What does it mean when folks say that universe is not "Locally real"?

Question

I've read somewhat about the matter but can't quite picture it. Is this a property that only applies at the quantum level and not the classical level like us?

So far I've seen some rather strange takes on what it means. Some suggesting the moon doesn't exist unless observed, some saying that it implies consciousness is fundamental (though that comes up every time something new in quantum physics gets announced), and others saying that stuff isn't real unless observed, and more than that some say it's evidence of anti-realism. It seems like trying to get to the bottom of it all is more work than understanding the subject.

From what I read, it means that quantum particles don't have to be close to each other to impact each other once they are entangled. Furthermore, the realism implies that they have definite properties before observation, but it's more like that they are slot machines that spit out a result when observed. Does this mean conscious observation or just stuff interacting with each other? Is there any cause for concern for me at the classical level or is this just cool applications for computing by being able to beam info across gaps in an instant?

I'm struggling to grasp what it means.

Answer 1

In quantum physics, “observation” refers to any interaction that causes quantum outcomes to become definite. This doesn't involve human consciousness; instead, it includes interactions with measuring devices or the environment that record results. Entanglement creates correlations between particles when they are initially prepared, but measuring one particle doesn’t transmit information instantaneously to the other. Instead, it reveals a relationship that was already established.

Quantum mechanics challenges the classical idea that certain properties exist independently of measurement. For example, properties like spin directions aren't fixed until they're measured. However, this doesn't mean that everyday objects disappear when not observed. At the macroscopic level, processes like decoherence ensure that objects have stable, definite properties, making quantum "weirdness" negligible in our daily experiences.

I hope this helps you to grasp.

Answer 2

I have to preface that terms like these often have subtle meanings that depend on context, and terms composed of words (like "local realism") often mean more than just a combination of what the words mean separately.

But broadly speaking, "realism" is the idea that the world consists of activities and states independently of "observation" or "consciousness", and that there is a system of understanding these things in which there would be no contradiction between what different people know about them.

Realism generally corresponds with popular common sense, but it can differ from alternatives which generally fall into two broad groups, either denying that a system of understanding free of contradiction between individuals can exist, or that only knowledge is real and that it is produced by the activity of conscious observers (or occasionally, the activity of measuring equipment).

"Locality" meanwhile is the idea that activities and states occur or exist in a particular place, not everywhere at once, and that alterations do not influence the entire universe at once. "Non-locality" is not usually the diametric opposite of locality, but instead some kind of subtle modification - for example, it might suggest that things exist or alter in two places at once (e.g. action-at-a-distance), or that what are being regarded as two places are actually one indivisible place (but with the indivisible place still far more local than global, certainly not as extensive as the entire universe).

When Einstein searched for a "locally real" interpretation of QM, he was primarily rejecting action-at-a-distance, and rejecting the strange unreality of the so-called Copenhagen interpretation in which nothing could be real until "measured" (popular at the time, but less so nowadays).

What exactly Einstein might have considered permissible was probably far less rigid and specific than his rejection and reaction against these two specific explanations. JS Bell was later thought to have proved "local realism" impossible, but he didn't thereby prove correct the positions reviled by Einstein, but instead shifted attention onto "non-local realism" in forms which do not propose action-at-a-distance.

Answer 3

You have very good reason to be confused about this topic. Most of the precise formulations of this are very technical and mathematical, and hard to understand if you’re not deep in the field of quantum foundations. And the remaining non-technical discussion is almost inherently doomed by the imprecision associated with the “measurement problem”.

One way to gain some precision is to retreat to an operational viewpoint, treating quantum systems like a black box, and refusing to discuss or even question what might be happening inside. Then we can be precise about our operations on that box (classical input choices) and the classical outcomes (measurement results). We can compare the mathematics of quantum theory with the classical inputs and outputs of experiment, and judge whether the mathematics works. (It does!)

But “realism”, in this context, means going deeper than this operational viewpoint, asking about what is going on in the “box”, in the quantum systems themselves. I suppose an anti-realist would have to deny there was anything going on at all, but far more common is just to ignore this question of what might really be happening. Realism is then just the (trivial?) position that something is really happening between our inputs and outputs. (I think a much better replacement word for “realism” here would just be “scientific”!)

