irotsoma comments on What's a scientific fact that sounds 100% real but is made up?

[–] irotsoma@piefed.blahaj.zone 0 points 5 months ago* (2 children)

Not quite. The idea is that once entangled, they stay entangled after being separated physically. So changes made to one happen to the other. Of course, the distance we've successfully maintained entanglement is limited still because lots of things can happen while moving them, but theoretically there is no maximum physical distance. I think the current record was less than 100 miles. So if the particles are entangled, then separated by a kilometer then the state of one particle is altered, the second particle is instantaneously altered from our frame of reference. You don't change the value and then move them, you move them first and then change the value.

Problem is that measuring the current state of one particle inherently changes the current state of both particles instantaneously. So, how do you tell the person with the other particle the state you measured. And when the other side measures that change, they then change it again in both particles.

Basically, we haven't 100% figured out exactly how to take advantage of this instantaneous change without exchanging additional information which is then limited by the speed of light. So potentially, yes it's possible that instantaneous communication might be possible if we can figure out a way to measure without changing the state. Possibly by measuring some side effect of the change like a medium that reacts when the particle changes state that can be measured without affecting the entangled particles.

And while it is true that entanglement doesn't violate the speed of light, due to relativity, it does appear as though it does to humans on Earth, and for all practical purposes we might use it for, it does as long as the observers at both ends are in our familiar frame of reference on Earth. So, if the particle could be safely moved to a space station around Jupiter for example, without messing up the entanglement, and we figure out a way to read the state without affecting the particles' states, then there could be instantaneous communication between Earth and Jupiter, instead of taking 35-52 minutes at light speed.

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[–] Deconceptualist@leminal.space 2 points 5 months ago* (last edited 5 months ago) (1 child)

[We're not still playing OP's game here, right? I'm keeping that to top-level comments only, so the rest is serious]

So if the particles are entangled, then separated by a kilometer then the state of one particle is altered, the second particle is instantaneously altered

That's not how entanglement works. A pair of entangled particles (as a system) are in a superposition of states. When you measure the state of the near particle, the superposition collapses and you instantly know the far one has the opposite state.

But you don't get to "alter" or choose the state of either particle; the outcome of the collapse is random and you just have to measure it. If you tried to impose a particular state onto the system you would sever the delicate entanglement.

Because of this, you have to encode information onto the system locally when the particles are first entangled, i.e. before you separate them. Note, the information couldn't be stored directly in the quantum states, but rather in how you arrange a set of entangled particles (a simple data structure like an array would suffice).

After separation by some distance, you observe the particle set on your side and read out the pattern of their states. As expected, that instantly tells you the remote pattern as well. However, by doing this you don't actually learn anything new that you didn't already know when the entangled particles were together locally in the first place!

Logically you cannot do all the encoding/arranging work later when the particles are already far apart, because you would lack the information to create the same arrangement on both ends of the communication (necessary to read any message, like ordered letters in a word). As you say, you would need a secondary slower-than-light communication method to share that info, and that defeats the whole point.

If it did work like you suggest where you can arbitrarily write data to your local particle set and have the entanglement transmit it remotely, then you wouldn't need a secondary verification method at all. You would know your message was received because your faraway partner would reply via the same tech in a way that only makes sense if they read it (plus maybe a few parity bits for error correction).

Unfortunately the entangled system is inherently too fragile for that scenario. Any meaningful perturbation (including measurement) collapses the necessary superposition at the heart of it. Yes that collapse can potentially reveal information, however it's limited to info that was already known in a shared local past.

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[–] irotsoma@piefed.blahaj.zone 0 points 5 months ago

Yeah I was glossing over a lot of the more hypothetical stuff and went off a tangent probably, but the two points I was making are

Data transfer in quantum communications does not require only setting a state and then they maintain that state and that's it. Quantum teleportation allows for "modifying" the state later by entangling one side to another particle whose state you want to communicate.
The fact that the system of modern quantum communications relies on classical communication to know how to then measure the newly entangled state is not the only reason that quantum entanglement/teleportation doesn't violate the speed of light. This side is still not as solid in understanding, but the information of the entanglement of the third particle is available seemingly instantaneously. It just requires 2 bits of information to know what measurement to take to get the new state. And you can't measure all 4 possible ways because the first time you measure, the state is "modified" and you dont get a second chance since measurement on either side "modifies" the state seemingly randomly. But what you can do to prove that the state changed relatively instantaneously, is to set up a system where you entangle lots of particles. Then at exactly the same time entangle them to a third particle and randomly choose a method of measurement on the output side. Then throw out the results of output side that don't match the 2 bits you send over classical means when they arrive. Those are the ones that should match. This doesn't solve the issue of instantaneous communication, but does prove that if you had instant knowledge of which measurement to take, the information was available instantaneously. And this is glossing over some details in interpretation of the results, but the data itself is there if you knew how to interpret it.

Currently, it's potentially useful in it's current form because the quibit contains and "transmits" more information than the 2 bits sent over classical means if we can make the entanglement more reliable.

I was also out of date about the successful distance of quantum entanglement/teleportation. It has been done with satellites up to about 1,000km. Particles entangled, one half launched into orbit. Earthbound particle is entangled to a third particle which "changes/destroys" the previous state. 2 bits per particle are sent to the satellite via the internet or other classical means to know which of 4 measurement to take. The information from the third particle state is then read on the satellite. This has about a 90% accuracy rate currently, mostly due to the entanglement being broken or the state being changed by outside forces before the classical data are received.

Anyway, these are all glossing over some details and there is still a major piece we're missing to make actual instantaneous communication, but it's the best I can do in simple terms. And it's definitely possible that there is no way for the output side to be read correctly without those 2 bits of classically transmitted information, at least for in our relative realm of existance. This is all also admittedly a decade or so out of date information since I haven't kept up with it lately.

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