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After reading this question, I feel I understand why quantum mechanics is so confusing (and so often confused by the media): It can be either local (if A causes B, then there must be time for a signal to travel at the speed of light from A to B) or real (the definition of which is a bit fuzzier to me), but not both. The confusion stems from the fact that it makes more intuitive sense for a layperson (like me) to view it as non-local: I.E., that faster-than-light communication is possible.

However, scientists seem to agree on the other option, the "non-real" option. My question is this: Is this an option? The way I interpreted the answer to the linked question is that any consistent theory of quantum mechanics can be either non-local or non-real. Therefore, if you get enough scientists together, they can form a theory based on non-locality, and it will explain the universe just as accurately as the current theories based on non-realism, just in a different way.

If this is true, then why have scientists agreed on non-realism? If this is not true, where did I go wrong in my interpretation?

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    $\begingroup$ Closely related: physics.stackexchange.com/a/7277/80818 $\endgroup$ – tok3rat0r Aug 3 '15 at 19:24
  • $\begingroup$ I read "this question" and I agree that quantum mechanics is very confusing. I think that the best comment I have seen so far involves the fact that humans experience the world in a classical sense, and are trying to describe a non-classical phenomenon. Due to this, it's no wonder that no explanation so far has been seen as completely adequate. $\endgroup$ – David White Aug 3 '15 at 20:13
  • $\begingroup$ The realism part involves measuresement of quantum objects, which neglects experience in that macroscopic objects do have both a well defined position and momentum. Heisenberg just says you can't measure both when looking at quantum particles because the act of measuring changes (part of) the state of the particle. Measuring means influencing the particle. $\endgroup$ – jjack Aug 3 '15 at 21:10
  • $\begingroup$ citeseerx.ist.psu.edu/viewdoc/… for a relational quantum mechanic (local, non-real) interpretation. $\endgroup$ – Ilya Grushevskiy Apr 28 '17 at 17:20
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The answer to the linked question is misleading (and I've left a comment there). You don't get to choose between nonlocal and non-real. While experiments violating Bell's inequalities do imply that no local, hidden-variables (i.e. real) theory is consistent with QM, that theorem must be taken in context with an earlier argument it's supposed to be a reply to, namely, the famous paper by Einstein, Podolsky, and Rosen (EPR). That paper argues that local QM theory implies that there are hidden variables (not quite the same as saying that local QM is always realistic, but it is in the context of entangled states). Taken together, the logical conclusion is that no local theory consistent with QM works - you're stuck with nonlocality.

You still have a choice between realistic and nonrealistic theories (or perhaps theories that are not completely realistic but do contain some hidden variables), but to produce predictions consistent with Bell's inequalities they will have to be nonlocal. The classic examples are Bohmian "pilot wave theory", which is realistic but highly nonlocal, and traditional QM, which is nonrealistic, but with its nonlocality apparently (?) being limited to some esoteric situations like particle pairs being generated in entangled states. Note that both those theories give the same predictions, so in that sense they are indistinguishable. That means it's probably an empty question to ask whether "reality" is realistic or unrealistic, or at least you can't tell based on those theories, since both the realistic theory and the nonrealistic theory provide equally good representations of reality.

To answer your question about why scientists have settled on the unrealistic theory (i.e. QM), it's because, compared with the only viable realistic theory (so far), which is Bohmian mechanics, it was developed first, it's easier to use for computation, and it has proven highly useful and successful. Bohm never intended pilot wave theory for everyday use; he just wanted to demonstrate that a realistic theory could be made.

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  • $\begingroup$ Why is relational quantum mechanics, which preserves locality but is non-real, so out of fashion? $\endgroup$ – Ilya Grushevskiy Apr 28 '17 at 17:19
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It isn't as simple as realism versus B. Why?

Because you can be a realist about different things.

When you do a spin "measurement" on a spin 1/2 particle you get two results. And ... you leave the particle in a new state. We know this because if you prepare a thousand systems identically they have a whole fraction that give you one result and a complementary fraction that gives the other result.

But after the "measurement" you get a new thing, a thing that for sure gives the same result if you measure it again. So the original gave that result just a fraction of the time, but the new post-measurement result is different since it gives that result 100% for sure. So measuring it has actually polarized the particle into a new state that is different than it was before.

This is common in everything we call measurement in quantum mechanics. Yet even though this interaction changes thing, since the word measurement is used people want to pretend that it revealed something that already existed.

Even people that want to call themselves realists sometimes want to pretend that, even though it clearly is false. So some people aim for a kind of realism where they want the knowledge (which of the two results you get) to be determined by some spooky realist kind of thing.

Other realists are willing to be realist about whatever it takes to get results that agree with experiment. They are willing to allow the results of those "measurements" to be determined by realistic things that also include things more than just some preexisting up/down.

After all if you write down the Schrödinger equation for the actual experimental set up you will see that there are different devices that can take the same beam of identically prepared particles and get the correct fraction having the correct results (deflect the beam into distinct separate beams with the current total probability current). But yet these different devices send different portions of the probability current onto the different paths.

This isn't a question of interpretation, it's just how the Schrödinger equation evolves an incoming beam and how the probability current density has streamlines that go to the corresponding deflected beams.

So why should we expect a realistic picture to say that they are sorted based on their secret spin cheat sheet when ones with different secret positions (if the positions are realistic) are getting sorted into different beams while the spin becomes changed (polarized) by different devices?

You don't have to be realistic about position, but if you are there is no room left to be realistic about all these different devices merely revealing secret up/down results for spin. Instead you have to have a spin state that is realistic as an object and have it realistically be polarized and have it diverge these beams in different ways for these different devices. Because that is what the Schrödinger equation requires.

If you tell a realist what to be realistic about, then show it doesn't work, you just showed that you were bossy, not that realism doesn't work.

That said, the initial state can be nonlocal to begin with, so I don't know why anyone asked for a local theory to begin with. I guess if you thought the result of up/down was passively revealed them maybe it made sense to just as sign that to each particle in some kind of predetermination.

But predetermination is another issue, you can't control for predetermination if your attempted controls were predetermined as well. If someone picks there favorite kind of realism, then pretends that is the only realism and then calls something locality and then pretends predeterminism isn't allowed. Then they can contradict observation.

There is a sense where everything is local, they are local enough to have a PDE, but the domain is configuration space. And again, the initial conditions are that way, so I'm not sure how to get around that.

But to address your question directly now that we have the background. People have a good reason to prefer nonrealism. Just not a knock out like they might think they do.

What is the point of a realistic nonlocal theory? If someone far away can mess with your realist stuff then how can you make any local predictions unless you average away the influence far away people can have on your stuff. And once you've done that your effective theory discounts the effective reality of all that stuff, so why bother calling it real when it doesn't affect the statistical results you get averaging away the effects of stuff far away?

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