4 ms·

No, it really is. Consider a sealed room with an experimenter looking at a box with Schrödinger's famous cat in it. The question we usually ask is whether the

by btilly 3y ago

No, it really is. Consider a sealed room with an experimenter looking at a box with Schrödinger's famous cat in it. The question we usually ask is whether the cat is alive, dead, or in a superposition before the box is opened. Schrödinger's famous equation predicts that if the cat itself can be described by quantum mechanics, then it must be in a superposition. If the experimenter can be described by quantum mechanics as well, then the experimenter must also go into a superposition upon opening the cat's box. And, thanks to thermodynamics, there is no experiment that is doable by the experimenter from which the existence of collapse can be demonstrated. Therefore the claim that there is a collapse at all is an entirely unnecessary hypotheses. All other interpretations of QM have to invent explanations for an event (collapse) that no experimental evidence exists for.

tsimionescu 3y ago
That's all well and good, but it then predicts that all possible outcomes have equal probability, and this is measurably false. Say you design the experiment such that the observer will see the cat is alive if 2 particles both have spin up, and dead if any particle has spin down. Say the Schrodinger equation will assign equal amplitudes to the 4 possible states (up-up, up-down, down-down, down-up), and let's ignore the composite states (e.g. 1/sqrt(2)up-up + 1/sqrt(2)up-down). So, the cat-alive (up-up) state has an amplitude which is 3* the amplitude of the cat-dead state (up-down + down-up + down-down). In a naive interpretation of MWI that only used the Schrodinger equation, there are two versions of the observer, so the probability that the observer sees one outcome versus the other is obviously 1/2: you either happen to be the version that sees the cat alive, or you happen to be the one that sees it dead. This reasoning will work if we repeat the experiment many times: since the repetitions are independent, if I repeat it 10 times, I expect that I will happen to be one of the observers who sees the cat alive about 5 times, and dead about 5 times as well. In your interpretation, the amplitude of the Schrodinger equation is irrelevant, as long as it is greater than 0: all possible events happen. If I actually do the experiment though, I will see the cat alive only about 2.5/10 times, since the total amplitude of the wavefunction for all states where the cat is alive is much lower than the total amplitude of all states where the cat is dead. So, the actual MWI says that, while there are two kinds of worlds, they are not equally likely. In the multitude of all worlds, the prevalence of worlds where the cat is alive is proportional to the wavefunction amplitude of the cat-alive state (about 1/4) and the ones where the cat is dead follows the same logic (about 3/4). So, given that I am one observer in one of the many worlds, the chance I am the observer in a world where the cat is alive is only 1/4. But this connection between the number of worlds and the amplitude of the wavefunction is an additional assumption atop the Schrodinger equation. Sure, the wavefunction doesn't collapse, but it splits according to the exact same formula as the collapse in the CI (Born's rule). And I again want to mention that even this is not enough. If |cat-alive> and |cat-dead> are solutions to the Schrodinger equation, then so is x|cat-alive>+y|cat-dead>, for an infinite number of x and y real numbers. The MWI has to explain why no observer ever actually perceives such a state (how this state would look like to to an observer is not even definable). Decoherence solves this, and it was an extremely important contribution, but it also adds an additional assumption (that CI also needs): some pre-existing classical-like background.
- btilly 3y ago
  Yes, you can construct a naive version of the MWI that produces answers in disagreement with experiment. But that naive version of the MWI also doesn't match the predictions of attempting to model both cat and experimenter with QM. This is the essence of a straw man argument. OK, your ridiculous version of the theory doesn't work. Now what happens if you look at the actual theory under discussion?
  - tsimionescu 3y ago
    My point is that the actual version of the MWI requires the Born rule (which can't be derived from the Schrodinger equation, and which is also known as the measurement postulate) just as much as any other interpretation. I wasn't building a strawman, I was trying to explain why the simple explanation you had given in the previous post (which is the commonly presented explanation of the MWI in many popular channels) doesn't actually work. You were the one who was claiming that the MWI simply says that the observer is in a superposition itself, which is indeed what the Schrodinger equation predicts. But this entirely leaves out the other half (why does the observer in fact observe a single outcome , with some probability X) and I was explaining how the same maths as the oh-so-hated collapse sneak back in through there. Basically, the MWI and the CI agree that from the point of view of the observer something happens when the observer opens the box which they can't predict deterministically. The collapse versions of the CI say that this event actually changes the wavefunction, it collapses it, and all other possible results don't happen. The MWI says that this is just how it looks like to one observer, and all other results happen to other observers, in a very precise proportion. The "shut up and calculate" version of CI says that it's unscientific to even discuss this distinction, since a single observer anyway observes a single thing, talking about observations that didn't happen and how real or false they are is unscientific speculation.
    - btilly 3y ago
      You've said nothing suggesting that the explanation doesn't work. Whether you believe in collapse now, collapse later, or collapse never but the Born rule works, you get the same exact predictions. Therefore no experiment done to date represents evidence that there actually is a collapse. And evidence that the experimenter is modeled by QM is evidence against a collapse. This is all true, and is all verifiable from QM. And whether collapse handles later or never, the math behind the MWI explains why we'd think we'd observed what we observed in the absence of a collapse.

