# Quantum Mechanic Interpretations

Context I was watching this video "Do we have to accept Quantum weirdness?" and it said how currently there are several interpretations of quantum mechanics that are consistent with the facts.

Question For quantum mechanics, how does one decide on which is the "correct" interpretation? What experiments could be done that could show one is correct and the other fails to predict such results?

Thanks

• Personally I would be happy with any interpretation that simply makes sense. According to this criteria, we have no interpretation of QM. See arxiv.org/abs/quant-ph/0209123 – Stéphane Rollandin Jan 29 '18 at 15:56

That is precisely the problem. David Mermin said somewhere that every year a new interpretation of quantum mechanics is introduce and none are ever ruled out. This clearly points to the fact that interpretations of quantum mechanics are not Popper falsifiable. Thus, according to strict definition, interpretations of quantum mechanics do not qualify as a science. It lies in the domain of philosophy.

Now, this does not mean that somebody would not some day come up with a way to test (some of) these interpretations experimentally. However, I wouldn't hold my breath.

• This is incorrect. Firstly, anybody who thought a bit about the topic, e.g. today's philosopher's of science, will agree that falsificationism is totally outdated. (It assumes e.g. that experiments cannot contain mistakes.) Secondly, there are always several possible formulations/interpretations of a confirmed physical theory, but the discussion which one helps us to understand nature best is also relevant for science. Mermin, by the way, is a very bad man to quote here because he actually is in strong favor of a certain interpretation (QBism). – Luke Aug 18 '17 at 9:38
• @Luke Philosophers like being radical. If you support falisfiability that you should be a radical falsificationalist. When you finally get that it's childish you should drop the principle altogether... Come on, that's childish too. Interpretations of QM are not really the reformulations - people don't just rewrite theory, they claim something about ontology. – OON Aug 18 '17 at 11:14
• @Luke: Popper falsification is alive and well in the scientific community, despite what philosophers may have to say on the topic. How else would one know whether one's theory is any good? I can understand that people working in a field that cannot make headway through experimentation would feel ambivalent about falsification, but that doesn't change the situation. – flippiefanus Aug 19 '17 at 5:19
• @Luke: It is precisely because Mermin is actively doing research on the interpretations of quantum mechanics that his quote is so pertinent. He knows what he is talking about. – flippiefanus Aug 19 '17 at 5:22

There are important insights that I would like to convey here:

1. All/Most interpretations of quantum mechanics predict exactly the same for all possible experiments.
2. They do, however, strongly differ in the picture of the world they convey, and in the amount of conceptual clarity.
3. One should choose an interpretation/formulation of quantum mechanics according to how useful, understandable and conceptually clear it is to us.

For point 1, the "interpretations" like many-worlds, Copenhagen/ operationalist quantum mechanics, de-Broglie-Bohm, ... have in common that they all give at the level of experiments a formalism with which you can calculate probabilities. These have been shown to be right over and over again in myriads of situations.

For point 2, you might want to read up about the measurement problem. This was a problem that came up in the early formulation of quantum mechanics by Bohr and Heisenberg (which is basically still in the textbooks), but it is not a problem in practice, because still the effective measurement formalism of Copenhagen quantum mechanics works great. But it is a problem in principle or for the picture of the world, it makes the theory less understandable, or less mathematically and conceptually precise. Most interpretations of quantum mechanics try to solve the measurement problem and do so better or worse. And by the way, one could really phrase the measurement problem briefly as: Copenhagen quantum mechanics only works on an operational level and does not tell you how nature really behaves.

For point 3, since there is no experiment which will tell us which interpretation to choose, we are mainly free to choose what we like best. This is not so new to physics and nothing which should be foisted off to the philosophers, because equations alone are not a physical theory, you need words to connect it to the world. The reasons for which we choose an interpretation can be manifold and this is the point in the on-going discussion about interpretations. There is no real consensus about the question "What is a good physical theory?". I would say, personal opinion, that understandable, mathematically and conceptually clear interpretations should be favored. They are easier to teach and, since they are equivalent, give you a good grasp of what happens in atoms and such.

And for the end: No, quantum "weirdness" (which is a bad word) cannot be avoided, since Bell's inequality and the experiments really reveal non-local causality, in fact, and you have to do quite some strange things to try and get around this.

From the point of view of physics the quantum theory requires absolutely no interpretation. It's formulated in the most direct way - you ask "What is a probability to measure something?" and the quantum theory gives you a way to calculate the answer.

When people discuss "interpretations" they actually fall into two categories,

1). Interpretations in the sense of the ontological implications of the quantum theory if you accept it as fundamental which is all fun but can be neither disproved nor verified (because "by construction" everything is restricted to the non-observable) and therefore has no relation to physics as empirical science. You will never know which one is correct.

