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Quantum wave functions represent a probability distribution for the characteristics of an object in quantum mechanics, such as a particle or photon. When you me

by simonh 4y ago

Quantum wave functions represent a probability distribution for the characteristics of an object in quantum mechanics, such as a particle or photon. When you measure it, that probability distribution resolves to a discrete value. So basically when you measure the spin of a particle, god rolls some dice to determine what it is. That’s one interpretation anyway.

dclowd9901 4y ago
So you’re talking about the collapse of the wave function. If I collapse the wave function on one entangled particle, is the other entangled particle collapsed as well?
- andreareina 4y ago
  Experiments upholding Bell's inequalities imply it is.
  - dclowd9901 4y ago
    So couldn’t you send a bunch of entangled particles somewhere and collapse the wave function on them in sequence in some way as to communicate faster than the speed of light?
    - roywiggins 4y ago
      All you can do with your half of the pair is observe its state after the wave function collapsed. Whether it collapsed because you just poked it or because the other half of the pair was poked first can't be known without comparing notes afterward using classical means. https://en.m.wikipedia.org/wiki/No-communication_theorem https://en.m.wikipedia.org/wiki/No-communication_theorem
      - GoblinSlayer 4y ago
        An interferometer can differentiate that. If a photon is in superposition, it exits interferometer on a certain path, if it's collapsed, it exits on a random path. If you send 100 such photons, you can send 1 bit this way with good precision.
        Workaccount2 4y ago
        Although not informed enough to comment on this, I do know that if it was true, it would break relativity and that would have come across my feed at some point. So I would say you are missing something.
        GoblinSlayer 4y ago
        I made it up. Any nonlocality breaks relativity, this doesn't break it further. You can still say the same thing - it's okay because why not. Or you mean it will be a paradox if they receive a message in the past? I guess it can be prevented if the entangled photon doesn't interfere with itself.
        Workaccount2 4y ago
        Information cannot move faster than the speed of light. If the non-locality has a random output, then it doesn't violate relativity because random noise cannot carry information (by definition it's no longer random if it does).
        GoblinSlayer 4y ago
        That's the trick - to replace random process with deterministic process which is still sensitive to collapse. Or rather make collapse switch from deterministic to random process.
        Workaccount2 4y ago
        Whether or not the photon is collapsed isn't the process that makes entangled particles "spooky" though. (Sorry I should have originally stated this) Its that they have opposite states when you collapse them. So collapsing one doesn't collapse the other. Its just that when you collapse one (by measuring it) you can know the state of the other (even if it's on the other side of the universe). The real "spookiness" is that the particles don't have a predetermined state "underneath" their superposition. When you collapse one its odds of being heads or tails are 50/50 up to the moment you collapse it. And when you collapse it you know what reading you will get when you collapse the other one.
    - cycomanic 4y ago
      No, because the only way to determine "who collapsed the wavefunction first" is to communicate subluminal. In other words you can't detect the event of the wavefunction collapsing.
    - dreamcompiler 4y ago
      Think of the two entangled particles as telegraph operators that can communicate with each other infinitely fast -- the speed of light is no obstacle for them. But you cannot hand one of them a message to be transmitted because they simply ignore you.* You can ask either of them if they recently sent or received a message from the other, and what that message was, and they will tell you the truth. But they will only tell you this over subluminal channels, and they will not accept a message to be transmitted superluminally. We can prove they are communicating superluminally because of the Bell inequality, but we cannot control what they are communicating. *More precisely, the act of handing one of them a message causes that one to communicate some random message to the other.
      - dclowd9901 4y ago
        Thanks for the explanation, I think I understand better the underlying theory

by simonh 4y ago

dclowd9901 4y ago

So you’re talking about the collapse of the wave function. If I collapse the wave function on one entangled particle, is the other entangled particle collapsed as well?

andreareina 4y ago
Experiments upholding Bell's inequalities imply it is.
- dclowd9901 4y ago
  So couldn’t you send a bunch of entangled particles somewhere and collapse the wave function on them in sequence in some way as to communicate faster than the speed of light?
  - roywiggins 4y ago
    All you can do with your half of the pair is observe its state after the wave function collapsed. Whether it collapsed because you just poked it or because the other half of the pair was poked first can't be known without comparing notes afterward using classical means. https://en.m.wikipedia.org/wiki/No-communication_theorem https://en.m.wikipedia.org/wiki/No-communication_theorem
    - GoblinSlayer 4y ago
      An interferometer can differentiate that. If a photon is in superposition, it exits interferometer on a certain path, if it's collapsed, it exits on a random path. If you send 100 such photons, you can send 1 bit this way with good precision.
      - Workaccount2 4y ago
        Although not informed enough to comment on this, I do know that if it was true, it would break relativity and that would have come across my feed at some point. So I would say you are missing something.
        GoblinSlayer 4y ago
        I made it up. Any nonlocality breaks relativity, this doesn't break it further. You can still say the same thing - it's okay because why not. Or you mean it will be a paradox if they receive a message in the past? I guess it can be prevented if the entangled photon doesn't interfere with itself.
        Workaccount2 4y ago
        Information cannot move faster than the speed of light. If the non-locality has a random output, then it doesn't violate relativity because random noise cannot carry information (by definition it's no longer random if it does).
        GoblinSlayer 4y ago
        That's the trick - to replace random process with deterministic process which is still sensitive to collapse. Or rather make collapse switch from deterministic to random process.
        Workaccount2 4y ago
        Whether or not the photon is collapsed isn't the process that makes entangled particles "spooky" though. (Sorry I should have originally stated this) Its that they have opposite states when you collapse them. So collapsing one doesn't collapse the other. Its just that when you collapse one (by measuring it) you can know the state of the other (even if it's on the other side of the universe). The real "spookiness" is that the particles don't have a predetermined state "underneath" their superposition. When you collapse one its odds of being heads or tails are 50/50 up to the moment you collapse it. And when you collapse it you know what reading you will get when you collapse the other one.
  - cycomanic 4y ago
    No, because the only way to determine "who collapsed the wavefunction first" is to communicate subluminal. In other words you can't detect the event of the wavefunction collapsing.
  - dreamcompiler 4y ago
    Think of the two entangled particles as telegraph operators that can communicate with each other infinitely fast -- the speed of light is no obstacle for them. But you cannot hand one of them a message to be transmitted because they simply ignore you.* You can ask either of them if they recently sent or received a message from the other, and what that message was, and they will tell you the truth. But they will only tell you this over subluminal channels, and they will not accept a message to be transmitted superluminally. We can prove they are communicating superluminally because of the Bell inequality, but we cannot control what they are communicating. *More precisely, the act of handing one of them a message causes that one to communicate some random message to the other.
    - dclowd9901 4y ago
      Thanks for the explanation, I think I understand better the underlying theory