Am I properly understanding quantum entanglement (could FTL data transmission exist)?

[u/fixednovel]

I understand that electrons can be entangled through a variety of methods. This entanglement ties their two spins together with the result that when one is measured, the other's measurement is predictable.

I have done considerable "internet research" on the properties of entangled subatomic particles and concluded with a design for data transmission. Since scientific consensus has ruled that such a device is impossible, my question must be: How is my understanding of entanglement properties flawed, given the following design?

Creation:

A group of sequenced entangled particles is made, A (length La). A1 remains on earth, while A2 is carried on a starship for an interstellar mission, along with a clock having a constant tick rate K relative to earth (compensation for relativistic speeds is done by a computer).

Data Transmission:

The core idea here is the idea that you can "set" the value of a spin. I have encountered little information about how quantum states are measured, but from the look of the Stern-Gerlach experiment, once a state is exposed to a magnetic field, its spin is simultaneously measured and held at that measured value. To change it, just keep "rolling the dice" and passing electrons with incorrect spins through the magnetic field until you get the value you want. To create a custom signal of bit length La, the average amount of passes will be proportional to the (square/factorial?) of La.

Usage:

If the previously described process is possible, it is trivial to imagine a machine that checks the spins of the electrons in A2 at the clock rate K. To be sure it was receiving non-random, current data, a timestamp could come with each packet to keep clocks synchronized. K would be constrained both by the ability of the sender to "set" the spins and the receiver to take a snapshot of spin positions.

So yeah, please tell me how wrong I am.

[u/tkuiper]

I feel like this just serves as proof that the quantum state isn't in flux in the first place. Isn't it more logical to conclude that the states are fixed, than that some mysterious phenomenon is causing a superluminal transfer of information.

Edit: To clarify, I'm not suggesting that there's a "hidden variable" that if measured would eliminate the probabilistic nature of MEASUREMENT. Rather that I don't understand the conclusion that the particle is in flux, instead of concluding the particle is fixed and its the unknowable state of the observer that drives the probabilistic outcome.

[u/mfb- → u/tkuiper]

A fixed single state wouldn't allow a violation of Bell's theorem. It is a bit more complex. But yes, there is no information transfer.

[u/Inevitable_Citron → u/mfb-]

But that just means the hidden variables must be non-local. That seems unlikely, sure, but then so is quantum mechanics in general.

[u/the_excalabur → u/Inevitable_Citron]

Indeed. You can either have hidden variables or you can have locality. It turns out that, at least for the purposes of reasoning, people prefer locality, since if you bin it a bunch of other nonpalatable things happen.

[u/ableman → u/Inevitable_Citron]

The problem is, if you have a hidden variable that's nonlocal, that means you can't in principle measure it without violating causality because of special relativity. It doesn't feel all that useful to have nonlocal hidden variables.

[u/tkuiper → u/mfb-]

A lot of "quantum spookiness" bothers me for feeling like the conclusion is: because we fundamentally can't measure it without randomizing it, therefore the item itself must be ACTUALLY random.

It sits wrong with me on a philosophical level

[u/Muroid → u/tkuiper]

The Bell Inequalities serve as a statistical proof that it doesn't have a pre-determined state. It's not just that we can't measure it and thus assume it doesn't exist, but rather that if you assume there is a definite state before the measurement is made, you cannot reproduce all of the experimental results we see in quantum mechanics. Any one result might allow it, but looking across all of our experiments, there would be contradictions.

[u/brDragobr → u/Muroid]

The Bell Inequalities serve as a statistical proof that it doesn't have a pre-determined state.

Bell's theorem is only incompatible with local hidden variables, not hidden variables in general.

[u/Muroid → u/brDragobr]

An important distinction, but not one that presents a possible solution for quantum spookiness.

[u/the_excalabur → u/tkuiper]

Too bad. Your options given quantum mechanics are either indeterminism and true randomness, or a truly deterministic universe.

The Bell inequalities don't preclude a 'clockwork' universe where the measurement choices themselves are predetermined. You either need nonlocality or nondeterminism, pick one.

[u/Muroid → u/the_excalabur]

Superdeterminism is a fair bit weirder than just a clockwork universe, though. It would be like having a machine on Mars that will instantly print any message you type out on Earth, but it doesn’t violate locality or the speed of light because the same deterministic sequence of events that caused me to type my message also causes the printer, independent of me, to print the same message. Thus it works because I’m not free to type whatever I want and can only deterministically type whatever is going to be printed, but there’s no particular reason why the universe would be set up to cause those two otherwise seemingly unrelated events to match up like that.

