# Can someone clarify whether the recent experiment closing all remaining loopholes to Bell's Theorem really shut the door on local realism for good?

I saw this recent article on Phys.org that purports to close all remaining loopholes that previous experiments on violations of Bell's inequality left open. My question is, does this really close the door on any possibility of local realism being true? I know these articles tend to have some exaggeration added in for good effect, but I wanted to understand whether things have really changed from the standpoint of different interpretations of Quantum Mechanics.

Both locality and realism seem very intuitive (especially the latter, which many physicists/philosophers/ordinary people have a very hard time giving up), and Bell himself said it best:

"For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational that I think that when Einstein saw that, and the others refused to see it, he was the rational man. The other people, although history has justified them, were burying their heads in the sand. ... So for me, it is a pity that Einstein's idea doesn't work. The reasonable thing just doesn't work."

I know he was a strong proponent of the Bohmian Interpretation and strongly felt that QM would at its core end up being formulated in an observer independent way that would ensure the elements of the theory correspond to actual things in the external world. I'm very much of the same persuasion, but I wanted to get some input from people who know much more about the subject than I do. What are the options? Does this experiment mean anything with respect to Bohmian mechanics/MWI/other "realist" interpretations of QM? And what of this recent speculation into ER = EPR? Does that idea actually suggest that communication could be facilitated through subatomic wormholes, thereby bypassing any worries of nonlocality?

Any input is greatly appreciated!

• That's quite a broad question. Can you try to ask something more precise? This currently invites adherents of all interpretation to give their personal view on the results, which would be a discussion, not a question with an exact answer. – ACuriousMind Oct 22 '15 at 14:50
• You can always come up with loopholes, for example by postulating that there are magical space unicorns that tamper with our experiments but disappear whenever we look at them. Also, I've never seen a coherent definition of realism, so I'm not sure why anyone has a hard time giving it up. – user10851 Oct 22 '15 at 15:49
• @ChrisWhite While this is true, most (all?) Bell experiments up to now had the problem that they had loopholes which were well contained within standard physics (choice of measurement not spacelike separated, postselected outcomes, ...). However, there is probably no clear-cut boundary to magical space unicorns (e.g.: are our random choices of measurement truely random?) – Norbert Schuch Oct 22 '15 at 16:40
• @ChrisWhite as far as a definition of realism goes, probably the fact that there is a mind-independent reality out there that has absolutely nothing to do with whether or not thinking humans exist. I'd say that's pretty damn coherent. In fact, something like "there exists a mind-independent reality" is something I've seen time and again brought up as an assumption that one would have to have before doing scientific observation/experimentation and trying to understand the world – Pete1187 Oct 22 '15 at 17:24
• Actual preprint: arxiv.org/abs/1508.05949 . (Pet peeve: the pop sci article you've linked to is about the paper, but it is the paper that make the interesting claims and it is the paper that any one hoping to answer the question must read. A link to the article is almost useless unless it links to the paper or preprint.) – dmckee --- ex-moderator kitten Oct 22 '15 at 19:42

to close all remaining loopholes that previous experiments on violations of Bell's inequality left open.

When they say closing a loophole, the point is to get better experimental results to verify an inequality predicted by quantum mechanics.

So quantum mechanics predicts an inequality and some imaginary theories (which don't agree with quantum mechanics) don't predict it. The magical theories that don't predict it, disagree with quantum mechanics, and disagree because they have certain ... silly ... features.

1. They have a fake determinism, specifically they pretend to be deterministic about some things (like objects to be measured) but are explicitly not deterministic about other things (like the devices used to measure things). You can see this directly in Bell's paper when he assumes the theory to be criticised does not allow the hidden variable $\lambda$ to affect which direct the spin is measured.

2. They are not contextual. Which means the result can depend on the hidden variable and on the orientation of the measurement device but not on the state of the device or even on which device type you use. Despite the fact that quantum mechanics explicitly predicts different correlations between say spin and position when you use actual devices designed in different ways. You see this in Bell's paper when the result is supposed to be a function of just the orientation and the hidden variable of the thing to be measured.

3. They are local. Despite the fact that it isn't even clear how to specify initial conditions for a local theory.

So the point is that a local and non contextual theory with fake determinism will have one (wrong) result for the inequality. Therefore any that agrees with quantum mechanics will either have to have a real determinism, or have no determinism (and it isn't scientifically possible to distinguish between those two options, only fake determinism has scientific predictions), or must be non local (which means you cam specify initial conditions), or must be contextual (which means you can correctly predict what happens in real devices).

If you want to agree with quantum mechanics you definitely want to be contextual by really looking at actual experimental setups when needed. And you definitely want to be nonlocal so you can specify initial conditions. Determinism or no determinism is up to you, any theory of one type has a theory of the other type that makes the same predictions so calm down and don't worry.

My question is, does this really close the door on any possibility of local realism being true?

No. What it does is close the door on local and non contextual theories with fake determinism.

I wanted to understand whether things have really changed from the standpoint of different interpretations of Quantum Mechanics.

Absolutely zero people wanted a local and non contextual theory with fake determinism. So it's purely academic.

Both locality and realism seem very intuitive

How can local realism seem intuitive? How would you even specify the initial conditions in a local theory? And you can be realist on say position and then to get the correct correlations for contextual measurements you have to realize that anything incompatible with your realist parts is produced by the interaction.

We already objectively know that interactions create spin eigenstates that weren't there before.