For example if you work with the quantum cosmology the question "Does the universal quantum state exist in some non-specified sense of the word "exist"?" can have all sort of answers because you may introduce all types of "existence" completely disconnected with the observable picture.

2). Claims about some more fundamental layer beyond the quantum theory that in principle leads to the observable deviation. Now you can't really say that those are just some interpretations of the quantum theory but some new hypotheses. Posing them as just interpretations is dishonest.

For example the objective collapse idea is not actually an interpretation of quantum mechanics but something different because it introduces the new process that leads to potentially observable deviations from the quantum theory (which I will not comment now).

Now there's common trap when the creator may think that the "interpretation" falls into the first category whereas it actually belongs to the second one. For example if you assume that the MWI branching picture is not some very coarse-grained approximation of the universal wave-function in the idealized situation but something exact you immediately introduce some non-quantum macroscopic layer with potential observable consequences.

Similar case is with DeBroglie-Bohm interpretation. If you regroup the quantum equations in a certain way that's just reformulation. But whenever you claim that something posseses classical properties non-existent in the standard quantum theory you can get observable consequences. To check that of course you should provide specific theory (which is problematic in the Bohmian case because of its inherent issues in the relativistic case)

• The 'interpretation' is actually necessary to make it totally precise how the connection between the something you want to measure in real life and the operators in your theory works. This was nicely put by John Bell in his paper "Against measurement". It explains that, of course, the quantum formalism works for all/most practical purposes, but for a good fundamental theory, "professional physicists ought to do better" (quote by Bell). – Luke Sep 6 '17 at 15:51
• @Luke In all talks about "interpretations of the quantum mechanics" no one discusses why this specific experiment should correspond to that specific observable. This is the only interpretational part you need to supplement the formalism with to make it work. The QM (and many technical questions in it!) is very simple to understand when you accept that observables correspond to the experiments we may perform on the object whereas Bell wanted them to correspond to the properties of the object itself. "Bell wants"$\neq$"necessary", "makes Bell sad"$\neq$"wrong"! – OON Sep 7 '17 at 10:05
• You should really read Bell's paper "Against measurement" and maybe take a look at this one, arxiv.org/abs/quant-ph/0308038, which addresses in many details in a certain "interpretation" how the correspondence between observables and experiments actually might work. – Luke Sep 7 '17 at 10:24

Interpretations tie the mathematics to the real world.

There are many reasons why interpretations are important, including

1. Humans need to find an explanation that they personally feel comfortable with. Manipulating equations without understanding is hard and not satisfying. There is a psychological need for interpretation. Psychology however has proven to be a poor guide in the interpretation of Quantum Mechanics

2. Philosophical principles historically have been at the heart of the development of Physics – for example, Galileo’s Principle of Relativity was used by Galileo and Newton to argue against Aristotle.

3. Physics is unfinished business. Research into topics such as Quantum Gravity has reached the point where it is not possible to directly test theories; interpretations should increase our understanding of Physics and point the way forward towards new and better theories.

So, which is the "best" interpretation? If you are only interested in using the equations of QM, then which ever interpretation you are most comfortable with is probably the best. Chemists, for example, do not need to spend time trying to understand every subtlety of QM.

If you plan to produce a viable theory of Quantum Gravity, IMHO you probably need to pick the correct interpretation. How can you tell what that is? That is a topic for a large book, and experiments are of no help. But since you ask, I would suggest starting with

• No changes to standard QM (not really an interpretation if you do that),
• Not sticking with an interpretation simply because bits look like the macroscopic world. For example, IMHO pilot waves are ruled out since Bell's inequality suggests that a particle does not have defined momentum and position and so cannot have a hidden trajectory. However there always seem to arguments to resurrect even bad ideas, so
• (Occam's razor) If the interpretation gets complex just to get around a known phenomenon, it's probably wrong.

Different interpretations are suitable at different phenomena to varying degree. In any case, the quantitative analysis of QM is flawless.

Pilot wave theory explains double slit experiment better than others. Specially collapse of the wave when you place a detector. Taking an example of an electron being looked at with laser, the laser has to reflect off the electron in order to see the electron. Laser is a wave, a wave can not reflect off a wave, it can reflect off a particle, so, in order to be seen, the electron has to give up (collapse/decoher) the wave nature and exhibit particle nature - so the interference pattern disappears.

But in case of entanglement, pilot wave suggests an active, superluminal wave link between the two particles. That is hard to accept.

Ordinary QM, in general fits most phenomena with some obvious issues. For one, it uses a mathematical convenience called superposition. Which is neither provable, nor falsifiable experimentally, so not sure if superposition can be considered proper science. Therefore all explanations rooted in superposition become a matter of belief. That is why, those who believe it, do so staunchly.

Many worlds in my opinion is so unreal/bizarre that I would consider it an escape route rather than an interpretation.