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It's almost impossible to answer that question. Which is one motivation behind hidden variables theories of Quantum Theory that don't elevate observation to hav

It's almost impossible to answer that question. Which is one motivation behind hidden variables theories of Quantum Theory that don't elevate observation to having a role in quantum phenomena.

deleted 9y ago
[deleted]
lisper 9y ago
To the contrary, it is very easy to answer: measurement does not cause the collapse of the wave function. This is a reasonable approximation to the truth in many common situations, but it is not the truth. The truth is that measurement and entanglement are the same physical phenomenon. See: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
- meri_dian 9y ago
  Thanks for posting that. Can you reconcile these two parts for me though: "It accounts for the apparent contradiction between quantum theory, which says that entropy is conserved in unitary transformations, and the apparent increase in entropy that arises from the randomness in quantum measurements." And "randomness is not an essential cornerstone of quantum measurement but rather an illusion created by it." So is it random or is it not, in your view?
  - lisper 9y ago
    > So is it random or is it not, in your view? It depends on your perspective. From the perspective of a classical entity, yes, it's random. From the perspective of the quantum wave function, no, it's not.
- 77pt77 9y ago
  > or the video version: Both links are identical!
  - lisper 9y ago
    Oops, sorry. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
- kgwgk 9y ago
  > measurement does not cause the collapse of the wave function. But you provide an example of measurement causing the collapse of the wave function yourself! "Now let us add a detector at the slits to determine which way the photon went. To describe this situation mathematically we have to add a description of the state of the detector: (ΨU |DU> + ΨL |DL>)/√2 where |DU> is the state of the detector when it has detected a photon at the upper slit and |DL> is the state of the detector when it has detected a photon at the lower slit. Now the probability density is: [|ΨU |^2 + |ΨL|^2 + (ΨUΨL <DU|DL> + ΨLΨU <DL|DU>)]/2" That describes the quantum system formed by the photon and the detector. And when you reduce the density matrix to its diagonal, setting the interference terms to zero in a non-unitary way, you are collapsing it to a "classical" mixture of two states corresponding to the possible outcomes of the measurement (as described by von Neumann 85 years ago).
  - lisper 9y ago
    > when you reduce the density matrix to its diagonal, setting the interference terms to zero in a non-unitary way, you are collapsing it to a "classical" mixture of two states corresponding to the possible outcomes of the measurement (as described by von Neumann 85 years ago). Yes, that's true, but that is not the same thing as "measurement causing the collapse of the wave function". Collapse is not a physical phenomenon, it a a mathematical approximation. When you measure a particle, nothing happens to that particle as a result of the measurement. It does not suddenly "decide" to go through one slit or the other. Anything that "happened" to the particle happened not as a result of the measurement but as a result of the initial entanglement with the ancilla.
    - kgwgk 9y ago
      You had a wave function for the particle/detector system, you performed a measurement, then you didn’t have any longer the wave function that you previously had. I think that’s what von Neumann would call “measurement causing the collapse of the wave function”.
      - lisper 9y ago

It's almost impossible to answer that question. Which is one motivation behind hidden variables theories of Quantum Theory that don't elevate observation to having a role in quantum phenomena.

deleted 9y ago

[deleted]

lisper 9y ago

To the contrary, it is very easy to answer: measurement does not cause the collapse of the wave function. This is a reasonable approximation to the truth in many common situations, but it is not the truth. The truth is that measurement and entanglement are the same physical phenomenon. See: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf

meri_dian 9y ago
Thanks for posting that. Can you reconcile these two parts for me though: "It accounts for the apparent contradiction between quantum theory, which says that entropy is conserved in unitary transformations, and the apparent increase in entropy that arises from the randomness in quantum measurements." And "randomness is not an essential cornerstone of quantum measurement but rather an illusion created by it." So is it random or is it not, in your view?
- lisper 9y ago
  > So is it random or is it not, in your view? It depends on your perspective. From the perspective of a classical entity, yes, it's random. From the perspective of the quantum wave function, no, it's not.
77pt77 9y ago
> or the video version: Both links are identical!
- lisper 9y ago
  Oops, sorry. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
kgwgk 9y ago
> measurement does not cause the collapse of the wave function. But you provide an example of measurement causing the collapse of the wave function yourself! "Now let us add a detector at the slits to determine which way the photon went. To describe this situation mathematically we have to add a description of the state of the detector: (ΨU |DU> + ΨL |DL>)/√2 where |DU> is the state of the detector when it has detected a photon at the upper slit and |DL> is the state of the detector when it has detected a photon at the lower slit. Now the probability density is: [|ΨU |^2 + |ΨL|^2 + (ΨUΨL <DU|DL> + ΨLΨU <DL|DU>)]/2" That describes the quantum system formed by the photon and the detector. And when you reduce the density matrix to its diagonal, setting the interference terms to zero in a non-unitary way, you are collapsing it to a "classical" mixture of two states corresponding to the possible outcomes of the measurement (as described by von Neumann 85 years ago).
- lisper 9y ago
  > when you reduce the density matrix to its diagonal, setting the interference terms to zero in a non-unitary way, you are collapsing it to a "classical" mixture of two states corresponding to the possible outcomes of the measurement (as described by von Neumann 85 years ago). Yes, that's true, but that is not the same thing as "measurement causing the collapse of the wave function". Collapse is not a physical phenomenon, it a a mathematical approximation. When you measure a particle, nothing happens to that particle as a result of the measurement. It does not suddenly "decide" to go through one slit or the other. Anything that "happened" to the particle happened not as a result of the measurement but as a result of the initial entanglement with the ancilla.
  - kgwgk 9y ago
    You had a wave function for the particle/detector system, you performed a measurement, then you didn’t have any longer the wave function that you previously had. I think that’s what von Neumann would call “measurement causing the collapse of the wave function”.
    - lisper 9y ago