Case in point, interact in the $\hat z$ direction first, then the $\hat z$ direction second, then the $\hat x$ direction third. Repeat and note that the second $\hat z$ interaction always agrees with the one immediately before it. So clearly the $\hat z$ interaction puts it into a state that reliably produces a particular outcome for $\hat z$ interactions.

This time interact in the $\hat z$ direction first, then the $\hat x$ direction second, then the $\hat z$ direction third. Repeat and note that the second $\hat z$ interaction only agrees with one before it 50% of the time (of spin one half). So clearly the $\hat x$ interaction changed it into a state that no longer reliably produces a particular outcome for $\hat z$ interactions.

So interactions change things. No complicated measurements required. Realism can not be about an interaction that just passively reveals a property without changing things. Any interaction for two things that don't commute changes things. Full stop.

I know he was a strong proponent of the Bohmian Interpretation and strongly felt that QM would at its core end up being formulated in an observer independent way that would ensure the elements of the theory correspond to actual things in the external world.

Bell spoke often about people misunderstanding him. He indeed liked the dBB theory. And that theory is realist about position, but not about spin. No theory is realist about spin because different components of spin fail to commute with each other so clearly spin interactions change things and aren't passive measurement processes that reveal properties without changing things

I'm very much of the same persuasion,

What persuasion? Clearly if you realize spin interactions change things then you know they aren't measuring something that already existed while failing to change things.

What are the options?

Same as always. Don't use local theories that are non contextual and have fake determinism. But no one does, so that was easy.

Does this experiment mean anything with respect to Bohmian mechanics/MWI/other "realist" interpretations of QM?

Nope. Those theories are designed to predict the same things as quantum mechanics so they were also confirmed by the recent experiments.

And what of this recent speculation into ER = EPR? Does that idea actually suggest that communication could be facilitated through subatomic wormholes, thereby bypassing any worries of nonlocality?

That's a very different question, and should be asked separately.

Let's assume for the sake of argument that the experimentalists here did a good job, didn't make any obvious mistakes, and will be confirmed by future experiments of this type**. My understanding is that other Bell tests that close these loopholes are currently being performed or are even completed and being analyzed, so in the unlikely event that this isn't the case we should know soon.

Well, the first thing to realize (as you probably do know) is that few, if any, physicists are actually surprised by this result. There is a good reason for this: even in Bell experiments with loopholes, there is no generally accepted mechanism by which these loopholes would ever actually be exploited by nature. To put it another way: if we did not believe any results in which two measured quantities, within the same light cone, could conceivably conspire with each other to trick us, the vast majority of physics knowledge would go out the window. The extraordinary thing about Bell's result is that it does let us rule out a class of theories with a generality that is (as far as I can tell, at least) pretty much unmatched in physics.

As Chris White points out, you can always find "loopholes" in a Bell test, like any experiment, in that it must rely on certain assumptions. For example, the processes that lead to the choice of measurement basis must be random. I don't know the details of Hanson's random number generation, but similar ones have, for example, used weak laser pulses at each of the spacelike separated locations and counted an even or odd number of photons. Since these devices have been within each other's light cones at various points before, one could worry in principle that they are able to somehow conspire with each other, even though of course no one has any real proposal for why this could possibly happen. There are also loopholes in an experiment like this in that they assume the photons they create are shooting off at near the speed of light as photons normally do. If they were instead somehow "hanging around" near where they were created for awhile, before zipping off to the detectors, they could have a chance to communicate and avoid the closing of the locality loophole. Maybe future experiments will try to close these and other even more improbable loopholes, but presumably there will always be some way to cast doubt on the results if you really want to.

So why care about this paper? Well, maybe you shouldn't, but to be fair the locality and detection efficiency loophole are, arguably, different than the loopholes I've mentioned above in that they are relatively consistent with our current principles of physics. So, whether it is really justified or not, in the minds of people who think about this sort of thing they have come to be considered a line in the sand between a fake result being forbidden "just" by all our understandings of the tools we use to measure it, and a fake result being forbidden by the most basic principles we believe the physical world obeys. By that standard this result is significant. But at the same time you should remember what I said earlier: I claim that, in terms of statistical significance plus minimal number of assumptions, few if any physical principles have been tested as stringently as Bell inequality violation already. So if one doubts the outcomes of all these tests one should also ask which (if any!) experimental results you actually trust by the same standard.

**Update (11/11/15): two new papers today report strong loophole-free Bell inequality violations with entangled photons.

Please find below what I wrote elsewhere based on their preprint, not the article in Nature ( I don't know if there is substantial difference): "I have not studied the paper in detail, but would like to make some comments based on what the authors write in their article. First, it was noted in another thread that the probability $p$=0.019/0.039 is not very impressive. Second, authors write: "Our observation of a loophole-free Bell inequality violation thus rules out all local theories that accept ... that the outputs are final once recorded in the electronics." On the other hand, as I wrote here a few times, unitary evolution of quantum mechanics is, strictly speaking, incompatible with final outcomes of measurement, as far as I understand (for example, due to Poincare recurrence). Therefore, the authors' experimental results can only rule out local realistic theories that predict deviations from unitary evolution. For example, the local realistic theories of my article http://link.springer.com/content/pdf/10.1140/epjc/s10052-013-2371-4.pdf (Eur. Phys. J. C (2013) 73:2371) have the same evolution as unitary evolution of some quantum field theories."