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The basis of measurement is provided by method of measurement: if you measure coordinate, you measure in coordinate basis, if you measure energy, you measure in

The basis of measurement is provided by method of measurement: if you measure coordinate, you measure in coordinate basis, if you measure energy, you measure in energy basis. It's not specific to MWI.

tsimionescu 2y ago
That's OK for things like the Copenhagen Interpretation, which don't consider the wave function to be anything more than a method for computing probabilities of measurement. However, the MWI is claiming that the real world is described by the Universe wave function, and that classical properties we're used to seeing are merely a consequence of the decoherence of pure quantum states as they get entangled with the general environment. However, this process doesn't set any preferred basis. So this doesn't explain why all of the classical world appears to have definite properties in the same basis. That is, decoherence explains why we don't see phenomena like self-interference when throwing a basketball at a double slit experiment. But it doesn't explain why we can observe an electron in multiple places at once if we measure it in an energy basis, but we can never observe a basketball in multiple places at once if we measure it in an energy basis. How does the classical world's preferred basis emerge out of the wave function where any basis is just as valid? This is why, even with the MWI, the wave function alone is not a satisfactory explanation for the universe - it doesn't explain, without additional postulates, how the illusion of a classical world can form. Contrast this to other counter-intuitive theories like special relativity. Special relativity says that lengths contract in the direction of motion, which we don't observe in normal experiments such as when running really fast or even when driving in our car. But we have a good account of this: the contraction is so small that it just can't be noticed. So there is no real conflict between our most immediate perceptions and the theory, because the effects of the theory are just too small for us to perceive. But the effects that the wave function description of the universe predicts are actually massive: if basket balls could be in multiple places at once just as long as they have a definite kinetic energy level, then we would notice that very clearly.
- GoblinSlayer 2y ago
  It's up to theoretization, feels like classical physics happens due to fermions forming large spatial structures like molecules. It doesn't work as well for bosons: electromagnetic wave is as much as they can muster, and AFAIK it doesn't experience self-decoherence and is just a bunch of photons that happen to fly nearby. So without molecules there would be no classical physics, other things being equal. >So there is no real conflict between our most immediate perceptions and the theory, because the effects of the theory are just too small for us to perceive. Roger Penrose couldn't wrap his head around presentistic special relativity, Lorentz synchronism specifically, cf Andromeda paradox.
  - tsimionescu 2y ago
    I don't understand your first paragraph at all, so I can't answer it. For the observation on Roger Penrose, you've missed my point. Sure, the consequences of special relativity are highly unintuitive if you try to think about them. But it doesn't contradict any day to day observations you directly make with your senses: it accurately predicts the exact numbers your intuition (or Newtonian mechanics) would predict for day to day phenomena. The problems only appear for phenomena that you can't observe easily, like spaceships moving at significant fractions of the speed of light. I've never been on one of these, so it's not surprising that it might behave differently to what I would expect. In contrast, the Schrodinger equation predicts that if I throw a ball, it will end up in many different locations at once. This is obviously wrong. So, you need to introduce another postulates (the Born rule, and the concept of a measurement, or at least the concept of an environment) for QM to not be in stark conflict with our day to day experience.

tsimionescu 2y ago

That's OK for things like the Copenhagen Interpretation, which don't consider the wave function to be anything more than a method for computing probabilities of measurement. However, the MWI is claiming that the real world is described by the Universe wave function, and that classical properties we're used to seeing are merely a consequence of the decoherence of pure quantum states as they get entangled with the general environment. However, this process doesn't set any preferred basis. So this doesn't explain why all of the classical world appears to have definite properties in the same basis. That is, decoherence explains why we don't see phenomena like self-interference when throwing a basketball at a double slit experiment. But it doesn't explain why we can observe an electron in multiple places at once if we measure it in an energy basis, but we can never observe a basketball in multiple places at once if we measure it in an energy basis. How does the classical world's preferred basis emerge out of the wave function where any basis is just as valid? This is why, even with the MWI, the wave function alone is not a satisfactory explanation for the universe - it doesn't explain, without additional postulates, how the illusion of a classical world can form. Contrast this to other counter-intuitive theories like special relativity. Special relativity says that lengths contract in the direction of motion, which we don't observe in normal experiments such as when running really fast or even when driving in our car. But we have a good account of this: the contraction is so small that it just can't be noticed. So there is no real conflict between our most immediate perceptions and the theory, because the effects of the theory are just too small for us to perceive. But the effects that the wave function description of the universe predicts are actually massive: if basket balls could be in multiple places at once just as long as they have a definite kinetic energy level, then we would notice that very clearly.

GoblinSlayer 2y ago
It's up to theoretization, feels like classical physics happens due to fermions forming large spatial structures like molecules. It doesn't work as well for bosons: electromagnetic wave is as much as they can muster, and AFAIK it doesn't experience self-decoherence and is just a bunch of photons that happen to fly nearby. So without molecules there would be no classical physics, other things being equal. >So there is no real conflict between our most immediate perceptions and the theory, because the effects of the theory are just too small for us to perceive. Roger Penrose couldn't wrap his head around presentistic special relativity, Lorentz synchronism specifically, cf Andromeda paradox.
- tsimionescu 2y ago
  I don't understand your first paragraph at all, so I can't answer it. For the observation on Roger Penrose, you've missed my point. Sure, the consequences of special relativity are highly unintuitive if you try to think about them. But it doesn't contradict any day to day observations you directly make with your senses: it accurately predicts the exact numbers your intuition (or Newtonian mechanics) would predict for day to day phenomena. The problems only appear for phenomena that you can't observe easily, like spaceships moving at significant fractions of the speed of light. I've never been on one of these, so it's not surprising that it might behave differently to what I would expect. In contrast, the Schrodinger equation predicts that if I throw a ball, it will end up in many different locations at once. This is obviously wrong. So, you need to introduce another postulates (the Born rule, and the concept of a measurement, or at least the concept of an environment) for QM to not be in stark conflict with our day to day experience.