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Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light? If me observing the parti

by qq12as 6y ago

Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light? If me observing the particle in Australia alters the probability distribution of your particle in USA, can't I only observe the particle when I want to communicate 1 and never observe it when I want to communicate 0? Edit: thanks a lot for the answers! I guess it boils down to the fact that the Australian guy cannot condition his decision on the (unknown) spin of his particle -- if he could (eg: had access to the local hidden information) then he would be able to update the USA's probability distribution instantaneously and use it to communicate

EliRivers 6y ago
When the person at the other end looks at their particle, and sees it either 1 or 0 and has no other information, how do they know if you've looked at your particle or not?
- deleted 6y ago
  [deleted]
- fjfaase 6y ago
  And even if they would know, due to the distance and general relativity, there is no concept of one of the two persons looking first. It could be that from Alice's point of view Bob looked later, while from Bob's point of view Alice looked later. So, in which direction did the information go?
mppm 6y ago
Initially, your particle in Australia and its entangled twin in the USA exist in a superposition of 0 and 1. When you "measure" the state of your particle, you force it to assume a definite state, and entanglement forces the other particle to assume e.g. the opposite definite state ("spooky action at a distance"). This allows you to synchronize information across large distances, but you cannot send anything, because you cannot chose the outcome of the quantum measurement.
- willis936 6y ago
  If you can guarantee a random distribution of the color of the balls, it seems like a decent way to handle secure key echange.
  - mppm 6y ago
    Yes, that is in fact one of the practical ways to do quantum key distribution.
- teekert 6y ago
  What if we manage to get our whole world into a superposition and only collapse to the world where the particles did contain the message we wanted? Sorry... I've been reading a lot of Greg Egan lately ;)
317070 6y ago
I have a bag with two balls, one red, one blue. I randomly send one of the balls to Australia, one of the balls to the USA. The moment you look at your ball in Australia, you also know the color of the ball in the USA. How would you use that fact to send information? The crux with quantum mechanics is that you can show (very easily, understandable by the layman, look up the Bell inequality) that in the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well. However, the basic setup still applies. You cannot send information by merely observing something.
- smusamashah 6y ago
  How does a ball know that it has been observed and in effect changing the color of the other paired ball?
  - EliRivers 6y ago
    That's the "spooky action at a distance" (in the case of the entangled particles, obviously; with the two balls it's not so spooky). Spooky.
    - nemothekid 6y ago
      Maybe this is a basic question - but what I don’t understand is why this is called “spooky” action. My intuition is you have two particles, and you don’t know what concrete states they are in, but you know all possible states (that may be represented as some sort of system of equations). By observing a single particle you unlock a variable in that system of equations and can therefore solve the whole thing. To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed. The concept of superposition seems like an overly complex description for this phenomenon. I understand my view is wrong, but I don’t understand how I’m wrong
      - rrss 6y ago
        What you are describing is a hidden variable theory - i.e. there is some concrete state of the particles, but it is hidden. John Bell demonstrated that in order for a hidden variable theory to make predictions in agreement with quantum mechanics, it must have nonlocal interactions, which means any workable hidden variable theory must also be pretty spooky. https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem
phkahler 6y ago
>> Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light? Because physicists can not tell the difference between a particle whose "wave function collapsed" and one that didn't. The faster than light communication would be equivalent to modulating the collapsedness of a stream of particles by measuring-or-not their entangled counterparts. Since the is no discernable difference between particles pre and post collapse, no information can be transmitted.
GoblinSlayer 6y ago
Entanglement works like synchronized clocks. You can't send information, but you can coordinate actions to a similar effect. Much like you can coordinate actions of separate groups using synchronized clocks with no communication except for passing time.
- tabtab 6y ago
  That's the "hidden local variables" theory. Experiments seem to suggest it's wrong. How one pair is observed appears to affect (change) the other instantaneously. But, not in a way we can use to our advantage to get instant communication.
  - GoblinSlayer 6y ago
    Exact mechanism is different from synchronized clocks, it's an illustration how you can have correlation without communication and how you can use it in practice, an this correlation works with both entanglement and synchronized clocks.
    - tabtab 6y ago
      It seems some aspects can be explained by (modelled as) synchronization, but not all. "Spooky action at a distance" resembling faster-than-light communication still exists. The two top explanations are either some info can travel faster than light (from our perspective), or the universe forks into copies when needed.
Gunax 6y ago
This is my favourite explaination. I have had this bookmarked for over 10 years and keep coming back to it: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html
lisper 6y ago
This is a Google Tech Talk I gave about this about 8 years ago: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc It is based on this (unpublished) paper: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf The key insight is that measurement and entanglement are actually the same physical phenomenon. A measurement is nothing more than a very large network of mutual entanglements.
deleted 6y ago
[deleted]
8note 6y ago
you have to send classical information to get useful stuff out of the entanglement like quantum teleportation. The simplest one is that you need to be able to tell the partner when you've made your changes and that you're ready for them to measure

by qq12as 6y ago

EliRivers 6y ago

When the person at the other end looks at their particle, and sees it either 1 or 0 and has no other information, how do they know if you've looked at your particle or not?

