So is quantum entanglement actually FTL "communication" or is it mundane pre-determination?
So is quantum entanglement actually FTL “communication” or is it mundane pre-determination?
In short:
- It is not "mundane pre-determination".
- Whether it is "actually" some form of FTL communication depends on which interpretation of quantum mechanics you prefer.
- However, regardless of interpretation, if there is some form of FTL communication between the parties, then it is completely inaccessible to us.
That said, you're certainly correct in one respect: there's a lot of very poor descriptions of what entanglement is and how it works in the popular-science press. In particular, these two paragraphs do a very good job of summing up a very common misconception that arises from poorly-written material:
The usual analogies I hear are things like, well if you have a blue and red marble, you scramble them in a bag and then take one blindly, and give the other to your friend, if you look at your marble and see blue, you know your friend must have red.
To me this sounds like "boringly classical pre-determination." Like this whole spooky action at a distance is really just "the spins were already opposite to begin with and we just separated them," as if we had a machine that spat out pairs of opposite-colored marbles and now we're surprised that they're always opposites regardless of how far apart we are when we see them.
Your interpretation of that description is indeed correct. However, quantum-mechanical entanglement goes much further than that property, and this is exactly the content of Bell's theorem.
To be more precise, Bell's theorem is a description of systems that use "boringly classical pre-determination" (known in the technical lingo as Hidden-Variable Theories) to produce correlated outcomes, and it makes quantitative statements about which kinds and what amounts of correlations you can expect from such a system.
Bell's bigger argument then goes on to construct quantum-mechanical states that break those bounds, and which therefore (provably) cannot be explained with "boringly classical pre-determination" at all. And, when we talk about Bell-test experiments, we refer to experiments that implement those states and show that you do indeed get more correlations than classical pre-determination can produce.
This does raise the question about how your initial model,
you have a blue and red marble, you scramble them in a bag and then take one blindly,
fails to describe the quantum systems at play. The answer is that this classical model is unable to describe superposition states between the blue-marble and red-marble states of each bag, and for the quantum-mechanical description to work at a level where it can exceed the classical bounds on correlations, it needs to be able to measure on the superposition state $$ |\text{blue marble}⟩ + |\text{red marble}⟩ $$ and to distinguish it from the 'conjugate' superposition state $$ |\text{blue marble}⟩ - |\text{red marble}⟩, $$ where that $-$ sign is a truly new, fully quantum-mechanical ontological feature that doesn't exist in classical mechanics.
OK, so it's not classical pre-determination. But is it faster-than-light communication, then?
Well... also no, this time because of another theorem of quantum mechanics - the No-Communication Theorem. This theorem states that, if you have any arbitrary entangled states linking two parties, and you allow them to perform any arbitrary physical operation on them, it is provably impossible to transmit a message in any way other than classical communication.
But that's at the level of the (ostensibly human) operators of the experiment, though. Do the particles themselves communicate?
And the answer here is that yes, it's not inconceivable that they do. More to the point, it is a consistent interpretation of quantum mechanics to suppose that there is a hidden-variable theory that underpins QM. Since Bell's theorem rules out hidden-variable theories which are 'local' and 'realist', the price for doing so is that those variables need to be 'non-local', which basically means that changes in those variables can propagate at FTL speeds.
However, the price for that is that the No-Communication Theorem requires that, to be consistent with QM, such a hidden-variable theory needs to somehow make that FTL communication completely inaccessible above the quantum-mechanical layer of the description. And that then means that, when you build those theories, they come out looking extremely contrived and artificial.
And here is where it gets subjective: most working quantum mechanicists are extremely uncomfortable with those theories as models of what reality is "really like". There are very good reasons to be uncomfortable with them, but $-$ as in all things where the interpretations of quantum mechanics are concerned $-$ this is ultimately a subjective thing.
Now as for the bulk of the text that you've written $-$
Normally this discussion goes nowhere and someone inevitably says "Well you just have to learn quantum mechanics." I always find this answer deeply unsatisfying.
Your observation is correct: this is indeed unsatisfying. But the hard truth is that, while it would be wonderful to know what's "really going on" between two entangled particles, we simply don't know. There's a broad array of proposals of how one should understand this, but they all have serious drawbacks and for each of them there's multiple reasons to think that it's completely bonkers.
We do know a lot about entanglement:
- we know how to formulate the quantum-mechanical laws that define it, and which are unbeaten in their ability to match experiments
- we know how to operate on those quantum-mechanical laws and the concepts within them
- we know how to perform experiments that are able to probe those laws and their differences to more 'reasonable' theories
- we know for sure that it goes well beyond 'classical pre-determination', and we have verified that experimentally multiple times
- we also know for sure that it does not allow us, as operators, to use it for FTL communication
- we also know that if it does involve some "back-end" FTL communication between the systems, then that requires additional "protective" layers in the theory that can fairly be described as completely bonkers
- (but then again, every way to interpret QM does things that can fairly be described as completely bonkers)
But as to what's "really going on", we just don't know.
Judging from your text, it sounds like you already have a pretty decent understanding of the epistemology around entanglement, and that you've reached the kind of grounded frustration at the weirdness of the theory that's widely shared among working physicists. We know it's weird. We agree that it ultimately makes very little sense. We absolutely would like a better answer. We are actively searching for better answers, and we are making decent progress $-$ we are indeed advancing the state of the theory, bit by bit, and we're getting better and better at examining single quantum systems in ways that allow us to test QM in finer and finer ways. But we've yet to find that better answer.
