger equation).

That's what usual QT says without assuming Bohmian trajectories on top of the usual QT formalism. So what's gained by dBB compared to the standard "shutup and calculate" interpretation?

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.

ad 1) I don't know. It depends on what happened to the system during the measurement, i.e., on the specific apparatus you used to measure the observable.

ad 2) Do you have a specific example? I guess you refer to single particles. Then such an example was how to measure momentum of a particle. For simplicity let's assume we know which particle we have, i.e., its mass and electric charge. Then you can e.g., use a bubble chamber (it's just the most simple example that comes to my mind; nowadays one uses all kinds of electronics, but that doesn't matter for the principle argument) in a magnetic field. The particle leaves a track in the bubble chamber (why it does so was derived by Mott from quantum mechanics as early as 1929); then you can measure the curvature of the track, and with the given mass, charge, and the magnetic field strength you know which momentum the particle had when entering the bubble chamber. Of course, in a way you measured position and inferred from this the momentum of the particle. All this doesn't need any additions to standard "shutup-and-calculate" QT.

 

[mikeyork]

Many of the difficulties in finding consensus around QM have their origin in the relationship between underlying reality and the observer's space-time context. Unravelling that relationship will, I suspect, resolve many of the paradoxes that challenge our common sense (such as the contradiction between non-locality and special relativity).

 

[microsansfil]

We are not capable of interpreting QM if we think classically, that's for sure!

Sure; Neither with ontological interpretation
Patrick

 

[rkastner]

Many of the difficulties in finding consensus around QM have their origin in the relationship between underlying reality and the observer's space-time context. Unravelling that relationship will, I suspect, resolve many of the paradoxes that challenge our common sense (such as the contradiction between non-locality and special relativity).

I offer an account of that relationship that reconciles QM with relativity by acknowledging that QM processes are not spacetime processes (relativistic light-speed limitation applying only to spacetime processes). But that doesn't mean QM processes are not 'real'. The point is that we may need to examine the usual uncritical assumption that 'real' = 'existing in spacetime'. Spacetime may be just the 'tip of the iceberg'. (I discuss this in my new book for the layperson, Understanding Our Unsen Reality--chapter 1 available for free on amazon)

 

[rkastner]

Sure; Neither with ontological interpretation
Patrick

We can certainly provide an ontology for QM. It's just not a classical one. I've offered one. See my post above.

 

[rkastner]

That's what usual QT says without assuming Bohmian trajectories on top of the usual QT formalism. So what's gained by dBB compared to the standard "shutup and calculate" interpretation?
ad 1) I don't know. It depends on what happened to the system during the measurement, i.e., on the specific apparatus you used to measure the observable.

ad 2) Do you have a specific example? I guess you refer to single particles. Then such an example was how to measure momentum of a particle. For simplicity let's assume we know which particle we have, i.e., its mass and electric charge. Then you can e.g., use a bubble chamber (it's just the most simple example that comes to my mind; nowadays one uses all kinds of electronics, but that doesn't matter for the principle argument) in a magnetic field. The particle leaves a track in the bubble chamber (why it does so was derived by Mott from quantum mechanics as early as 1929); then you can measure the curvature of the track, and with the given mass, charge, and the magnetic field strength you know which momentum the particle had when entering the bubble chamber. Of course, in a way you measured position and inferred from this the momentum of the particle. All this doesn't need any additions to standard "shutup-and-calculate" QT.

deB/B may be falsified; see http://arxiv.org/pdf/1410.2014v1.pdf
However there is a nice physical account of entanglement experiments via TI. See Chapter 5 of my CUP book.

 

[vanhees71]

Very interesting, but it's not a real surprise since dBB is known to have notorious trouble with relativistic QFT. Photons are good candidates for falsifying it, because there's not even a position operator for photons.

 

[microsansfil]

We can certainly provide an ontology for QM.

Of course. It is your faith.

Patrick

 

[C Davidson]

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.

Except if you go with Einstein, who said the biggest loophole in QM is that quantum theory is incomplete. The Universe is real and local and no Bell theorem test to date disproves that. Look up T-duality and understand 1/R space first. String theory has a way out. For the "shut up and calculate" crowd, what if the math produces the right answer, but you have the wrong model?

 

[DrChinese]

For the "shut up and calculate" crowd, what if the math produces the right answer, but you have the wrong model?

I can't see how that would bother them. In that view, there is no model other than the math itself.

 

[vanhees71]

If the math produces the right answer, it's the right model. QT can be formulated adequately only in mathematical form. There's no other intuition than the math!

 

[zonde]

deB/B may be falsified; see http://arxiv.org/pdf/1410.2014v1.pdf

This paper is faulty. It attributes to deBB open contradictions with relativity. In derivation length contraction is ignored. But this is not accurate. You still have to use Lorentz transformations when you calculate something in moving frame or alternatively you can derive specific laws for bodies at motion in preferred frame.