deleted 6y ago
[deleted]
fjfaase 6y ago
And even if they would know, due to the distance and general relativity, there is no concept of one of the two persons looking first. It could be that from Alice's point of view Bob looked later, while from Bob's point of view Alice looked later. So, in which direction did the information go?

mppm 6y ago

Initially, your particle in Australia and its entangled twin in the USA exist in a superposition of 0 and 1. When you "measure" the state of your particle, you force it to assume a definite state, and entanglement forces the other particle to assume e.g. the opposite definite state ("spooky action at a distance"). This allows you to synchronize information across large distances, but you cannot send anything, because you cannot chose the outcome of the quantum measurement.

willis936 6y ago
If you can guarantee a random distribution of the color of the balls, it seems like a decent way to handle secure key echange.
- mppm 6y ago
  Yes, that is in fact one of the practical ways to do quantum key distribution.
teekert 6y ago
What if we manage to get our whole world into a superposition and only collapse to the world where the particles did contain the message we wanted? Sorry... I've been reading a lot of Greg Egan lately ;)

317070 6y ago

I have a bag with two balls, one red, one blue. I randomly send one of the balls to Australia, one of the balls to the USA. The moment you look at your ball in Australia, you also know the color of the ball in the USA. How would you use that fact to send information? The crux with quantum mechanics is that you can show (very easily, understandable by the layman, look up the Bell inequality) that in the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well. However, the basic setup still applies. You cannot send information by merely observing something.

smusamashah 6y ago
How does a ball know that it has been observed and in effect changing the color of the other paired ball?
- EliRivers 6y ago
  That's the "spooky action at a distance" (in the case of the entangled particles, obviously; with the two balls it's not so spooky). Spooky.
  - nemothekid 6y ago
    Maybe this is a basic question - but what I don’t understand is why this is called “spooky” action. My intuition is you have two particles, and you don’t know what concrete states they are in, but you know all possible states (that may be represented as some sort of system of equations). By observing a single particle you unlock a variable in that system of equations and can therefore solve the whole thing. To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed. The concept of superposition seems like an overly complex description for this phenomenon. I understand my view is wrong, but I don’t understand how I’m wrong
    - rrss 6y ago
      What you are describing is a hidden variable theory - i.e. there is some concrete state of the particles, but it is hidden. John Bell demonstrated that in order for a hidden variable theory to make predictions in agreement with quantum mechanics, it must have nonlocal interactions, which means any workable hidden variable theory must also be pretty spooky. https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem

phkahler 6y ago

>> Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light? Because physicists can not tell the difference between a particle whose "wave function collapsed" and one that didn't. The faster than light communication would be equivalent to modulating the collapsedness of a stream of particles by measuring-or-not their entangled counterparts. Since the is no discernable difference between particles pre and post collapse, no information can be transmitted.

GoblinSlayer 6y ago

Entanglement works like synchronized clocks. You can't send information, but you can coordinate actions to a similar effect. Much like you can coordinate actions of separate groups using synchronized clocks with no communication except for passing time.

tabtab 6y ago
That's the "hidden local variables" theory. Experiments seem to suggest it's wrong. How one pair is observed appears to affect (change) the other instantaneously. But, not in a way we can use to our advantage to get instant communication.
- GoblinSlayer 6y ago
  Exact mechanism is different from synchronized clocks, it's an illustration how you can have correlation without communication and how you can use it in practice, an this correlation works with both entanglement and synchronized clocks.
  - tabtab 6y ago
    It seems some aspects can be explained by (modelled as) synchronization, but not all. "Spooky action at a distance" resembling faster-than-light communication still exists. The two top explanations are either some info can travel faster than light (from our perspective), or the universe forks into copies when needed.

Gunax 6y ago

This is my favourite explaination. I have had this bookmarked for over 10 years and keep coming back to it: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html

lisper 6y ago

This is a Google Tech Talk I gave about this about 8 years ago: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc It is based on this (unpublished) paper: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf The key insight is that measurement and entanglement are actually the same physical phenomenon. A measurement is nothing more than a very large network of mutual entanglements.

deleted 6y ago

[deleted]

8note 6y ago

you have to send classical information to get useful stuff out of the entanglement like quantum teleportation. The simplest one is that you need to be able to tell the partner when you've made your changes and that you're ready for them to measure