51 ms·

It is late - but while watching an episode of spacetime+ the host stated an entangled pair could be millions of miles apart and one could immediately cause a ch

by czbond 5y ago

It is late - but while watching an episode of spacetime+ the host stated an entangled pair could be millions of miles apart and one could immediately cause a change in the other. This would be faster than light

ackbar03 5y ago
As an arm-chair quantum physicist, I found this article helpful https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light-communication/?sh=1826d6ee3a1e https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-r... If the entangled pair is measured precisely at the same time, both parties get the same value instantly. However this fact itself does not allow for actual communication/information passing. For that there needs to be a way to actually control the state. Or something like that Lol
- dfasdf1231 5y ago
  Lets say I'm in NY, now I go to ask to join a cooking course, they say me: "we will let you know if you're accepted or not". Then I take a plane to the other side of the planet, Japan. We have some pending information, a link between the cooking course acceptation and me. The link will keep existing even if I take a trip to Mars. Then I got accepted in the cooking course, the information about the acceptance changes instantaneously in the whole universe, including in my head (currently in Japan, thousands of miles away from the place when the cooking course decision to accept me occurs). Is this quantum entanglement somehow? It seems to check all the boxes, including that the information changes instantaneously in both sides (I got accepted! even if now I'm in Japan), but I wouldn't know about it till they send me the news using a standard non-quantum channel. FTL comms aren't possible at all here, etc. Then I ask, entanglement occurs continously at levels in universe, when two entities somehow get to share a question, a pending information? Of course I think I'm missing something (a degree or PhD in physics for starters), that's why I'm asking. Thanks in advance.
roywiggins 5y ago
It doesn't work like that: you can only measure the state, not set it. Measuring the state does change something (it collapses the wave function, in the Copenhagen interpetation) but not in a way that's measurable by the person at the other end. You can't tell if the wave function collapsed just by measuring one of the entangled particles. There's a theorem in QM the guarantees that you can't use measurements on entangled systems to communicate: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
- ackbar03 5y ago
  Since you clearly know more than me what your talking about, is it correct to interpret the significance of china's quantum satellite communication as one of quantum cryptography (based on states changing post measurement) rather than potential high speed communication?
  - roywiggins 5y ago
    There's quantum superdense coding, which in theory could allow higher bandwidth (transmitting one qbit can be used to transmit two classical bits) but I believe the main point of quantum channels is for cryptography. If entanglement allowed FTL communication you'd see high-frequency traders already using it!
giantrobot 5y ago
That's an incorrect explanation. I find the envelope analogy to be the clearest. I have two cards, a red card and a blue card. I have only one of each. I put each into a totally opaque envelope. I have created an entangled pair. I give you an envelope and you pay SpaceX a few million dollars for a trip to Mars. You have an equal probability of having a red or blue card in your envelope. Once you settle in on Mars and recover a bit from bone and muscle density loss from six months in zero-g you open your envelope. You have the red card. You've just measured the state of the entangled pair. You instantly know I have the blue card but your measurement didn't cause some FTL communication. Because our entangled pair had exclusive states you gain some instant knowledge of my envelope contents. While that's a cool effect it still required classical slower-than-light transmission of the envelope. I also have no knowledge that you opened your envelope until I receive your classical communication saying so. Even if you were just across the room the information about the color of your card would still only travel at the speed of light. Even though you could instantly know the state of my card once you know yours you can only communicate that knowledge to me at the speed of light.
- czbond 5y ago
  That is a clear explanation - thank you. And I probably misunderstood the exact Spacetime semantics of the situation. Thank you

by czbond 5y ago

ackbar03 5y ago

As an arm-chair quantum physicist, I found this article helpful https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light-communication/?sh=1826d6ee3a1e https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-r... If the entangled pair is measured precisely at the same time, both parties get the same value instantly. However this fact itself does not allow for actual communication/information passing. For that there needs to be a way to actually control the state. Or something like that Lol

dfasdf1231 5y ago
Lets say I'm in NY, now I go to ask to join a cooking course, they say me: "we will let you know if you're accepted or not". Then I take a plane to the other side of the planet, Japan. We have some pending information, a link between the cooking course acceptation and me. The link will keep existing even if I take a trip to Mars. Then I got accepted in the cooking course, the information about the acceptance changes instantaneously in the whole universe, including in my head (currently in Japan, thousands of miles away from the place when the cooking course decision to accept me occurs). Is this quantum entanglement somehow? It seems to check all the boxes, including that the information changes instantaneously in both sides (I got accepted! even if now I'm in Japan), but I wouldn't know about it till they send me the news using a standard non-quantum channel. FTL comms aren't possible at all here, etc. Then I ask, entanglement occurs continously at levels in universe, when two entities somehow get to share a question, a pending information? Of course I think I'm missing something (a degree or PhD in physics for starters), that's why I'm asking. Thanks in advance.

roywiggins 5y ago

It doesn't work like that: you can only measure the state, not set it. Measuring the state does change something (it collapses the wave function, in the Copenhagen interpetation) but not in a way that's measurable by the person at the other end. You can't tell if the wave function collapsed just by measuring one of the entangled particles. There's a theorem in QM the guarantees that you can't use measurements on entangled systems to communicate: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem

ackbar03 5y ago
Since you clearly know more than me what your talking about, is it correct to interpret the significance of china's quantum satellite communication as one of quantum cryptography (based on states changing post measurement) rather than potential high speed communication?
- roywiggins 5y ago
  There's quantum superdense coding, which in theory could allow higher bandwidth (transmitting one qbit can be used to transmit two classical bits) but I believe the main point of quantum channels is for cryptography. If entanglement allowed FTL communication you'd see high-frequency traders already using it!

giantrobot 5y ago

That's an incorrect explanation. I find the envelope analogy to be the clearest. I have two cards, a red card and a blue card. I have only one of each. I put each into a totally opaque envelope. I have created an entangled pair. I give you an envelope and you pay SpaceX a few million dollars for a trip to Mars. You have an equal probability of having a red or blue card in your envelope. Once you settle in on Mars and recover a bit from bone and muscle density loss from six months in zero-g you open your envelope. You have the red card. You've just measured the state of the entangled pair. You instantly know I have the blue card but your measurement didn't cause some FTL communication. Because our entangled pair had exclusive states you gain some instant knowledge of my envelope contents. While that's a cool effect it still required classical slower-than-light transmission of the envelope. I also have no knowledge that you opened your envelope until I receive your classical communication saying so. Even if you were just across the room the information about the color of your card would still only travel at the speed of light. Even though you could instantly know the state of my card once you know yours you can only communicate that knowledge to me at the speed of light.

czbond 5y ago
That is a clear explanation - thank you. And I probably misunderstood the exact Spacetime semantics of the situation. Thank you