This is where the imprecision starts, because we’re already used to talking about classical-stuff (in the operational viewpoint), and now we’re asking about quantum-stuff. And most people, even physicists, sadly, have a sort of “shifty split” in their mind as they talk about these two things as if they’re sometimes different and sometimes the same. (The phrase “Shifty Split” comes from John Bell.) On one hand, it seems reasonable that there aren’t two sorts of stuff – there’s just one, and it must somehow all be quantum-stuff, even the classical-stuff. But on the other hand, you can’t formulate operational quantum theory without the classical-stuff, which “measures” the quantum-stuff!

After all, every quantum textbook will tell you there’s somehow a difference between an interaction and a measurement… but what could this “difference” possibly be? As most physicists see it, an interaction occurs between some quantum-stuff and other quantum-stuff. But a measurement occurs between quantum-stuff and classical-stuff. But it’s really all the same stuff…. ?!?! This is the measurement problem in a nutshell. It’s incoherent without two sorts of stuff, but few people think there are two sorts of stuff. Thus the “shifty split”, where the line between classical-stuff and quantum-stuff blurs and changes in a shifty manner as people talk about different aspects of the problem. No wonder it sounds so confusing.

Anyways, it is still possible to be careful and precise about what sort of properties the quantum-stuff has to have in order to explain what we see. This is what John Bell did; he pointed out that if the quantum-stuff was subject to a package of reasonable assumptions (which he called “local causality”, not “local realism”), then some quantum predictions would have to be wrong. They’re not wrong, so the package of assumptions must be partly wrong. And so, thanks to Bell’s Theorem and the experiments, people can now confidently say that nature violates “local causality”. This then (unfortunately) got shortened to “locality”, and someone else (even more unfortunately) started tacking “realism” on the end.

So, in summary there is a precise meaning in which quantum systems violate “local causality”, and the framework for even thinking about those systems is “realism”. That’s what’s meant by claims that the universe isn’t “locally real”. But what is going on is still completely unknown; there is no consensus at all, and the language people use to talk about is often inherently contradictory because of the shifty split as combined with the measurement problem.

Answer 4

If you want to understand this, you need to first understand what John Bell's local causality, implying the Bell inequalities, is. Bell put forward some plausible assumptions about a class of physics theories for systems with two distant parts with spin which work in the spirit of relativity theory, in that influences from one place to another travel forward in time at most with speed of light, and there can't be any influences faster than that or back in time or some other weird way. From these assumptions, he derived the Bell inequalities, which are relations that results of measurement on those two systems have to obey. He also showed that quantum theory violates them, so quantum theory is not locally causal.

Experiments since 80's have shown that the Bell inequalities are, in agreement with quantum theory, violated in our world; more specifically, changes in experimental setup here affect what is measured there, without any continuously propagating causal link in the sense of relativity. So Bell's local causality is violated just as quantum theory predicts, and many people express this shortly by statements such as "influences in our world can be non-local" or "our world is non-local".

This irks some people who have a different idea of the word "local" in quantum theory and do not agree with such characterizations of quantum theory. It seems they have invented the term "local realism" to describe what John Bell calls "local causality", most likely to save, in light of the Bell theorem and results of related experiments, the statement that quantum theory is local.

They have a point in that Bell's analysis works not only with relativistic causality, but also with probability over so-called hidden variables, which are supposed to exist before the measurement, and determine the actual result of measurement, before the measurement is done. So Bell assumes the system studied has some real state which determines what will happen in a measurement. If this assumption is dropped, and the usual stochastic view of orthodox quantum theory is adopted, where results of measurement are not determined by anything before that measurement is done, they just "happen" and bring new fact into the world, then the Bell inequalities do not follow, and their violation has nothing to do with this other view of locality in quantum theory, and we can continue saying that quantum theory is a local theory and the world can still be local, despite the Bell inequalities being violated.

However, this pedantry/renaming risks trivialization/downplay of the Bell theorem and the experiments it motivated. They really are an advance in understanding quantum theory and the world, and are an endless source of wonder for "how could that be" and they provoke people to search for deeper explanation.

Answer 5

As others have said, the phrase "local realism" takes on different shades of meaning in the hands of different authors.

But a pretty good rule of thumb is that local realism means this:

The things we can measure are well modeled as random variables, with a joint probability distribution that cannot change instantly in response to distant events.

Answer 6

I am surprised that nobody provided an equation yet. "Local realism" is given by the factorizability of probabilities: $$P(a,b|x,y,\lambda)=P(a|x\lambda)P(b|y,\lambda)$$ where $P(w|z)$ is the conditional probability of measuring $w$ given $z$, $a$ and $b$ are results that can be obtained by Alice and Bob respectively in an entanglement experiment, $x$ and $y$ are the settings of their respective measurement devices and $\lambda$ are the initial variables of your system (including hidden variables). This equality does not hold in quantum mechanics.

