7 ms·

One think I've been wondering about spooky action is if entanglement reactions are bound to the speed of light. As in, if one particle of the entangled pair is

One think I've been wondering about spooky action is if entanglement reactions are bound to the speed of light. As in, if one particle of the entangled pair is manipulated, is there a "signal" that is "transmitted" at some speed? Or is it "instant" in the sense that it transcends lightspeed? Or does this question even make sense in this context? Quantum stuff gets weird fast since we have very little basis in which to intuit it.

danbruc 10y ago
It is instantly but you can not use it to transmit information. Say you have a pair of entangled spins in a superposition of the two anti-parallel states first spin up and second spin down or first spin down and second spin up with equal probability for both states, i.e. each spin is up or down with a probability of 50 % but the two spins always have opposing orientations. If you measure one of the spins, then there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured because the result of the measurement somehow had not yet enough time to reached the second spin, supposedly because this state change is limited to travel at the speed of light. It is as if the spins had secretly picked on of the two possible outcomes when they were created but you just do not know which one until you perform a measurement. But this is not the case, this is what is called a local hidden variable theory and is ruled out by Bell test experiments. The spins have no definite orientation until you measure the first one but after the first measurement both spins instantaneously have a definit orientation no matter how far separated.
- platz 10y ago
  a side note, there isn't a universal "spin orientation detector" that reveals a spin orientation on the unit circle. You have to choose how to align the magnets so that the spin measured is either "up" or "down"; you could align the magnets any way you like, but they are fixed along an axis for the duration of the observation
- tedsanders 10y ago
  There is no proof or strong evidence that this occurs instantly. Some interpretations of quantum mechanics still satisfy locality. https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_mechanics https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m...
  - danbruc 10y ago
    One has to be careful what one means here. I meant it in the following specific sense and we have good experimental evidence for this. [...] there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured [...] If an interpretation makes use of non-locality to explain this fact is a different matter, but experimentally it looks exactly like measurements instantaneously affecting entangled partners.
    - deleted 10y ago
      [deleted]
btilly 10y ago
The idea works like this. A pair of particles is entangled then separated to positions L and L'. At L we can observe A or B. At L' we can observe A' or B'. Until the moment of observation, it is indeterminate which. L and L' are too far apart and close in time for information to be communicated by light. The point of the EPR experiment is that when we observe A at L we will observe A' at L'. And when we observe B at L we will observe B at B'. In other words "collapse is instantaneous". In the many worlds interpretation, nothing is actually happening faster than light. At L we split into an A and a B observer. At L' we split into an A' and a B' observer. And then when those observers meet up, QM just describes how those observers match up to generate two consistent stories. However this is a very, very important for theoretical physics. General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them.
- danbruc 10y ago
  General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them. But to add, there are at least ideas, it is not the case that we made no progress at all. I would like to highlight the ER=EPR conjecture [1] by Leonard Susskind and Juan Maldacena. It roughly states that entangled particles are connected by worm holes and that they are therefore not actually separated in space. Personally I prefer a less sensationalist sounding description, that entanglement defines what it means for points in space to be close together or far apart. This avoids constantly visualizing pairs of particle connected by glowing worm holes from science fiction movies. [1] https://en.wikipedia.org/wiki/ER%3DEPR https://en.wikipedia.org/wiki/ER%3DEPR
  - intrasight 10y ago
    You don't need a wormhole, as there are no particles, and therefore nothing is "separated".
    - danbruc 10y ago
      I have no idea what you mean, there are obviously particles, we have an entire standard model full of them.
cgc0 10y ago
Although I'll certainly butcher the phrasing, my understanding is: measuring a property of one of the entangled particles lets you know the same property of the other particle (an experiment showed this happens faster than the speed of light by measuring the two particles when they were 1.3km apart). This is particularly mysterious because it seems that the property of the particle is not determined before this measurement. Here are some quotes from a piece on the experiment I mentioned above. from https://www.sciencenews.org/blog/context/entanglement-spooky-not-action-distance https://www.sciencenews.org/blog/context/entanglement-spooky... > Einstein insisted that the handedness of the glove must be determined in advance by some physical law. He was perplexed by the possibility that Alice’s choice of hand could have something to do with whether Bob’s glove would fit his fingers. > Suppose you prepared entangled photons and sent them to Alice and Bob in such a way that if Alice measured hers to be vertically polarized, she instantly knows that Bob’s will be horizontally polarized. . . . > . . . The photon does not have an orientation until Alice detects it. Same for Bob’s. But once Alice makes a measurement, the outcome of Bob’s measurement is certain. > Here’s where a lot of confusion clouds entanglement commentary. Contrary to what you might have read in a magazine with “New” and “Yorker” in the title, Alice’s measurement does not “instantaneously” influence Bob’s photon. No signal is sent, no influence transmitted. For all Alice knows, Bob might have measured his photon first. In fact, if the measurements are made at nearly the same time, there might be no objective way to say who made the first measurement. (A space traveler flying along at nearly the speed of light might see Bob’s measurement first, while another traveler flying in a different direction would see Alice’s first.)
return0 10y ago
They are not bound by speed of light. Nothing is exchanged or transmitted, the collapse of wavefunction is genuinely instant in both places. The drawback is that you cannot choose or predict which outcome you will measure, so you can't really use it to transmit information instantly.