A couple of thoughts.
No, entanglement is not (does not allow for) FTL communication. With this I mean that, even if you and me share an entangled system, there is no way for me to alter what you will observe on your side. Yes, the operations I perform on my part of the system will affect what you will observe on your part, but there is no way for you to realize that this correlation exists without knowing what happened on my side as well. In other words, there is no way for me to send you information using only our shared "entanglement link".
To me this sounds like "boringly classical pre-determination." Like this whole spooky action at a distance is really just "the spins were already opposite to begin with and we just separated them," as if we had a machine that spat out pairs of opposite-colored marbles and now we're surprised that they're always opposites regardless of how far apart we are when we see them.
The key point here is that it is possible to show (via Bell inequalities for example) that sometimes you and me can observe correlations between our observations that cannot be explained by any classical theory like the one you mention.
More Precisely, the gist is that if we denote with $x$ and $y$ our measurement choices (e.g. $x=0$ means I push button $A$, $x=1$ means I push button $B$, $y=0$ means you push button $A'$ etc.), and with $a$ and $b$ our observations (say $a=0$ means I see output $0$, $b=1$ means you observe output $1$ on your side, etc.), then quantum mechanics allows for correlations $p(ab|xy)$ that cannot be explained by any classical theory, in the sense that there is no way to write such correlations as: $$p(ab|xy)=\sum_\lambda p_\lambda p(ab|xy,\lambda).$$ This expression means that there is no way to describe our observations by invoking some pre-existing correlation between our shared system. For example, your proposed explanation with the marbles is such a case, with $\lambda$ representing the pre-shared correlation between the colors of our marbles.
It is worth noting that the possibility of (independently) choosing different measurement settings $x,y$ is crucial in this argument. If we consider fixed, pre-determined measurements, then it is always possible to find a "classical description" of the correlations (see e.g. this post about this).
So... what's going on? I'm not stupid but I'm also frustrated that these two interpretations both seem to be wrong and yet no one can actually seem to address the core point of confusion despite Googling this question across blogs, news articles, Reddit, Physics forums, Quora, etc. It's always the same back-and-forth. "There's no communication... but it's not like the states were known to begin with either." How on earth is there a third possibility to this?
The frustration is understandable. The way I personally like to state the apparent paradox is as follows: how can there be correlations unexplainable by a common cause, and yet that cannot be used to transmit information?. Note that if we admit the possibility of a common cause, that is we consider "classical" situations, then it is no surprise that we can share correlations that nevertheless do not allow for communication (your marble example is again such a case).
But it's also important to realise that this is not simply some mathematical shenanigans that people like to wonder about in the abstract. These sorts of correlations are routinely observed in the real world, and as weird as that sounds, quantum mechanics is still the best and most "natural" way we know to explain these observations.
"How on earth is there a third possibility to this?" That is exactly the $64,000 question of entanglement, but observations show that there is indeed a third possibility. It is possible for experiments to show correlations after the fact without it being possible to ever use this to communicate information FTL. (And don't confuse it with the red and blue marble, look up "Bertlmann's socks" to show why that's not it.) It means there is some kind of holistic quality of the system that maintains correlations across huge distances, yet this holistic character cannot be understood as a form of communication from one part to the other because it is maintained in ways that cannot be used to transmit information. (Interestingly, it can be used to encode information, in such a way that to intercept that information is to prevent its transfer. That's quantum cryptology.)
To go further, you have to start thinking about what information actually is, and where it lives. Does it live in a system, or in someone's head? If I read a book, is that information carried in the book and I'm just "extracting" it, or am I an active participant because I need to interpret what I read? In the formal sense, there is less information in a book than in a list of random letters the same length (because I would need more yes/no questions answered to be able to know the second, whereas redundancies in the first make it discernible with fewer yes/no questions), so you can see that "what is information" is tricky.
But I think this fact is telling us something: you could have one part of a pair of entangled particles, and you could choose any way you like a direction. You could then test the spin of your particle along that direction, and send a single bit of information (slower than light) to the other person, and with that bit tell them how to put their entangled particle into the same spin state as yours (same axis direction, same up or down state along that axis). This holds even though you are only communicating one bit of information, yet you can choose any axis direction you like!
That sounds like communicating more than one bit, but here's the catch: they would put their particle into the same state as yours, but never have any way of knowing what that state was! In particular, they could never know the axis direction you chose. This is related to the fact that when you do a spin measurement, you don't get to know what the state was, you only get to know if it is up or down along some axis chosen by you. Plus, you only know it afterward, so much of that original information is "lost" (if it was ever really there in the first place, that's the issue of quantum information). So the limits of what you can know, versus what "information" is in a quantum state prior to measurement, is the crux of the matter. It's all connected to the quantum measurement paradox, and the issue of whether a wave function is a real thing that represents information we don't have complete access to in each individual case, or if it is just a rule about correlations in ensembles. Classically, we never make a distinction between those two possibilities (like a deck of cards we just haven't looked at yet so treat as random), but quantum mechanics challenges us to decide if that distinction needs to be made.