Hacker News Reader

Comment by ridgeguy | Hacker News Reader

Amateur here, certainly. But I recall that one of the two consequences of Bell's inequality (shown to be valid AFAIK) is that there isn't an "objective reality". Kind of like nature makes it up depending on what the observer is up to. Yes? No? Maybe?

View on HN

Not really, no, but I can see why you might think that, because Bell's theorem is often described as saying that quantum mechanics contradicts "local realism", but "realism" in this sense has a precise technical meaning, which is that a physical system at some time has a complete set of well-defined values for all the possible measurements that we might do on it (and "local" just means that physical effects can't propagate faster than the speed of light). It was known since the beginning that quantum mechanics, as a theory, doesn't have this feature because of complementary observables, aka the uncertainty principle, which says that the more precisely some quantities are known, the less precisely the theory determines other quantities, e.g. in the case of a spin-1/2 particle, knowing the value of its spin along one axis means that it's value along any axis at right-angles is completely uncertain - it can be up or down with equal probability.

Nevertheless, it was thought (e.g. in the Einstein Podolsky Rosen paper in 1935) that it might be possible to formulate a theory that could reproduce all the correct predictions of quantum mechanics, while also ascribing simultaneous well-defined values to all the physical quantities possessed by a quantum system, i.e a locally realistic theory. These are also known as local "hidden variable" theories, where the idea was that some of the values of the variables might be unobserved simply because of measurement practicalities - we can't measure the spin of a particle along two orthogonal axes simultaneously because the measurement needs a magnetic field gradient along the direction we're measuring in.

Bell derived an inequality that any locally realistic theory must satisfy, and showed that quantum mechanics in fact violates this inequality, so no locally realistic theory can reproduce the predictions of quantum mechanics. Alain Aspect and others later implemented Bell's thought experiment in the lab and showed that the physical world obeys quantum mechanics, and so is not describable by a locally realistic theory.

In my view, none of that shows that there is "no objective reality". Rather, it shows that objective reality is as far as we can tell quantum mechanical, and not locally realistic in the sense described above. It's certainly the case that quantum mechanics requires a modification of the classical concepts of reality, i.e. of classical ideas about what a physical system is, but you would only accept that conclusion if you agree that quantum mechanics is telling you something objective about reality... At least according to how I understand those words.

So I think what people really mean when they say quantum mechanics shows there's no objective reality is just that it contradicts classical conceptions of physical systems, which is clearly true but sounds less sexy and mysterious.

The term is an error due to messy history. Copenhagen program being the first, influenced early quantum physics, quantum behavior was unreal, classical behavior was real. When the term local realism was introduced, it was intended to be philosophic realism, but was confused with classical behavior, because historical baggage was messy.

In EPR, the setup is that there are two labs doing measurements outside of each other's lights cone. The outcome in one lab allows a perfect prediction of what happens in the other. This means that it is not possible that something random is going on in unless there is some nonlocal coordination between the two. This suggests that there is some actual fact of the matter as to how the experiment will turn out. That is, they argued that QM+locality = extra information beyond the wave function to determine outcomes. Bell then saw Bohm's theory and wondered about getting rid of the nonlocality. Bell showed that QM+extra info determining outcomes = nonlocal. In short, EPR + Bell shows that if QM predictions are correct (the predictions, not the theory), then there is something nonlocal going on. The lab experiments confirmed this and nature is indeed nonlocal.

Thus, there is no local theory that has definite experimental results compatible with what is actually demonstrated in labs. Many worlds, to the extent that one can apply any notion of locality to it, avoids this by not having singular, definitive experimental results (all results happen).

> The outcome in one lab allows a perfect prediction of what happens in the other.

I guess you know this, but just to clarify, that's only if the same measurement is performed in the other lab. If the other lab measures an orthogonal spin component, that result can't be predicted at all (I'm assuming entangled spin-1/2 particles for simplicity). It's more precise to say that measurement in the first lab tells you the state in the second lab, and with that information the probabilities for the various possible measurement results in the other lab can be predicted. In particular, if the other lab measures the spin along the same axis, the results can be perfectly correlated, as you say.

