# By what logic does one come to support a certain interpretation of quantum mechanics?

I have been on Wikipedia reading up on the various interpretations of quantum mechanics. I am entering my final semester as an undergraduate and have taken a QM course, so I am familiar with the basic theory, although I can't say I've done extensive reading on these.

However, it is my understanding that different interpretations of QM do not yield falsifiable predictions, so how can anyone really support any particular one? For example, I have heard people ridicule the Copenhagen interpretation, saying how preposterous it is that the possible outcomes/eigenstates that are NOT measured simply cease to exist, that they disappear from the universe... but then these scientists will claim that the universe splits at the moment of measurement, creating a universe for each possible outcome/eigenstate. Have they not made as bold (and perhaps ridiculous) of a claim as those who support the Copenhagen interpretation? No experiment will verify one or the other.

So my question is, by what logic does one come to support a particular interpretation of quantum mechanics?

• This question is, imo , a near duplicate of physics.stackexchange.com/q/133906 and if you look at the list on the right of previously asked questions you might get an idea of how different people think about this issue. I think it is more a discussion or opinion question than a straight q and a about an aspect of physics. – user108787 Jul 15 '16 at 20:06

The answer given above explains that there are some experiments that might be used to distinguish between different interpretations. However, it is a myth that experiments are the only way in which one can distinguish between theories in science. Many theories are discarded without any experimental tests of their consequences. A theory can be discarded for internal inconsistency, for clashing with another explanation, or because it is ad hoc.

A theory is ad hoc if it was invented to solve a particular problem and has no implications beyond that problem. Suppose that I feel uncomfortable about curved spacetime. I might say that spacetime is flat but that invisible, undetectable pixies push bodies around as if general relativity was true. Nobody would take such a theory seriously for very long, although it is not experimentally distinguishable from general relativity. There would not be extensive debates about the existence of the pixies. The theory would have been invented solely to dodge the straightforward implications of general relativity. The pixy theory is a bad explanation, it proposes that the pixies will push bodies around according to the equations of general relativity without explaining why or how they would do that.

The Copenhagen interpretation (CI) is in the position of the pixy theory. The CI amounts in substance to saying that quantum mechanics is true, except when it happens to imply that macroscopic objects exist in multiple versions. This leads to a theory that is non-local, non-Lorentz invariant and tries to build a distinction between macroscopic and microscopic objects into fundamental physics. This is entirely unnecessary since decoherence that different versions of macroscopic objects are dynamically isolated from one another without such ad hoc tricks. Descriptions of the CI that take it seriously as an explanation of how the world works make it sound ridiculous. So you have two options.

(1) Throw out the CI.

(2) Throw out the scientific standard that an idea should be stated clearly and judged by whether it solves problems, explains stuff, is self consistent and consistent with experimental evidence.

Some commentators above have expressed dissatisfaction with the lack of a quantum field theory version of Everett. Since there are papers describing quantum field theory in Everettian terms, this objection makes no sense:

http://arxiv.org/abs/quant-ph/0204024

• Indeed, Everett and QFT go well together. One can bitch about Deutsch, but you would never have people like Steven Hawking, Sidney Coleman etc. all experts in QFT and quantum gravity coming out in strong support of the MWI. – Count Iblis Jul 16 '16 at 20:13
• I don't think the person complaining about Deutsch knows anything about him since Deutsch is not ignorant of QFT. He wrote papers on QFT with Candelas and Sciama. – alanf Jul 17 '16 at 0:14

It is often claimed that the various interpretations of quantum mechanics are equivalent, in the sense that they yield the same predictions for the outcome of experiments. But for the 3 most popular interpretations, the Copenhagen Interpretation, the Many Worlds Interpretation and the De Broglie–Bohm theory this is only true when making certain (hidden) assumptions, like e.g. that the measurement apparatus is always macroscopic and will thus itself always decohere irreversibly.

