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> What causes it to occur or not occur? Measurements cause it to occur. What causes entanglement to occur? > quantum eraser Not sure how it works, but Wikipe

• lmm 7y ago
  > Measurements cause it to occur. Ok, but what's the objective physical distinction between a measurement and a non-measurement? > What causes entanglement to occur? Various specific kinds of normal physical interactions, the details of which are an established part of QM. > After the measurement I’m not in a superposition of the two entangled states but in one of the two What would you expect to be different if you were in a superposition of two states? > I do observe the photon in a particular spin state Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states.
  • edna314 7y ago
    > Ok, but what's the objective physical distinction between a measurement and a non-measurement? Various specific kinds of measuring devices the details of which are an established part of QM. > Various specific kinds of normal physical interactions, the details of which are an established part of QM. So, which is the interaction that causes entanglement? I was asking for this before. > What would you expect to be different if you were in a superposition of two states? Well, I would expect my measurement to also have all possible outcomes given by the superposition. > Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states. Only after a one of the entangled particles is measured. Either my brain must be measured or the result must be measured. In this sense entanglement doesn’t work without an ill defined concept of measurement either.
    • lmm 7y ago
      > Various specific kinds of measuring devices the details of which are an established part of QM. They're not though. There's no generally accepted definition of what is and isn't a measuring device. And, as per the quantum eraser experiment, the exact same equipment might be considered as a measuring device or not a measuring device, depending on what happens in the future. > So, which is the interaction that causes entanglement? I was asking for this before. The only fully accurate answer is "look at the Schrodinger equation". But, broadly, interacting with an object in superposition in a way that depends on that superposition will cause entanglement. For example, if a particle's spin is in superposition, another particle interacting it in a spin-dependent way will cause entanglement, but interacting with it in a spin-independent way will not create an entanglement. I appreciate that this must sound exactly as vague as the definition of a measurement. But for those familiar with QM it really isn't. If you look at textbooks, even those written from a Copenhagenist point of view, they're very clear on which physical circumstances give rise to entanglement and which don't. If you go through any undergrad-level QM textbook you'll have a clear understanding of what entanglement is and isn't. > Well, I would expect my measurement to also have all possible outcomes given by the superposition. Ok, but what would the subjective experience of that look like? > Only after a one of the entangled particles is measured. Either my brain must be measured or the result must be measured. In this sense entanglement doesn’t work without an ill defined concept of measurement either. Not true. Just by looking at the wavefunction, one can see that the wavefunction is a superposition of two distinct Everett branches that don't interact with each other - not because of some mysterious "collapse" phenomenon, but just as an emergent property of that actual wavefunction. Finding yourself in one specific branch is somewhat mysterious, but each individual branch being consistent with itself and not interacting with any other branch is absolutely normal QM.
      • edna314 7y ago
        > But, broadly, interacting with an object in superposition in a way that depends on that superposition will cause entanglement. This doesn't explain the cause of entanglement. It just explains the circumstances under which entanglement occurs. > I appreciate that this must sound exactly as vague as the definition of a measurement That's because it is beyond the mathematics. > But for those familiar with QM it really isn't. This is not true. I'm familiar with QM and it is vague to me. >they're very clear on which physical circumstances give rise to entanglement But, none explain the cause of entanglement. Just as none explains the cause of collapse of a wave function which is called a measurement. > Ok, but what would the subjective experience of that look like? I wouldn't know, because it's not what can be experienced. Some folks argue this is exactly what makes many worlds unscientific. > Just by looking at the wavefunction, Interesting. What kind of microscope are you using to look at wave functions? Or do you mean at the mathematical expression of the wavefunction? I'm not arguing that decorherence theories are mathematically sound and a neat theory. I'm just challenging that they offer some physical explanations beyond Copenhagen. They just take the unexplained things and replace them with other unexplained things. > emergent property Another empty word. > Finding yourself in one specific branch is somewhat mysterious Yes. And if you think about it, this is the same mystery which makes Copenhagen so unsatisfactory, just put into a different framework.