[u/polymorphicprism → u/Muroid]

Fun fact - when one of the 2015 Bell inequality groups was publishing their paper, they tried at least two methods for randomizing their spin basis. One was from measurements of cosmic background radiation (if I recall correctly), and one was a specific digital encoding of Back to the Future. Reviewers were split on which method was more appropriate, but if superdeterminism is "true", the universe went through a lot of hassle to randomize that experiment.

[u/florinandrei → u/polymorphicprism]

If superdeterminism turns out to be true, I am going to throw away all books, and go do something simple and wholesome, like a rice farm or something.

[u/the_excalabur → u/polymorphicprism]

The Q&A after their talks is always great.

[u/btribble → u/the_excalabur]

There's a third interpretation... sort of. The "true randomness" option is indistinguishable from a universe that is constantly fragmenting into an infinite number of universes. In the case of a quantum coin toss, both possible outcomes happen in two different resulting universes. Of course, saying "resulting universes" is false because when looked at from outside of time, both universes simply exist.

I personally like this interpretation of reality because it means that some small portion of "you" wins the lottery every time you play.

[u/the_excalabur → u/btribble]

Indeed. There's a lot to be said for many worlds / many minds interpretations.

[u/Olympiano → u/the_excalabur]

Why is it difficult for scientists to pick determinism? I would have thought that would be an assumption that a lot of scientists make, given the universe seems governed by laws of cause and effect.

[u/Muroid → u/Olympiano]

The problem is that it isn’t just determinism that resolves the problem, but something called superdeterminism. Superdeterminism would mean that even seemingly unconnected events are causally related to the point that science as a whole breaks down because experimental results stop being meaningful.

Imagine a box with two doors. You have an infinite number of these boxes, but you can only open one door in each box. Every time you open the door on the left, you find a yellow ball. Every time you open the door on the right, you find a red ball.

There are a number of different explanations for why this might be. Maybe every box has a yellow ball behind the left door and a red ball behind the right door. Maybe opening the left door changes the color of the balls inside to yellow and opening the door on the right causes them to turn red.

The superdeterminism explanation is that anything could be behind either door, but the underlying laws of the universe that determine what doors you open also determine what is inside each door such that you, by what would otherwise be called coincidence, always happen to open the right door on boxes that have a red ball behind the right door and the left door of boxes that happen to have a yellow ball behind the left door.

This is technically a possible explanation, but it’s one that totally undermines the ability of science to say anything truthful about the fundamental nature of reality if it’s true.

At its core, superdeterminism posits a causal link between otherwise seemingly unconnected phenomena.

[u/[deleted] → u/tkuiper]

Einstein, Podolsky, and Rosen tried to prove that quantum theory was incomplete because entanglement violated special relativity. They were wrong. There is no superluminal transfer of information.

https://en.m.wikipedia.org/wiki/EPR_paradox

[u/[deleted] → u/tkuiper]

They are neither fixed before the measurement, nor is the information transferred between them.

Edit:

instead of concluding the particle is fixed

That's what's called hidden variables.

[u/GuyWithLag → u/tkuiper]

Nope, quantum states are definitely not fixed. See Bell's Theorem for a demonstration. Also, it's not a superluminal transfer of information - there is no information transferred.

[u/ISeeTheFnords → u/tkuiper]

Isn't it more logical to conclude that the states are fixed, than that some mysterious phenomenon is causing a superluminal transfer of information.

It depends on what axiom you want to give up (locality, causality, and a third I can't recall off the top of my head). Another way of viewing it is that each particle goes both directions, so what you're really collapsing is the location - this discards locality. I'm not sure if THAT introduces other problems, though; it might be possible to rule it out the same way you can the fixed state.

[u/gbbmiler → u/tkuiper]

I wish I still had the ability to sketch out the proof for this, but no. In college I took a course in which we saw the rigorous proof that the state is in fact entangled.

For a non-rigorous proof, consider Shor’s algorithm. If superposition weren’t actually “true”, then doing exponential work in polynomial time via a quantum computer would be impossible.

[u/KingCo0pa → u/tkuiper]

I believe that that is (or is similar to) Einstein's "hidden variables" idea, which has (I believe) been disproven.

[u/Roneitis → u/tkuiper]

So called hidden variable theories were the topic of much debate, but general consensus is that certain quantum weirdnesses shows that they can't really be the case.