An experiment that can distinguish between the Copenhagen Interpretation (CI) and Many World Interpretation (MWI) has been proposed by David Deutsch. Note that falsifying MWI is easy, all you have to do is demonstrate in an experiment that an isolated systems undergoes decoherence that is not due to residual interactions with the environment. This is not as easy as it sounds as any anomalously fast decoherence can be interpreted as an interaction with new physical degrees of freedom (e.g. dark matter particles, interactions via extra dimensions etc. etc.) that then need to be tested for in different experiments.

But how can we experimentally falsify CI? David Deutsch has proposed the following (thought) experiment. Suppose that large scale quantum computing makes it possible to implement an AI in a quantum computer. You could even imagine uploading your brain continents to a quantum computer. Then suppose that you are alive inside a virtual quantum simulation where you set out to measure the z-component of a spin (that is then implemented by a qubit) that is polarized in the x-direction. Then consider a transform to the state this is the initial state except that you keep a record in your memory that a measurement was actually performed. It's easy to see that this transform is unitary, and it is known that a quantum computer can approximate any unitary transform to arbitrary accuracy.

So, this means that you can undo the measurement, while you can still verify that a measurement was actually carried out (the result of the measurement will necessarily be erased). The spin will be restored in the original state, and you can verify by measuring the spin in the x-direction, and repeating this whole experiment many times and verifying that you always find the same spin.

Now, if only one branch of the wavefunction is physically realized, then the unitary transform back to the original state except for keeping the information about the measurement being carried out, would not work. If you apply the unitary transform to only one branch, the state of the spin won't be restored to the initial state. So, this experiment would verify to you that when you measured the z-component of the spin that both possible outcomes were indeed physically realized.

De Broglie–Bohm theory is also said to be equivalent to Copenhagen Interpretation, but just like the case of the MWI, this isn't true either. De Broglie–Bohm theory is only equivalent to regular QM if one assumes quantum equilibrium, deviations from this state can be tested as it yields probabilities that deviate from the Born rule.

So, the answer to the question "by what logic does one come to support a particular interpretation of quantum mechanics?" is just as anything else in physics, the outcome of experiments. The only problem is that we don't have the results to rule out any of the existing interpretations, but that doesn't necessarily mean that they are all equally plausible given what we do know. One can consider thought experiments that will yield different outcomes depending on your interpretation, or you may interpret what is going on in some particular thought experiment differently depending on what your interpretation of QM is. Such arguments are valid arguments in physics.

• Whenever somebody needs a human brain as part of a physics experiment, I smell pseudo-science. Physics experiments, science in general never need a human observer, they are expressing explanations of natural phenomena as combinations of other phenomena. Technically a quantum computer doesn't even produce a result unless coupled to an open environment. It's not possible to undo a quantum measurement in this setup because no measurement has ever been made. The claim that remembering that a measurement was made is not a measurement is even more questionable. Then what, exactly, is "it"? – CuriousOne Jul 15 '16 at 22:35
• By the time a thought experiment requires an AI, uploaded into a quantum computer, you've lost my faith in obtaining meaningful conclusions. This reeks of clocks in the desert and other such fanciful thought experiments that put too much faith in reasoning ones way to the truth. – anon01 Jul 15 '16 at 23:41
• @CuriousOne Physics is local, so if you live in closed universe such that locally it looks like just like our local neighborhood, that cannot possibly affect experimental outcomes. If such a closed universe will eventually undergo a Poincaré recurrence $10^{\text{zillions}}$ years from now, how would that make measurements today not be "real measurements"? – Count Iblis Jul 15 '16 at 23:52
• @ConfusinglyCuriousTheThird This just illustrates the unavoidable consequence of unitary time evolution. You may not believe it, but then the counter experiment for that is to demonstrate that time evolution of a closed system is not unitary. But as long as we assume that time evolution is unitary then Deutsch' argument will hold. – Count Iblis Jul 15 '16 at 23:57
• Time evolution is only unitary in toy models of closed systems. Just like Poincare couldn't look beyond the tiny phase space of 19th century classical physics (don't try to use the same arguments on even a classical electromagnetic wave... you will get nothing but divergences!) Deutsch can't look beyond the non-relativistic single particle model he teaches to his students. – CuriousOne Jul 16 '16 at 1:25