5 ms·

I have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say t

I have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say that a select group of them were still entangled while others are not? Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles? *edit missed a word

ars 11y ago
No to both questions. Both would allow you to violate the speed of light if it was possible. Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle. Read also: https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem
- MichaelApproved 11y ago
  So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
  - ars 11y ago
    > So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? Correct. Entangled means these two particles have correlated measurements. If you don't have that second particle then there is nothing to correlate with. > This observer would never know it was in a superposition Superposition and entangled are two different things. A particle can be in a superposition and not be entangled. (But not the reverse.) > I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. Correct.
    - MichaelApproved 11y ago
      Thank you so much!
  - chriswarbo 11y ago
    > If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. Crucially, from an experimental point of view, we can tell whether a pair of particles was entangled by measuring both of them and comparing the results. To ensure that nothing else is messing with these results, eg. that there's no signal travelling between the particles, we can move them very far apart and arrange that each measurement takes place at almost the same time; that way, if any such signal existed, it wouldn't have time (even travelling at the speed of light) to allow the first measurement to influence the second measurement. Importantly, each person doing the individual measurements has no idea what the other person's result is. They only know what they themselves have measured. In order to compare the results, the experimenters must either meet to compare notes, or send a message (eg. via radio) telling the other. Hence, even if the entangled particles interact instantaneously, there's no way to use the result of a measurement without waiting for that subsequent radio message from your collaborator, and that message will only travel at the speed of light. The quantum interaction has not sent any information faster than light.
    - MichaelApproved 11y ago
- poelzi 11y ago
  theorems and bad assumptions. first: speed of light is derived from 2 energy states and affects ONLY matter with rest mass. Once you understand all the energies involved it becomes clear where the GR effect comes from. Einstein never said that speed of light is necessary for virtual particles. Photons are NOT particles but waves with boundary condition and therefor appear particle like. Photon entanglement is based on a string like (has 3 dimensional space of course) energy state in in our vaccum, but I can't explain here the BSM-SG photon model. Read it for yourself.

ars 11y ago

No to both questions. Both would allow you to violate the speed of light if it was possible. Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle. Read also: https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem

MichaelApproved 11y ago
So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
- ars 11y ago
  > So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? Correct. Entangled means these two particles have correlated measurements. If you don't have that second particle then there is nothing to correlate with. > This observer would never know it was in a superposition Superposition and entangled are two different things. A particle can be in a superposition and not be entangled. (But not the reverse.) > I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. Correct.
  - MichaelApproved 11y ago
    Thank you so much!
- chriswarbo 11y ago
  > If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. Crucially, from an experimental point of view, we can tell whether a pair of particles was entangled by measuring both of them and comparing the results. To ensure that nothing else is messing with these results, eg. that there's no signal travelling between the particles, we can move them very far apart and arrange that each measurement takes place at almost the same time; that way, if any such signal existed, it wouldn't have time (even travelling at the speed of light) to allow the first measurement to influence the second measurement. Importantly, each person doing the individual measurements has no idea what the other person's result is. They only know what they themselves have measured. In order to compare the results, the experimenters must either meet to compare notes, or send a message (eg. via radio) telling the other. Hence, even if the entangled particles interact instantaneously, there's no way to use the result of a measurement without waiting for that subsequent radio message from your collaborator, and that message will only travel at the speed of light. The quantum interaction has not sent any information faster than light.
  - MichaelApproved 11y ago
poelzi 11y ago
theorems and bad assumptions. first: speed of light is derived from 2 energy states and affects ONLY matter with rest mass. Once you understand all the energies involved it becomes clear where the GR effect comes from. Einstein never said that speed of light is necessary for virtual particles. Photons are NOT particles but waves with boundary condition and therefor appear particle like. Photon entanglement is based on a string like (has 3 dimensional space of course) energy state in in our vaccum, but I can't explain here the BSM-SG photon model. Read it for yourself.