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Comment by suifbwish | Hacker News Reader

Quantum communication requires a continually renewed source of mutually shared entangled pairs to send communication. When entangled states collapse, which they must do in order to send information, the particle pairs are no longer entangled and can thus no longer send information. These civilizations would have to find a way to keep a stable quantum pairing which can be observed without collapsing the quantum state, which is the equivalent of finding a way to stop black holes from emitting Hawking radiation; essentially they would need to find a way to stop entropy as we know it in order to make long distance quantum communication sustainable.

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> These advanced civilizations would require technology I can't think of how to implement

I mean, that's part of what makes them advanced civilizations.

Advancements in technology will probably not do away with what we already understand about physics, it will likely just add more details.

>Advancements in technology will probably not do away with what we already understand about physics, it will likely just add more details

Could have been said in the 1800's about why we'd never have quantum computers.

I don't get it - wasn't it concluded that instantaneous communication is impossible, aka you can't transmit information EVEN if you were to take a decade to move two entangled particles apart, all you would get is data from 10 years ago?

FTL communication is not possible. You have access to the entangled particle right now but, to send information, you are bound to the limits of the classical channel (the 10 year problem transmitting information about the entanglement)

This would be correct if causality is an absolut certainty in all the universe, the whole time,

but what if causality it actually isn't a certainty in some contexts? Tadah..you would get ala "FTL" comms enabled.

Isn’t this just saying that if you could send a signal faster than c, then you could send a signal faster than c? I don’t see your point.

Sorry, I didn't wanted to write it like that was psychosis somehow.

I meant, causality as I understand, it could be not a fully understood concept. We could still find evidence - in the future - that there are phenomena capable of "evade" the cause-effect principle.

https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano...

To add more speculation, the idea from the movie "Tenet" could be a way to effectively surpase C in long distance communications: reversed entropy matter.

You could just send a ship or some kind of vesel with reversed entropy to a star and it will arrive years later, but years sooner that it was launched.

Matter is more or less energy, what if this trick could be done with reversed entropy energy: radio, x-rays, gamma rays, etc.

You can reverse entropy with a refrigerator, but I don’t think that will allow FTL communication.

But what if you had a spinning black hole to play with? Couldn't you strategically place a bunch of satellites in various orbits around it and use the frame dragging effects in either direction to manipulate signals into arriving arrive somewhere else at arbitrary times by sending them around often enough?

I can think of two ways in which one would still be limited though. First is that one is limited to dragging signals only as far back as when the necessary machinery was first put in place. This is analog to the limitation of the wormhole time travel hack where you accelerate one close to c, then slow it back down and put it in place again to increase the distance in time to the other end that was stationary the whole time. But perhaps even worse would be the hard limits on how much this could be used before the whole apparatus itself turns into a black hole by the sheer energy being sent back from the far future. it would be like a time lense effectively. Or less spectacularly, it might just get fried. Or perhaps a failsafe would just dump the signal before things get critical. In any case, there would be a limit to how much or from how far in the future one could use such a thing, but other than that it should work, right?

Is it really "equivalent"? I get both problems are about entropy in the universe but I would have thought managing the state of a black is harder, due to its suction into oblivion and whatnot.

I don't know much about if it's possible to make entangled particles stable somehow, or whether you could maybe just have a lot of them so you never run out even if a lot become untangled naturally. Doesn't quantum key exchange use entangled particles? So it must be possible to keep them stable for at least a little while.

Place a beam splitter between you and fire a laser. Get entangled particles back.

Or place an entangled particle emitter between you

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