4 ms·

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 simp

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
  Thanks for the reply. I've been doing well with understanding the implications of the test and the after effects but I struggled with understanding the state of the two particles themselves. Namely, the trouble was with understanding that no one but the people who entangled the particles are actually aware that they are entangled. There is no way for a stranger to look at two particles (given they were entangled by someone else) that those particles are entangled. They'll look like two normal particles. The answers here have helped me understand this.
  - tim333 11y ago
    I think all particles are basically entangled. When you get a photon from the sun it's probability of it being in a given state depends to an extent on the state of all the other photons coming out of the sun. But it's too complicated to figure and so appears random. It's only in simple experiments where exactly two particles are entangled and don't interact with others that you can figure out and notice the effects.

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
Thanks for the reply. I've been doing well with understanding the implications of the test and the after effects but I struggled with understanding the state of the two particles themselves. Namely, the trouble was with understanding that no one but the people who entangled the particles are actually aware that they are entangled. There is no way for a stranger to look at two particles (given they were entangled by someone else) that those particles are entangled. They'll look like two normal particles. The answers here have helped me understand this.
- tim333 11y ago
  I think all particles are basically entangled. When you get a photon from the sun it's probability of it being in a given state depends to an extent on the state of all the other photons coming out of the sun. But it's too complicated to figure and so appears random. It's only in simple experiments where exactly two particles are entangled and don't interact with others that you can figure out and notice the effects.