4 ms·

> I doubt SETI could detect even our own signals, This is true. The signals we communicate with amongst ourselves would be too weak to detect using present equ

> I doubt SETI could detect even our own signals, This is true. The signals we communicate with amongst ourselves would be too weak to detect using present equipment at any distance. > when it’s encrypted digital data. I mean it’s just noise like any background signal.. This part is not true. A simple spectrum analyzer will show what radio bands are being communicated on and it's clear as day. > we people are very soon capable of communicating using quantum entanglement, This is very much not true. This is theoretically impossible. Unless we are very--very--wrong about quantum mechanics, we cannot use effects related to quantum entanglement to communicate.

TechBro8615 3y ago
> we cannot use effects related to quantum entanglement to communicate ELI5 to a non-physicist why not? I thought this was possible, if Alice and Bob share an entangled pair of a set of particles (i.e. each has a clique of particles entangled with the other), and they pre-arrange an algorithm for determining the meaning of different states of their particles. IIRC wasn't this the gist of the "wormhole" paper that went viral last year (which was criticized for being improperly named, but nonetheless demonstrated a method of "communication" between two locations given a prearranged algorithm for assigning meanings to different states of entanglement)?
- nwallin 3y ago
  Quantum entanglement is like two dice that "agree" with each other to always roll the same number. If Alice and Bob shake their dice in their hands, and roll it on the table, they'll always roll the same number. It does not allow Alice or Bob the place a die on the table on a specific number and have the other die roll over to that number. If Alice or Bob try to influence the number that the die rolls, it will break the connection between the two dice.
  - TechBro8615 3y ago
    But why can't Alice keep rolling the die until it gets to a state that she and Bob agree is a part of an encoding scheme? Is it because she needs to read the number on the die to know whether to stop rolling it, but by reading (measuring) the number, she is effectively re-rolling (collapsing) it, so she could never know when to stop rolling it? In other words, she could stop after she measures a state, but the current state of the system is N, she'd be stopping after N-1, and there is no way to coordinate with Bob to know the difference? The link I posted downthread seemed to suggest that the No Cloning Theorem is the constraint that makes this impossible. Any chance you can try to explain that within the context of the dice analogy?
    - nwallin 3y ago
      You can only roll the die once. Alice and Bob have a dozen dice. They agree that if they roll a 4, that's the signal for "attack at dawn" and Alice has decided that she wants Bob to attack at dawn. Alice rolls the 12 dice, and at the agreed upon time, Bob also rolls his 12 dice. The sequence they get is 1,5,3,2,2,4,6,5,1,2,1,6. The 6th die is a 4, which is the signal that Alice wants to send. How does Alice signal to Bob that she wants him to look at the 6th die? If Alice can send a signal to say, "look at the 6th die", why wouldn't she just send a signal that says, "attack at dawn"?
      - TechBro8615 3y ago
        Thanks for the explanation. :) It makes sense, at least insofar as any of this can possibly make sense...
- Tor3 3y ago
  Isn't the point of the Alice/Bob thing that none of them know the state of the particles before one of them actually check - and at that point they know the state of the other particle. But Alice can't set the state of her particle, which prevents using this to send information.
- p-e-w 3y ago
  Quantum entanglement means that pairs of particles can have correlated states regardless of the distance between them. It does not mean that if you push on one particle, the other one lights up. Imagine that two people are each given a coin with the property that whenever they flip those two coins simultaneously, they will either both come up heads, or both come up tails. How can you use that to communicate, that is, send information from one to the other? You can't. You can pre-arrange actions that you both will take when you see either heads or tails, but that's not communication.
  - TechBro8615 3y ago
    But if each side of the pair can only have a certain number of states, is it not possible to perturb one of them until it gets into a state such that you know based on entanglement that the other must be in its corollary state?
    - johncolanduoni 3y ago
      Given only an entangled qubit, the only thing you can do that will be “synchronized” with the other party is perform a measurement. Each measurement performed by either party will produce either a 0 or 1 without any “input” from the measurer or the other party. Any perturbation of the state that preserves the entanglement can only rotate it (which just alters what state your side considers 0 and 1).
      - TechBro8615 3y ago
        Thanks. While you were writing your comment I found this article [0] that does a decent job of explaining why my hypothetical system isn't possible. It first lays out the exact system I had in mind and then unfortunately debunks it :( I don't fully understand the subtleties, but the root problem seems to be that by measuring the state, you're perturbing the system and putting it into a new state, such that your measurement becomes irrelevant, as if you're always one step behind the message and can never catch up. > The only way that this problem could be circumvented is if there existed some way of making a quantum measurement that actually forced a particular outcome. (Note: this is not something permitted within the presently-known laws of physics.) [0] https://bigthink.com/starts-with-a-bang/quantum-entanglement-faster-than-light/ https://bigthink.com/starts-with-a-bang/quantum-entanglement...

