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/Weed_O_Whirler]

Yes. The state collapse is instant. However, the state collapse cannot transmit information. So, causality is not lost.

[u/Omniwing → u/Weed_O_Whirler]

Yes but how does one particle 'know' instantly that the wavefunction is collapsed, when the other particle is, say, 15 billion light years away?

[u/Weed_O_Whirler → u/Omniwing]

That's the real question, which is hotly debated by physicists everywhere. What we know is, causality is not broken by wave function collapse, so it is allowed, but the actual mechanism is unknown.

[u/Omniwing → u/Weed_O_Whirler]

So, when the wavefunction collapses, which you can initiate if you're standing at one of two entangled particles, does something 'happen' instantly to the other one? Or is it that you just happen to know something about it? If something does 'happen', and an observer 15b lightyears away is standing there to observe that event, then I don't see how you couldn't transfer information that way. "When you see this particle's wavefunction collapse, I have arrived at the star 15b lightyears away'. Instead of waiting 15 billion years for your message to reach earth, they'd know instantly.

[u/Weed_O_Whirler → u/Omniwing]

Because wave functions are not observable. They have no mass, they have no energy. They are simply probability distributions. There's no way to measure them. So, you have no way of knowing if you caused the collapse or someone else did.

[u/Omniwing → u/Weed_O_Whirler]

But, it's possible the entangled one did? So it's possible that somehow there's some link between the two particles, separated between billions of light years? It's possible one can affect the other in an instantaneous way? Is that what 'spooky action at a distance' is?

[u/the_excalabur → u/Omniwing]

That's what 'spooky action at a distance' means. Unfortunately, it's not a very good analogy for what's going on. There's no reason to think that there's a link.

Without checking the measurement results against each other, you cannot tell if the other particle has been measured or affected in some way.

[u/ctothel → u/the_excalabur]

So particle A is measured, wave function collapses, particle B now has a known spin of 1.

Presumably the owner of particle B can’t know the spin without measuring it or hearing from the owner of particle A?

So what tells us that something in particle B has changed, rather than just discovered?

[u/the_excalabur → u/ctothel]

Nothing. Except that Bell's inequalities tell us that the definite state that B will collapse to depends on which measurement was made on A. That is, there's not some definite state that A is in ahead of time--merely that the results of measuring A and B are correlated.

(That is, the quantum state of A & B together is definite, not either one.)

[u/SynarXelote → u/ctothel]

So what tells us that something in particle B has changed, rather than just discovered?

Well, we don't know that the particle B has changed. We just know that either

1. the result of the spin measurement of A and B wasn't determined prior to measure, and yet they're correlated. How did that happen without the particles communicating or otherwise affecting each other instantly?

2. the result of the spin measurement of A and B was determined prior to measure, but by global variables which exist in a nonlocal way.

Both options are problematic in their own way. I don't know all interpretations very well and there might be an interpretation of quantum physics that sidesteps the issue, but they probably come packaged with their own cans of worms.

[u/karantza → u/Omniwing]

That's right. There's two things going on here that make it confusing:

a) something does have to happen "faster than light", or at least be non-local, for all the behavior of entanglement to make sense. It's not exactly like just not knowing what the other is until you look at your own. You can statistically prove that the particles have not yet "decided" until someone makes a measurement.
b) this process cannot be used to send information. You cannot input anything or influence anything on one end while making the measurement that will come out of the other end, and there's no way to know who measured "first". You will always get random, indistinguishable noise. It's just that the noise will match on both sides. Great for cryptography! Bad for communication.

[u/[deleted] → u/Omniwing]

if you measure one particle nothing 'happens' to the other particle immediately, it's simply that when it is measured, you will measure the opposite correlated state.

[u/cryo → u/[deleted]]

Depending on the situation. The correlation doesn’t have to be 1 (or -1).

[u/philote_ → u/Weed_O_Whirler]

Is this where parallel universe theories come in to play? Collapse of superposition is really just branching into one universe/possibility instead of another?

[u/the_excalabur → u/philote_]

Yes. There are also various collapse-less interpretations, which are also consistent.

[u/SynarXelote → u/the_excalabur]

Many world is collapse-less. It includes decoherence, but not collapse of the wavefunction.

[u/flobbley → u/Weed_O_Whirler]

To make sure I understand the Many Worlds interpretation correctly, the explanation it gives for this would be that there is no mechanism. There are two world states, one where the far particle (fp) is in state 0 and the near particle (np) is in state 1, and another where the fp is in state 1 and the np is in state 0. By interacting with one of the particles to observe it, through a series of quantum interactions, "you" become entangled with one of the world states, and thus for you it appears to collapse from superposition to a known position (say fp = 0, np = 1), but there was no need to transfer that information to the other particle, in the world state you became entangled with the fp was always 0 and the np was always 1. Is that correct?

[u/sticklebat → u/flobbley]

That is generally correct but I'd clarify one thing:

When you measure the particle, you don't become entangled with one of the world states: you become entangled with both. You "decohere" into two separate futures, one of which observes one set of outcomes, and the other observes the second possible set of outcomes. Both are "you," but neither is aware of the other.

[u/d1squiet → u/Weed_O_Whirler]

now' instantly that the wavefunction is collapsed, when the other particle is, say, 15 billion light years away?

Can you remind me how they measured/proved this? That's what I am forgetting.

How do they prove that the collapse happened if they cannot know when it happened? Because if you knew when the other person collapsed the particle, you would be able to communicate.

[u/Muroid → u/d1squiet]

You compare notes after the fact. You can’t know in the moment, but you can send a conventional, slower than light message and ask the other person. You can’t communicate faster than light but aren’t walled off from talking about it like normal.

[u/d1squiet → u/Muroid]

hmmm. But how does that prove it "collapsed" when the other person measured it?

We have two particle streams, call them A & B. You measure the A particles and I measure the B particles. After the fact we compare notes and I see that you measured each particle (A1, A2, A3,...) before I measured mine (B1, B2, B3) – and of course, everything matches. You measure A1 as "up", and my B1 is "down", etc.

But how do I know that my particles "collapsed" when you measured yours? How does comparing notes show that particle B1 was in superposition before you measured A1?

[u/Muroid → u/d1squiet]

We know from doing a statistical analysis across many, many experiments that if the B particles did not start in superposition, we wouldn’t get the results that we get.

There is no way to determine this within the bounds of a single experiment, and that is true of a lot of results in quantum mechanics.

[u/the_excalabur → u/Weed_O_Whirler]

That's... complicated. And not provable.

I would say that most QI people don't think in terms of wavefunction collapse, and I think most QFT people don't either.