 

[atyy]

Very interesting, but it's not a real surprise since dBB is known to have notorious trouble with relativistic QFT. Photons are good candidates for falsifying it, because there's not even a position operator for photons.

But in fact standard QED is not relativistic, because of the Landau pole.

 

[vanhees71]

Well, at the energy-momentum scale defined by the Landau pole I'd not trust any of our contemporary QFTs. I don't see what this has to do with dBB or QED being "not relativistic". It's simply undefined at energy-momentum scales, where it is inapplicable. All QFTs are effective theories, no matter whether they are Dyson renormalizable or not.

 

[atyy]

Well, at the energy-momentum scale defined by the Landau pole I'd not trust any of our contemporary QFTs. I don't see what this has to do with dBB or QED being "not relativistic". It's simply undefined at energy-momentum scales, where it is inapplicable. All QFTs are effective theories, no matter whether they are Dyson renormalizable or not.

For example, is QED consistent with lattice QED at small but finite lattice spacing?

 

[vanhees71]

I'm not aware of anything problematic concerning lattice QED.

 

[haushofer]

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.

So you define physics as a science where phenomenology is more important than ontology? I'm fine with that, but I guess that choice is more or less physicist-dependent :P

Also, from the past we have seen that questions which seemed metaphysical then, turned out to be important to construct something which was called physics later. Think e.g. about the properties of spacetime. So I'm not convinved that one can make such a clear cut between "metaphysics" and "physics".

 

[vanhees71]

Well, in my opinion, the history of science shows that the phenomenological approach is the reason for the success of modern science. I'm not aware of any example, where "scholastic" thinking lead to a profound result in theoretical physics. Even a genius as Einstein has had no luck with the scholastic approach in his attempt to find a unified classical field theory of gravity and electromagnetism that substitutes quantum theory.

The properties of spacetime came into closer inspection by FitzGerald, Lorentz, Poincare and finally Einstein, entirely driven by the incompatibility of Maxwell theory with Galileo symmetry, and thus was very well based on empirical facts since Maxwell's electromagnetism was from the very beginning solidly based on the collected empirical knowledge about electromagnetic phenomena, particularly by the newest comprehensive investigations by Faraday.

In my opinion, the philsophy of science cannot provide much guidance to find new physical models or even theories, but it is of course important to analyze the physical models found by analysis of empirical findings and mathematical consistency in order to understand its wider cultural context.

From this point of view there's not much necessity in ontology, because the natural sciences are based on observations (or in terms of experiments which lead to quantitatively refined observations again) about really existing phenomena. Science doesn't tell you about something "deeper" than just the phenomena, and that's its "ontology" and nothing else.

 

[Jeff Rosenbury]

Isn't the idea that scientific explanations must be logically consistent a scholastic one? No experiment would be falsifiable without the ability to contradict.

 

[atyy]

I'm not aware of anything problematic concerning lattice QED.

At finite spacing lattice QED is non-relativistic, so it could mean that QED is consistent with a non-relativistic theory. Consequently, QED is not a good candidate to falsify dBB.

 

[A. Neumaier]

At finite spacing lattice QED is non-relativistic, so it could mean that QED is consistent with a non-relativistic theory.

There is nothing nonrelativistic about QED. It can be defined perturbatively from the start in a manifestly covariant form, using [URL='https://www.physicsforums.com/insights/causal-perturbation-theory/']causal perturbation theory[/URL]. Then there is no question of a cutoff or a lattice spacing.

 

[atyy]

There is nothing nonrelativistic about QED. It can be defined perturbatively from the start in a manifestly covariant form, using [URL='https://www.physicsforums.com/insights/causal-perturbation-theory/']causal perturbation theory[/URL]. Then there is no question of a cutoff or a lattice spacing.

No. Causal perturbation theory does not construct the theory.

 

[A. Neumaier]

No. Causal perturbation theory does not construct the theory.

Neither does lattice QED. Both construct approximations to QED.

Moreover, lattice QED is never used in practice. All high accuracy tests of QED are based on the covariant form and further approximations derived from it. Thus the physical QED is defined by the covariant definition, while lattice QED is a toy object only.

 

[Ralph Dratman]

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

Your idea sounds interesting, but I have no idea what you mean by spacetime-stamps-labeling. Explain, please.

 

[Dirk Pons]

The starting question related to whether the multiple interpretations of QM could be reconciled to each other or whether a 'completely different mathematical framework' would be required.

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 different interpretations of QM all start from the same basic mathematical principles, and then offer different ways to make sense thereof, hence 'interpretations'. But the basic premises are the same: particles are zero-dimensional points with intrinsic properties that manifest stochastically over space.

No one interpretation explains everything, and none give answers that are satisfactory from the perspective of physical realism (not the same as local realism).