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I've been digging a lot into quantum physics in the last few weeks. I didn't care much about the maths, just about what empirically happens to get a conceptual idea about quantum phenomena.

The most widely accepted interpretation of quantum mechanics seems to be the Copenhagen one. If I got it right, it's heavily relaying on the two following principles (among others):

  • Superposition: a quantum system is at the same time in all the states it could possibly be in. When it's measured, it instantaneously collapses in a single state.
  • Entanglement (aka "spooky action at a distance"): if two or more quantum systems are entangled, it means that some of their properties are correlated. When measuring a system, all the entangled ones collapse in a state coherent to the measured one. Simultaneously. No matter how far away they are to each other.

I'm not able to believe it. It allows some unrealistic paradoxes (e.g. Schrödinger's cat paradox), and I have the feeling that this interpretation (and its consequences) is what makes quantum mechanics look so weird, mysterious, unnatural and spooky to the public. Besides I've read from a few sources (like this Google Tech Talk) that this interpretation has proven to be broken: the math says everything is continuous and doesn't hint to anything like collapsing, and even more important, the quantum eraser experiment contradicts the Copenhagen interpretation.

The second most popular interpretation, many-worlds sounds a lot more natural to me, although it strongly smells like science fiction.

I believe there must be many interpretations that would hold better and would be a lot less weird than the two mentioned ones.

What I'm wondering is, then: why does the Copenhagen interpretation (and to a lesser degree the many-worlds one) remain the most accredited one?

Emilio Pisanty
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peoro
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    My bet would be on the fact that it is the most practical one. – Ignacio Vergara Kausel Oct 21 '13 at 14:20
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    Actually, I read an article several years ago that said that in the last decade MWI had passed Copenhagen as the most widely accepted interpretation among physicists. – RBarryYoung Oct 21 '13 at 15:34
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    ...though apparently, that is a matter of some dispute. – RBarryYoung Oct 21 '13 at 15:44
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    You've got it wrong. Superposition and entanglement are part of the theory (quantum mechanics) not a specific interpretation. – MBN Oct 21 '13 at 16:03
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    @MBN: of course, but the definition I reported for them is specific of Copenhagen interpretation. Most other interpretation define them in different ways, often without including the idea of wave collapsing. – peoro Oct 21 '13 at 16:13
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    So you have been "digging a lot into quantum physics in the last few weeks" and you "don't care much about the math". This leaves room only for opinion based discussion, so I vote to close. – my2cts Mar 19 '19 at 21:30
  • I'm voting to close this question as off-topic because it asks not about the Copenhagen interpretation but about a ridiculous straw-man interpretation in which a quantum system is "at the same time in all states it can possibly be in". – WillO Mar 19 '19 at 22:46
  • @WillO Why is that a ridiculous strawman? – user76284 Jun 07 '19 at 17:28
  • @user76284 : Because the very definition of a state requires that a system be in exactly one state, and no sensible innterpretation --- invluding Copenhagen --- has ever said otherwise. – WillO Jun 07 '19 at 17:47
  • I'm voting to close this question as off-topic for the same reason as before. – WillO Jun 07 '19 at 17:48
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    @WillO The statement is not far off from “if a physical system may be in one of many configurations then the most general state is a combination of all of these possibilities, where the amount in each configuration is specified by a complex number”, which as far as I can tell is accurate. And it makes no sense to close the question for only that reason. – user76284 Jun 07 '19 at 18:04
  • @user76284 : the number of planets in a solar system can be (among other things) 1 or 2 or 5 or 7. It can also be equal to any sum of those numbers, such as 14. But if somebody posts a question that relies on the premise that the number of planets in a solar system can have more than one value, I will vote to close that one too. – WillO Jun 07 '19 at 18:11

5 Answers5

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Why is the Copenhagen interpretation the most accepted one? I would say the answer is this:

  • it's the oldest more or less "complete" interpretation
  • hence you'll find it in many (all?) early text books, which is basically from where people writing modern text books copy from.
  • the overwhelming majority of physicists doesn't really care about the interpretation, since it (up to now) is only a matter of philosophy. We cannot know what interpretation is correct, because we can't measure differences, hence the interpretation question is a matter of taste rather than scientific knowledge.
  • most standard QM courses at university (at least the ones I know) don't bother with the interpretation. They just introduce the concepts, updates of knowledge, etc. and in that sense, the Copenhagen interpretation is just convenient.

This implies that if you ask a lot of physicists, some have never even thought about the matter. If interpretation is a matter of philosophy, why should we worry about it then? I can think of two points here:

a) By thinking also about interpretations of our theory we may come up with new theories that give us "nicer" interpretations of existing results, but they are essentially inequivalent to quantum mechanics. Bohmian mechanics from what little I understand about it is such a candidate, which might turn out to at one point make different predictions than classical quantum mechanics (up till now, it's just a different interpretation). This is of course a very good reason to think about it, because if quantum mechanics can not explain everything and there is a better theory, which can explain more with similarly "simple" assumptions, we want to have it.

b) It might help our understanding of "reality". This is only interesting, if you believe that your theory describes reality. If you believe that we only ever create effective models that are limited to a certain domain of our variables, then interpretations become uninteresting. Your model isn't the real deal after all, so why bother with something, you can't measure? It doesn't enhance our knowledge.

So, if you don't believe that science should (or even can) provide ontologic theories and if you don't think a better theory than quantum mechanics is maybe just beyond the horizon, then you don't care about interpretations of quantum mechanics. Otherwise, you should.

Martin
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Due to historical reasons, physicists who do not have a strong preference for a particular interpretation default to the Copenhagen one, despite some of its pseudo-mythical outgrowth - which you can just ignore if you are in the 'shut up and calculate' camp.