by btilly 3y ago

tsimionescu 3y ago

That's all well and good, but it then predicts that all possible outcomes have equal probability, and this is measurably false. Say you design the experiment such that the observer will see the cat is alive if 2 particles both have spin up, and dead if any particle has spin down. Say the Schrodinger equation will assign equal amplitudes to the 4 possible states (up-up, up-down, down-down, down-up), and let's ignore the composite states (e.g. 1/sqrt(2)up-up + 1/sqrt(2)up-down). So, the cat-alive (up-up) state has an amplitude which is 3* the amplitude of the cat-dead state (up-down + down-up + down-down). In a naive interpretation of MWI that only used the Schrodinger equation, there are two versions of the observer, so the probability that the observer sees one outcome versus the other is obviously 1/2: you either happen to be the version that sees the cat alive, or you happen to be the one that sees it dead. This reasoning will work if we repeat the experiment many times: since the repetitions are independent, if I repeat it 10 times, I expect that I will happen to be one of the observers who sees the cat alive about 5 times, and dead about 5 times as well. In your interpretation, the amplitude of the Schrodinger equation is irrelevant, as long as it is greater than 0: all possible events happen. If I actually do the experiment though, I will see the cat alive only about 2.5/10 times, since the total amplitude of the wavefunction for all states where the cat is alive is much lower than the total amplitude of all states where the cat is dead. So, the actual MWI says that, while there are two kinds of worlds, they are not equally likely. In the multitude of all worlds, the prevalence of worlds where the cat is alive is proportional to the wavefunction amplitude of the cat-alive state (about 1/4) and the ones where the cat is dead follows the same logic (about 3/4). So, given that I am one observer in one of the many worlds, the chance I am the observer in a world where the cat is alive is only 1/4. But this connection between the number of worlds and the amplitude of the wavefunction is an additional assumption atop the Schrodinger equation. Sure, the wavefunction doesn't collapse, but it splits according to the exact same formula as the collapse in the CI (Born's rule). And I again want to mention that even this is not enough. If |cat-alive> and |cat-dead> are solutions to the Schrodinger equation, then so is x|cat-alive>+y|cat-dead>, for an infinite number of x and y real numbers. The MWI has to explain why no observer ever actually perceives such a state (how this state would look like to to an observer is not even definable). Decoherence solves this, and it was an extremely important contribution, but it also adds an additional assumption (that CI also needs): some pre-existing classical-like background.

btilly 3y ago
Yes, you can construct a naive version of the MWI that produces answers in disagreement with experiment. But that naive version of the MWI also doesn't match the predictions of attempting to model both cat and experimenter with QM. This is the essence of a straw man argument. OK, your ridiculous version of the theory doesn't work. Now what happens if you look at the actual theory under discussion?
- tsimionescu 3y ago
  My point is that the actual version of the MWI requires the Born rule (which can't be derived from the Schrodinger equation, and which is also known as the measurement postulate) just as much as any other interpretation. I wasn't building a strawman, I was trying to explain why the simple explanation you had given in the previous post (which is the commonly presented explanation of the MWI in many popular channels) doesn't actually work. You were the one who was claiming that the MWI simply says that the observer is in a superposition itself, which is indeed what the Schrodinger equation predicts. But this entirely leaves out the other half (why does the observer in fact observe a single outcome , with some probability X) and I was explaining how the same maths as the oh-so-hated collapse sneak back in through there. Basically, the MWI and the CI agree that from the point of view of the observer something happens when the observer opens the box which they can't predict deterministically. The collapse versions of the CI say that this event actually changes the wavefunction, it collapses it, and all other possible results don't happen. The MWI says that this is just how it looks like to one observer, and all other results happen to other observers, in a very precise proportion. The "shut up and calculate" version of CI says that it's unscientific to even discuss this distinction, since a single observer anyway observes a single thing, talking about observations that didn't happen and how real or false they are is unscientific speculation.
  - btilly 3y ago
    You've said nothing suggesting that the explanation doesn't work. Whether you believe in collapse now, collapse later, or collapse never but the Born rule works, you get the same exact predictions. Therefore no experiment done to date represents evidence that there actually is a collapse. And evidence that the experimenter is modeled by QM is evidence against a collapse. This is all true, and is all verifiable from QM. And whether collapse handles later or never, the math behind the MWI explains why we'd think we'd observed what we observed in the absence of a collapse.