danbruc 10y ago

It is instantly but you can not use it to transmit information. Say you have a pair of entangled spins in a superposition of the two anti-parallel states first spin up and second spin down or first spin down and second spin up with equal probability for both states, i.e. each spin is up or down with a probability of 50 % but the two spins always have opposing orientations. If you measure one of the spins, then there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured because the result of the measurement somehow had not yet enough time to reached the second spin, supposedly because this state change is limited to travel at the speed of light. It is as if the spins had secretly picked on of the two possible outcomes when they were created but you just do not know which one until you perform a measurement. But this is not the case, this is what is called a local hidden variable theory and is ruled out by Bell test experiments. The spins have no definite orientation until you measure the first one but after the first measurement both spins instantaneously have a definit orientation no matter how far separated.

platz 10y ago
a side note, there isn't a universal "spin orientation detector" that reveals a spin orientation on the unit circle. You have to choose how to align the magnets so that the spin measured is either "up" or "down"; you could align the magnets any way you like, but they are fixed along an axis for the duration of the observation
tedsanders 10y ago
There is no proof or strong evidence that this occurs instantly. Some interpretations of quantum mechanics still satisfy locality. https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_mechanics https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m...
- danbruc 10y ago
  One has to be careful what one means here. I meant it in the following specific sense and we have good experimental evidence for this. [...] there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured [...] If an interpretation makes use of non-locality to explain this fact is a different matter, but experimentally it looks exactly like measurements instantaneously affecting entangled partners.
  - deleted 10y ago
    [deleted]

btilly 10y ago

The idea works like this. A pair of particles is entangled then separated to positions L and L'. At L we can observe A or B. At L' we can observe A' or B'. Until the moment of observation, it is indeterminate which. L and L' are too far apart and close in time for information to be communicated by light. The point of the EPR experiment is that when we observe A at L we will observe A' at L'. And when we observe B at L we will observe B at B'. In other words "collapse is instantaneous". In the many worlds interpretation, nothing is actually happening faster than light. At L we split into an A and a B observer. At L' we split into an A' and a B' observer. And then when those observers meet up, QM just describes how those observers match up to generate two consistent stories. However this is a very, very important for theoretical physics. General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them.

danbruc 10y ago
General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them. But to add, there are at least ideas, it is not the case that we made no progress at all. I would like to highlight the ER=EPR conjecture [1] by Leonard Susskind and Juan Maldacena. It roughly states that entangled particles are connected by worm holes and that they are therefore not actually separated in space. Personally I prefer a less sensationalist sounding description, that entanglement defines what it means for points in space to be close together or far apart. This avoids constantly visualizing pairs of particle connected by glowing worm holes from science fiction movies. [1] https://en.wikipedia.org/wiki/ER%3DEPR https://en.wikipedia.org/wiki/ER%3DEPR
- intrasight 10y ago
  You don't need a wormhole, as there are no particles, and therefore nothing is "separated".
  - danbruc 10y ago
    I have no idea what you mean, there are obviously particles, we have an entire standard model full of them.

cgc0 10y ago

Although I'll certainly butcher the phrasing, my understanding is: measuring a property of one of the entangled particles lets you know the same property of the other particle (an experiment showed this happens faster than the speed of light by measuring the two particles when they were 1.3km apart). This is particularly mysterious because it seems that the property of the particle is not determined before this measurement. Here are some quotes from a piece on the experiment I mentioned above. from https://www.sciencenews.org/blog/context/entanglement-spooky-not-action-distance https://www.sciencenews.org/blog/context/entanglement-spooky... > Einstein insisted that the handedness of the glove must be determined in advance by some physical law. He was perplexed by the possibility that Alice’s choice of hand could have something to do with whether Bob’s glove would fit his fingers. > Suppose you prepared entangled photons and sent them to Alice and Bob in such a way that if Alice measured hers to be vertically polarized, she instantly knows that Bob’s will be horizontally polarized. . . . > . . . The photon does not have an orientation until Alice detects it. Same for Bob’s. But once Alice makes a measurement, the outcome of Bob’s measurement is certain. > Here’s where a lot of confusion clouds entanglement commentary. Contrary to what you might have read in a magazine with “New” and “Yorker” in the title, Alice’s measurement does not “instantaneously” influence Bob’s photon. No signal is sent, no influence transmitted. For all Alice knows, Bob might have measured his photon first. In fact, if the measurements are made at nearly the same time, there might be no objective way to say who made the first measurement. (A space traveler flying along at nearly the speed of light might see Bob’s measurement first, while another traveler flying in a different direction would see Alice’s first.)

return0 10y ago

They are not bound by speed of light. Nothing is exchanged or transmitted, the collapse of wavefunction is genuinely instant in both places. The drawback is that you cannot choose or predict which outcome you will measure, so you can't really use it to transmit information instantly.