So there's some kind of nonlocality, but it's not the kind of nonlocality that makes problems with relativity, because the correlations can't be used to signal or cause any difference in the distant lab, only to predict, in general probabilistically, what would happen in the other lab if some measurements are performed. So entanglement allows this interesting middle ground between a local theory and a theory that's nonlocal in the sense that it would allow nonlocal causation, which is the kind of nonlocality that would worry Einstein. There should be different words for the different kinds of nonlocality, but maybe nonlocal correlation versus nonlocal causation serves the purpose

In EPR, it is critical that it is the same measurement. Bell explores doing different measurements. For EPR, they assumed that if you can predict with certainty what happens in a space-like separated region, then there must be a fact of the matter about it. Not being probabilistic was very important for that. Bell then showed that there cannot be a fact of the matter without there also being some nonlocal means going on in order to account for the QM predictions. It is critical to appreciate the two separate pieces of arguments, how they differ, and how jointly they do lead to some kind of nonlocality. Tim Maudlin has a, now old, book exploring these different levels of nonlocality in quantum mechanics.

I recently heard a talk from Tim Maudlin where he mentioned that foliations are the easiest and most natural structures to use to provide nonlocality and, if there is such a thing, maybe there is a clever way of using it to actually communicate and discover the foliation in some sense. He mentioned there is current research on using arrival times which are experimental results outside of the operator formalism, as far as I know. I found an article describing the research:

https://www.altpropulsion.com/ftl-quantum-communication-reth...

> In EPR, it is critical that it is the same measurement.

I must admit I haven't read the full EPR paper, only post-Bell expositions and excerpts. But you can have perfect spacelike correlations of the same measurement classically as well, e.g. if two particles having opposite (angular or linear) momenta are sent from the midpoint towards distant labs, measuring one momentum will tell you the other one. They must somehow discuss making different measurements no? Maybe they effectively discuss a protocol where the two labs agree on the same sequence of orthogonal measurements. I should read these sources sometime...

Thanks for the ftl reference. It would be astonishing if their hypotheses are borne out. I find it unlikely, but of course the experiments will have to decide, so I'll keep tabs on that. By "foliation" in this context I guess he means a foliation of spacetime amounting to an absolute reference frame. I've seen Tim Maudlin discuss something like that before.

By the way, the article you linked mentions a couple of times the importance of distinguishing signaling from causation or action, but doesn't seem to define how they're distinct. Do you know some more formal article discussing the proposed experiments? The sources given in the article are just to video interviews.

EPR's point is that there is nothing mysterious from a classical perspective of being able to deduce this. They were arguing against the presentation of QM as to there being no fact of the matter about what the momentum is before the measurement and that it randomly becomes whatever it becomes when measured. Their point is that if both particles are randomly collapsing into their choices, then they should disagree at some point unless there is some nonlocal causation happening. Einstein rejected nonlocal causation, reasonable given what he knew at the time, and thus the momentum measurement result must already be preordained by something and it is then like the classical setup.

Bell's work was to show that it had to be the nonlocal causation.

>Do you know some more formal article discussing the proposed experiments?

I do not know of an article, but Maudlin's book Quantum Non-locality and Relativity goes through the various notions of locality and what QM says about it. There is a chapter about signaling and another about causation. It also covers the GHZ scheme which is a non-probablistic version demonstrating non-locality. It is pretty clean.

>Do you know some more formal article discussing the proposed experiments?

I have not read them, but my understanding that Siddhant Das is pursuing these and here is a link to his Arxiv papers which talk about arrival time experiments though I do not know if it is directly about these.

https://arxiv.org/search/advanced?advanced=&terms-0-operator...

Thanks, I had a look at Maudlin's book. It seems the distinction between signaling and causation is that there might be some kind of nonlocal causation that we can't control and so can't use to send a signal.

Local causation is defined as in Bell's Theory of Local Beables, as the probability distribution for values at spacelike separated regions only being correlated with respect to the overlaps in their past lightcones. Or to put it the other way around, there's nonlocal causation if the probability distribution of values in one region depends on values in a spacelike separated region. That's what I'd call nonlocal correlation rather than causation but I guess that's just terminological.

This looks like a pertinent paper from Das but I haven't read through it yet

Arrival Time Distributions of Spin-1/2 Particles (2018) https://arxiv.org/abs/1802.07141

If MWI is true, then nature is local without extra information beyond wave function.

Reply on news.ycombinator.com