Why should this factorizability be valid? Well there are many ways to derive it. The local realism argument is based on defining probability assumptions for locality and for realism. However as you might have seen there are many here that do not agree on how to define locality or realism and this is still debated by professionals. Many will argue that you cannot separate local realism into locality+realism. John Bell himself did not like the term and used "local causality". People that do not want to argue on semantics just call that equation "local realism" and do not argue what that means and move on.

Answer 7

I will supplement my earlier answer with a specific example of what it means for local realism to fail.

Alice, Bob, Carol and Doug play a game. Alice whispers a secret to Bob, Bob whispers that secret to Carol, Carol whispers that secret to Doug, and Doug reports the secret back to Alice. If the secret hasn't changed along the way, they all win.

By eavesdropping on Alice and Bob, you've discovered that Bob passes on Alice's message correctly 99% of the time. Likewise for Bob and Carol, and for Carol and Doug. You therefore expect the players to be successful almost every time. In fact they succeed only 1% of the time. Alice almost always passes the message correctly to Bob, Bob almost always correctly passes it on to Carol, Carol almost always passes it on to Doug, but somehow by the time it gets to Doug, it's almost always incorrect. [Note: You have to choose who you're going to eavesdrop on; you don't have the equipment to eavesdrop on everyone at once.]

If Alice's message to Bob, Bob's message to Carol, Carol's message to Doug, and the overall success rate were classical random variables, obeying the laws that random variables are supposed to obey (you can Google for "Kolmogorov axioms"), the situation I've just described would be impossible. In fact, Doug would have to get the right message at least 97% of the time.

One possible explanation would be to say that your decision to eavesdrop on Alice and Bob somehow causes Carol to make mistakes (which you don't see, because you're eavesdropping on Alice, not Carol), and your decision to eavesdrop on Carol and Doug somehow causes Alice to make mistakes, etc. If the players are all far enough apart, this would mean that your decision to eavesdrop has non-local effects.

Believing in "local realism" means first of all, believing there are no non-local effects, so your eavesdropping decisions can't affect the results you're seeing. (That's the local part.) It also means that the success rates at each step along the chain behave like random variables, so that Doug has to get the right message most of the time. (That's the realism part.) If Doug in fact almost always gets the wrong message, then local realism is refuted.

Answer 8

I would like to talk about why the idea that consciousness is fundamental comes up in quantum mechanics.

In the late 1900's, physics was well on its way to completely describing the clockwork universe. But only if you left some things out of the universe. Consciousness was explained by religion.

Even today, we have no physical explanation for what a mind is or how it interacts with matter and energy. We have very clear evidence that it does exist and it does interact. I can ask you to throw me a ball, and you can choose to do so or not. There are questions if you truly have free will or not. But from my point of view, I certainly seem to. Assuming I do is no worse an approximation than point particles. Of course, this way of thinking might be as much in the wrong direction as saying whatever fire is, it interacts with matter and energy.

It is clear we have emotions, sensation, and self awareness. But we have no idea how physics might explain what they are or how they work. The best we will be able to do is explain how the brain works and how states of the brain map to states of the mind.

Religious explanations are no better. Instead of "Whatever it is, it arises from the brain", we get "Whatever it is, it arises from God". We still get no details or mechanisms.

And then around 1900, cracks appeared in classical physics. It could not explain why electrons orbits around a nucleus without radiating away energy and spiraling in. Or the photoelectric effect. Or Michelson-Morley. Quantum mechanics and relativity required new ways of thinking.

In particular, things were wave functions. Wave functions collapsed when observed. This implies the need for an observer.

If I was trying to work out how a completely new set of physical laws needed an observer, I would certainly seize on this as a way to get at the physics of consciousness. No wonder it was taught for the next 50 years. And it is too bad that nothing came of it. We learned nothing about what a mind is or how it works. And now we see the observer as simply the environment and entanglement.

Other approaches have been far more successful. People have made more progress than I would have expected mapping states of the brain (and the rest of the body) to states of the mind. For an excellent introduction, see Robert Sapolski's online course at Stanford, Human Behavioral Biology. It doesn't explain what I would like to know, but it is an impressive ongoing achievement.

Answer 9

I'll explain something about quantum theory and interpretations of quantum theory before coming to your question.

