Comment by roywiggins
Comment by roywiggins 12 hours ago
Entanglement isn't particularly useful for communication, you can't send bits without sending photons (or similar) physically. Quantum mechanics doesn't permit ansibles as far as anyone knows.
Comment by roywiggins 12 hours ago
Entanglement isn't particularly useful for communication, you can't send bits without sending photons (or similar) physically. Quantum mechanics doesn't permit ansibles as far as anyone knows.
Yes, they are fun, but not notably useful, at least not yet. And only useful for pretty specialized tasks like key exchange.
Isn't it true that in key exchange entanglement isn't used in any way shape or form for sending data, but only in making a determination that there was no eavesdropping on the transmission, because any eavesdropper would collapse the wave function.
So like you said, entanglement can't be used to send information, but it can be used to detect if the transmission was secure (I think)
There are many protocols for quantum key distribution/exchange so it's hard to answer fully without knowing which one you're talking about. That said their are protocols, like the one invented by Artur Ekert in 1991, which use entanglement in an essential way to transmit the key. Even in the absence of an evesdropper the protocol will not work without entanglement. It escapes the no-communication theorem by also requiring some classical communication.
Right, if you expand the scope of the discussion to other areas other than sending bits, there are various ways entanglement is used in various protocols. But none of them utilize entanglement to be able to get a bit from Alice to Bob faster than light can go.
The issue the person above is alluding to is known as the no communication theorem.
It has a wiki page
https://en.m.wikipedia.org/wiki/No-communication_theorem
But the upshot is basically that entanglement doesn't let you do anything unless you send some classical data as well.
Thanks. The reason still seem to be related to the uncertainty principle although I am not sure.
The same way they explain no-cloning but it seems to be analogous to identity within a system with interaction from neighboring data. Ultimately there is no pure independent state. Data always exists within context. Hence causality and spatial preservation (no instant physical teleportation as far as is currently understood). (in very layman's terms)
I am unable to grasp why FTL communication would break causality, it's like my brain just refuses to accept it. Seriously, I've had it explained several times over the years.
Its probably mostly because you have an intuitive idea that there is some concept of "now" which is independent of the observer.
In special relativity this global "now" isn't a thing. It doesn't exist. There is no global now. Different observers who are in different places and/or moving at different speeds will describe different events as simultaneous.
In particular say we have an observer who sees an event A happening at time 0, and a second event (call it B) at time t and the distance between them is greater than c t. Then you can find observers who see A happening first, B happening first or the two happening at the same time. However all observers will agree that the distance between the events was greater than c times the time between them.
This seems like it would cause problems with causality, but it doesn't because we need the distance to be greater than c times the time, which means no lightspeed signal could get from A to B. If you allow ftl communication then this "escape" doesn't work anymore, and causality can be explicitly broken.
When you communicate, you're sending energy - whether it's sound waves or radio waves or whatever. Energy can't travel faster than c through spacetime. Now if you manipulate spacetime, such as a wormhole or whatever, then the end result can effectively appear as if it's FTL but its still going at c, its just traveling through less/compressed space.
One intuitive explanation is that anything moving at the speed of light is experiencing no time at all (from the point of view of an observer) and if something is moving faster than light that means it's going backwards in time. (only massless virtual particles can)
If a clock stops or runs backwards that totally messes up "causality" which is about events interacting relative to a time order, and so time must exist for causality to make sense.
If you did manage FTL communication, it seems like it might let you detect the notional absolute rest frame, so you'd be breaking special relativity anyway.
You can't grasp this because there is no FTL communication. Quantum entanglement does not enable FTL communication, and wormholes etc. are entirely theoretical.
It doesn't even enable STL communication, other than eg superdense coding and similar. But that's not what people mean when they think entanglement can be used for communication.
There was a young lady named Bright,
Whose speed was much faster than light.
She went out one day
In a relative way
And returned on the previous night.
It's all a big ball of wibbly-wobbly, timey-wimey stuff.
I had to google ansible. https://en.wikipedia.org/wiki/Ansible
While the second and third parts if your comment are complete true, the first part
> Entanglement isn't particularly useful for communication
I would say is false. Entanglement lets you do some fun and theoretically useful stuff for communication tasks. At the most basic level sharing entanglement lets you upgrade a classical communication channels you have into a quantum one (sending 2 bits and burning an entangled pair lets you send a qubit). You can do increasing fancy stuff if you so wish, if you are sufficiently paranoid you might be interested in device independent cryptography, which is only possible because of entanglement.