TechBro8615 3y ago

> we cannot use effects related to quantum entanglement to communicate ELI5 to a non-physicist why not? I thought this was possible, if Alice and Bob share an entangled pair of a set of particles (i.e. each has a clique of particles entangled with the other), and they pre-arrange an algorithm for determining the meaning of different states of their particles. IIRC wasn't this the gist of the "wormhole" paper that went viral last year (which was criticized for being improperly named, but nonetheless demonstrated a method of "communication" between two locations given a prearranged algorithm for assigning meanings to different states of entanglement)?

nwallin 3y ago
Quantum entanglement is like two dice that "agree" with each other to always roll the same number. If Alice and Bob shake their dice in their hands, and roll it on the table, they'll always roll the same number. It does not allow Alice or Bob the place a die on the table on a specific number and have the other die roll over to that number. If Alice or Bob try to influence the number that the die rolls, it will break the connection between the two dice.
- TechBro8615 3y ago
  But why can't Alice keep rolling the die until it gets to a state that she and Bob agree is a part of an encoding scheme? Is it because she needs to read the number on the die to know whether to stop rolling it, but by reading (measuring) the number, she is effectively re-rolling (collapsing) it, so she could never know when to stop rolling it? In other words, she could stop after she measures a state, but the current state of the system is N, she'd be stopping after N-1, and there is no way to coordinate with Bob to know the difference? The link I posted downthread seemed to suggest that the No Cloning Theorem is the constraint that makes this impossible. Any chance you can try to explain that within the context of the dice analogy?
  - nwallin 3y ago
    You can only roll the die once. Alice and Bob have a dozen dice. They agree that if they roll a 4, that's the signal for "attack at dawn" and Alice has decided that she wants Bob to attack at dawn. Alice rolls the 12 dice, and at the agreed upon time, Bob also rolls his 12 dice. The sequence they get is 1,5,3,2,2,4,6,5,1,2,1,6. The 6th die is a 4, which is the signal that Alice wants to send. How does Alice signal to Bob that she wants him to look at the 6th die? If Alice can send a signal to say, "look at the 6th die", why wouldn't she just send a signal that says, "attack at dawn"?
    - TechBro8615 3y ago
      Thanks for the explanation. :) It makes sense, at least insofar as any of this can possibly make sense...
Tor3 3y ago
Isn't the point of the Alice/Bob thing that none of them know the state of the particles before one of them actually check - and at that point they know the state of the other particle. But Alice can't set the state of her particle, which prevents using this to send information.
p-e-w 3y ago
Quantum entanglement means that pairs of particles can have correlated states regardless of the distance between them. It does not mean that if you push on one particle, the other one lights up. Imagine that two people are each given a coin with the property that whenever they flip those two coins simultaneously, they will either both come up heads, or both come up tails. How can you use that to communicate, that is, send information from one to the other? You can't. You can pre-arrange actions that you both will take when you see either heads or tails, but that's not communication.
- TechBro8615 3y ago
  But if each side of the pair can only have a certain number of states, is it not possible to perturb one of them until it gets into a state such that you know based on entanglement that the other must be in its corollary state?
  - johncolanduoni 3y ago
    Given only an entangled qubit, the only thing you can do that will be “synchronized” with the other party is perform a measurement. Each measurement performed by either party will produce either a 0 or 1 without any “input” from the measurer or the other party. Any perturbation of the state that preserves the entanglement can only rotate it (which just alters what state your side considers 0 and 1).
    - TechBro8615 3y ago
      Thanks. While you were writing your comment I found this article [0] that does a decent job of explaining why my hypothetical system isn't possible. It first lays out the exact system I had in mind and then unfortunately debunks it :( I don't fully understand the subtleties, but the root problem seems to be that by measuring the state, you're perturbing the system and putting it into a new state, such that your measurement becomes irrelevant, as if you're always one step behind the message and can never catch up. > The only way that this problem could be circumvented is if there existed some way of making a quantum measurement that actually forced a particular outcome. (Note: this is not something permitted within the presently-known laws of physics.) [0] https://bigthink.com/starts-with-a-bang/quantum-entanglement-faster-than-light/ https://bigthink.com/starts-with-a-bang/quantum-entanglement...