In classical physics the evolution of a measurable quantity of a system is described by a variable that is also the value you get if you measure that quantity. For example, the $x$ position will be described by the variable $x(t)$ and when you measure the $x$ position you get the value $x(t)$.

In many quantum experiments the outcome depends on what is happening to all of the possible values of the measured quantity. This is called quantum interference. For an example see Section 2 of

https://arxiv.org/abs/math/9911150

According to the equations of motion of quantum theory when information is copied out of a quantum system interference is suppressed: this is called decoherence

https://arxiv.org/abs/1911.06282

Any system you see in everyday life has information copied out of it on scales of space and time a lot smaller than those over which they change significantly and on those scales interference is suppressed very effectively. For electrons in the atoms of those systems interference isn't suppressed. The extent of decoherence changes continuously with the strength of information copying interactions it isn't all or nothing. Decoherence doesn't eliminate the other possible values for a system, it just suppresses interference between them. As a result for systems on the scales of everyday life using the same kinds of arguments we would use for any other kind of theory, we find that quantum theory implies the existence of multiple non-interfering versions of those systems that obey classical physics to a good but not perfect approximation:

https://arxiv.org/abs/1111.2189

https://arxiv.org/abs/quant-ph/0104033

This is often called the many worlds interpretation (MWI) but it is just an implication of treating quantum theory as one would treat any other physical theory.

Many people don't like the MWI.

The standard textbook response to this issue is just to say that you should do calculations and make predictions without caring about what is happening in reality. This isn't viable for either theorists or experimentalists because to understand how to set up and interpret an experiment properly you have to understand what is happening in reality in that experiment according to the theory you're testing.

Other people have proposed modifications to quantum theory such as saying that some interaction eliminates all but one of the possible values, e.g. - spontaneous collapse theories

https://arxiv.org/abs/2310.14969

One theory that hasn't been fleshed out much is the idea that consciousness has something to do with collapse, e.g. - this paper by Penrose and Hameroff:

https://web.archive.org/web/20170813024119id_/http://www.quantumconsciousness.org/sites/default/files/Hameroff%20Penrose%20-%20Consciousness%20in%20the%20Universe-A%20Review%20of%20the%20Orch%20OR%20Theory%20-%202013%20-%20Physics%20of%20Life%20Reviews.pdf

I say this hasn't been fleshed out much because there is no equation of motion for the proposed theory in the paper above. Without such an equation this theory is difficult to test experimentally.

Collapse theories and other alternatives to unmodified quantum theory don't currently reproduce the predictions of relativistic quantum theories, which are the vast bulk of actual predictions of quantum theory:

https://arxiv.org/abs/2205.00568

One way that the word realism is used is to talk about whether quantum theory or any other theory actually describes how the world works.

Another way the word realism is used and local realism in particular is related to Bell's theorem. Bell's theorem constrains the correlations that can arise between two systems obeying the following constraints. (1) The evolution of the system is described by stochastic variables: that is classical variables whose values are chosen with some probability out of a particular set of options. (2) These variables evolve locally: one system can only change another by interactions not by influences magically changing a system from far away. (3) The measured system and measuring device don't conspire with one another to choose measurements and outcomes in advance. Quantum theory produces larger correlations than those allowed by Bell but those correlations can't be used to transmit information faster than light.

Local realism means any theory that satisfies (1) and (2). There are some theories that satisfy (1) and (2) but not (3). These theories are described as superdeterminism:

https://arxiv.org/abs/2010.01324

Superdeterminism hasn't been fleshed out much but if it was it might be a local realistic alternative to quantum theory. Theories like spontaneous collapse satisfy (1) and (3) but not (2).

Quantum theory without modifications, the MWI, isn't realistic since quantum theory describes the evolution of quantities in terms of observables not stochastic variables. The equations of motion for quantum theory that have survived testing the most are local. If you wanted to understand this more deeply you could read a book on quantum field theory such as "Quantum field theory for the gifted amateur" by Lancaster and Blundell or "The conceptual framework of quantum field theory" by Anthony Duncan. There is an explanation of how Bell correlations arise in the MWI. Information that produces the correlations is carried in decoherent systems in a form that can't be accessed without looking at correlations with other systems: locally inaccessible information:

https://arxiv.org/abs/quant-ph/9906007

https://arxiv.org/abs/1109.6223

https://arxiv.org/abs/2304.14959

Answer 10

Quantum mechanics isn't locally real.

This is roughly due to the fact that entangled (but widely separated) particles do not have definite properties until they are measured. That particles have definite properties before being measured is obviously true for classical physics, so classical physics is locally real.