5 ms·

I did not get the point: "No useful information can be sent in quantum entanglement". Has it been found how entanglement actually works?

attilakun 6y ago
If you are familiar with probability theory, think of it like this: whatever you do with an entangled particle, it will never change other entangled particles' marginal probability, only the joint.
- da39a3ee 6y ago
  That's very helpful thanks, I do understand what that means but I didn't know I should/could think of entanglement like that.
thereisnospork 6y ago
An attempt at lay terms: imagine particles are tiny hands, and they can switch between being left hands and right hands. So to 'entangle' two particles is to make them shake. This means we now know that one particle is a lefty and the other a righty (two lefts can't shake each other) but because they are so small we don't know which is which. Now (gently) separate the two particles and place each in a box. It is presently unknown which is lefty, and which is righty so at this point each particle is considered to be in a superposition of both lefty and righty. A la Schrodingers cat each particle '''hasn't decided''' if it is a left or right. All QE states is that when we look in one box and see a right hand we know the other box contains a left hand - regardless of the distance between them. There is no provision or mechanism for shaking one particle and having a change occur with the other (information transfer).
- sidcool 6y ago
  That's a good explanation. However, since the particles are in superposition, the act of observation must be conveyed to the other particle, right?
  - thereisnospork 6y ago
    To be clear I'm not a physicist, but I'll try not to get out of my depth: I would say that the unobserved hand is and always has been (post entanglement) in the handedness dictated by viewing the other and the superposition merely refers to the indeterminably of that state -- which is still indeterminable from the POV of the persons with the unobserved hand[0] despite the first hand being observed. The superposition of both particles however has collapsed from the POV of the observers. [0] Until they get a phonecall at less then or equal to the speed of light from the observers of the observed hand. You can also imagine this is a pair of entangled Schrodinger's cats - one alive and one dead. On observing the live one there isn't a force or mechanism[1] by which the other one is instantly killed; it was always dead. [1]If there is, then it transcends space and time such that the cat dies in the past and so appears to have always been dead in the present.
Strilanc 6y ago
We have a simple mathematical model (quantum mechanics) that predicts the results of experiments involving entanglement. Using that mathematical model you can prove the no communication theorem [1]. As far as we can tell, we know how entanglement works. 1: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
krzat 6y ago
In a way, entanglement is inevitable if you superposition is possible. For example: imagine particle decayed into two other particles. For conservation of momentum to hold, those two particles must go in opposite direction. So if you measure direction of one particle, you instantly know direction of the other one. Otherwise you would vitness universe in invalid state.
Gunax 6y ago
Yea it does. Try this explaination: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html

attilakun 6y ago

If you are familiar with probability theory, think of it like this: whatever you do with an entangled particle, it will never change other entangled particles' marginal probability, only the joint.

da39a3ee 6y ago
That's very helpful thanks, I do understand what that means but I didn't know I should/could think of entanglement like that.

thereisnospork 6y ago

An attempt at lay terms: imagine particles are tiny hands, and they can switch between being left hands and right hands. So to 'entangle' two particles is to make them shake. This means we now know that one particle is a lefty and the other a righty (two lefts can't shake each other) but because they are so small we don't know which is which. Now (gently) separate the two particles and place each in a box. It is presently unknown which is lefty, and which is righty so at this point each particle is considered to be in a superposition of both lefty and righty. A la Schrodingers cat each particle '''hasn't decided''' if it is a left or right. All QE states is that when we look in one box and see a right hand we know the other box contains a left hand - regardless of the distance between them. There is no provision or mechanism for shaking one particle and having a change occur with the other (information transfer).

sidcool 6y ago
That's a good explanation. However, since the particles are in superposition, the act of observation must be conveyed to the other particle, right?
- thereisnospork 6y ago
  To be clear I'm not a physicist, but I'll try not to get out of my depth: I would say that the unobserved hand is and always has been (post entanglement) in the handedness dictated by viewing the other and the superposition merely refers to the indeterminably of that state -- which is still indeterminable from the POV of the persons with the unobserved hand[0] despite the first hand being observed. The superposition of both particles however has collapsed from the POV of the observers. [0] Until they get a phonecall at less then or equal to the speed of light from the observers of the observed hand. You can also imagine this is a pair of entangled Schrodinger's cats - one alive and one dead. On observing the live one there isn't a force or mechanism[1] by which the other one is instantly killed; it was always dead. [1]If there is, then it transcends space and time such that the cat dies in the past and so appears to have always been dead in the present.

Strilanc 6y ago

We have a simple mathematical model (quantum mechanics) that predicts the results of experiments involving entanglement. Using that mathematical model you can prove the no communication theorem [1]. As far as we can tell, we know how entanglement works. 1: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem

krzat 6y ago

In a way, entanglement is inevitable if you superposition is possible. For example: imagine particle decayed into two other particles. For conservation of momentum to hold, those two particles must go in opposite direction. So if you measure direction of one particle, you instantly know direction of the other one. Otherwise you would vitness universe in invalid state.

Gunax 6y ago

Yea it does. Try this explaination: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html