It doesn't help that every other interpretation (at least those I know of) contains some flaw or quirk I find unacceptable as well, which would leave me with the statistical one (and perhaps consistent histories), basically not explaining anything at all.

The ones I like best are Cramer's transactional one and de Broglie's double solution, with the caveat that these should be backed by a theoretical framework beyond quantum mechanics, but aren't.

Personally, I'm one of these cranks who think that we should be able to back quantum mechanics with a realist theory (but a superdeterministic one): Start from de Broglie's double solution, throw in the geon model of elementary particles and ER=EPR and you're good to go.

Christoph
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  • "Start from de Broglie's double solution, throw in the geon model of elementary particles and ER=EPR and you're good to go." If it's that easy, please publish the paper and get a Nobel Prize! – sasquires Dec 10 '21 at 03:11
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I believe that Bohr's strong personality is the major reason for the popularity of the Copenhagen interpretation, and I agree that "Shut up and calculate!" is the default interpretation for those not concerned with ontology.

Re superposition: all it says is that if there are two possible states then their linear superposition is also a possible state. A system is in only one state, but you can express that state as a linear combination of other states. Think of them as coordinates.

Entanglement is a consequence. not a basic principle. See, e.g., https://en.wikipedia.org/wiki/EPR_paradox.

shmuel
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Fundamental flaws in basic reasoning and perpetuation of myth (largely perpetuated by pop culture physicists granting both fame and money). In science, the best interpretation is the one that looks at the physical truth and doesn’t make a grand over reaching proposition counterintuitive and against all reason.

So that if I see the two slits experiment, I notice that when a light passes through these two slits, I can observe an interference pattern, the only conclusion is that given the exact same slits, with the exact same light passing through, we should observe the exact same interference pattern.

If I then pass this light through a crystal, and get two independent beams with complimentary properties, all I can really say is that given the same crystal and the same light, I should see two independent beams with complimentary properties.

Additionally, just because a mathematical model works doesn’t mean it is a representation of the physical universe in any way. There are numerous valid mathematical means to show the revolution of the sun around the earth…. This is inventive, but not very useful to the physical reality of our solar system.

We can only conclude from a working mathematical model that we have discovered a mathematical model that works. (It’s a huge leap to say that our model defines reality!). For instance, I might build an accurate model of a town using glue and carved figures, but that in no way should ever mean that real towns are built using glue and carved figures.

Finally, any physicist that tries to tell you that there is no difference between saying “we cannot simultaneously observe a particles position and its momentum” and “photons do not have both position and momentum” is missing a critical point. Just because we cannot with our instruments observe a photons position and its momentum does not imply, suggest, or in any way indicate that photons do not have both a position and a momentum. And, btw…. We can… just simply catch a photon (quanta of light) inside a polarizing lens (your sunglasses)- and wow, we have a location- the spot the photon last touched, and a momentum- whatever the particular frequency was that was absorbed!

Einstein’s main criticism about QM then (and most reasonable people’s criticism now) was/is that quantum mechanics has great math but lousy physical interpretations. He also said that the nature of physics demanded that we look deeper than just a probability. That we needed to investigate something real, measure something real. That physics is about the real physical world. And that QM just completely forgets /loses sight of the physical world (admittedly not good for physics). http://arxiv.org/pdf/1107.3701.pdf

user38649
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  • You can't even know if photons and electrons exist. You just see some images which might have no physical reality. – jinawee Jan 31 '14 at 14:26
  • Nice answer, just... the impossibility of photon's position is not meant for the event of interaction - of course, when some atom gets excited, the EM energy is present there. The problem with photon's position is different: it is meant for "free-motion". The quantum theory of radiation does not suggest any simple way to talk about photon's position consistently during interval there is no absorption and no emission; it does not use this concept. – Ján Lalinský Jan 31 '14 at 15:00
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    I think you are missing the point with " wow, we have a location- the spot the photon last touched, and a momentum- whatever the particular frequency was that was absorbed! " The uncertainty is a probabilistic uncertainty and the observation of interactions does not contradict it. It would contradict it after an accumulation of interactions/measurements that would disagree with the proposition, i.e. one could predict exactly where in x,yz the the photon of momentum hnu/c would be before measuring it. – anna v Jan 31 '14 at 16:20
  • "ust because we cannot with our instruments observe a photons position and its momentum does not imply, suggest, or in any way indicate that photons do not have both a position and a momentum." Bell showed that the two cases are distinguishable and Aspect et. al showed that Einstein was wrong on this one. – dmckee --- ex-moderator kitten Sep 15 '14 at 01:27
  • Position and momentum are different bases. A state of definite momentum is a superposition of position basis states. In a very direct, real way, a particle cannot have both definite position and momentum. – apdnu Nov 08 '17 at 21:28
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Copenhagen is considered the de-facto interpretation of Quantum Mechanics for historical and sound practical reasons. It is the least speculative of the various interpretations (no need for a multitude of alternative Universes or interactions with consciousness or ...) and the product of much debate amongst the great minds of the time (Bohr, Heisenberg, ...)

The Google Tech Talk is very confused. I suggest that you ignore it.

DriveByPhysicist
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    There is a very real sense in which the de facto interpretation is "shut up and calculate", simply because most physicist don't need to interpret for many (or any) purposes. – dmckee --- ex-moderator kitten Sep 11 '14 at 14:58
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    Collapse of the wavefunction is no more or less speculative than MWI. Speculation implies that one could eventually find out the result. This doesn't happen with interpretations of quantum mechanics, which can never be tested empirically. –  Sep 11 '14 at 15:32