4 ms·

I agree with this. But it's worth pointing out that lots of good physicists don't. It's a statement of the Everett/relative state/many worlds interpretation,

I agree with this. But it's worth pointing out that lots of good physicists don't. It's a statement of the Everett/relative state/many worlds interpretation, which is simply too weird for many people to accept. That's why there are about a dozen other interpretations of quantum measurement theory, which are all weird in other ways that I can't accept. The weird part is precisely this: "you can get back to a state where you can no longer tell what the outcome of the 'measurement' was." In other words, at lunchtime you believed that a horizontally polarised photon hit your nose at 10am in morning, and you were right. Now it's dinner time, you don't believe that, and you would be wrong if you did. If the Everett interpretation doesn't pose a massive challenge to your ideas about reality and human identity, you haven't understood it. There are physics professors who picture photons choosing which way to go at a beam splitter, then transmitting the news backwards in time, because that seems more plausible to them. Of course, interpretations are not science. Everyone agrees how an experiment would go: any attempt to reverse the interaction of the photon with your nose and brain would fail, because thermodynamics. From a purely scientific viewpoint, it simply doesn't matter how many other yous are superposed in parallel universes, because their existence or lack of it has no consequences that (any of?) you can observe. But scientists are as fascinated by this as everyone else is.

lisper 5y ago
> It's a statement of the Everett/relative state/many worlds interpretation No, it isn't. I've said nothing about many-worlds, only reversibility. And on that point everyone agrees. > Now it's dinner time, you don't believe that You really need to read the link above. It goes into all that in great detail. But the TL;DR here is that if it's dinner time, you haven't actually reversed the measurement, notwithstanding your current mental state with respect to the photon.
- thisrod 5y ago
  > You really need to read the link above I don't like claims to authority, but maybe you should read the papers I've published about Bell inequalities too? :-p It sounds like you're saying that, in principle, physical processes are all reversible. (Although that is often thermodynamically impossible in practice.) You're also saying that it's impossible in principle for someone to learn the result of a measurement in the morning, then unlearn it when the measurement is reversed during the afternoon. I don't see how there could be a self-consistent interpretation of quantum measurement where both those things are true.
  - lisper 5y ago
    > maybe you should read the papers I've published How am I supposed to do that? You haven't provided and references and your profile is empty. > in principle, physical processes are all reversible Correct. This is a straightforward mathematical property of the Schroedinger equation. > it's impossible in principle for someone to learn the result of a measurement in the morning, then unlearn it when the measurement is reversed during the afternoon That's right. But that's not because it's impossible to reverse the measurement. It's because when you reverse a measurement you don't just "unlearn" the result. You really should read the reference I gave you.

lisper 5y ago

> It's a statement of the Everett/relative state/many worlds interpretation No, it isn't. I've said nothing about many-worlds, only reversibility. And on that point everyone agrees. > Now it's dinner time, you don't believe that You really need to read the link above. It goes into all that in great detail. But the TL;DR here is that if it's dinner time, you haven't actually reversed the measurement, notwithstanding your current mental state with respect to the photon.

thisrod 5y ago
> You really need to read the link above I don't like claims to authority, but maybe you should read the papers I've published about Bell inequalities too? :-p It sounds like you're saying that, in principle, physical processes are all reversible. (Although that is often thermodynamically impossible in practice.) You're also saying that it's impossible in principle for someone to learn the result of a measurement in the morning, then unlearn it when the measurement is reversed during the afternoon. I don't see how there could be a self-consistent interpretation of quantum measurement where both those things are true.
- lisper 5y ago
  > maybe you should read the papers I've published How am I supposed to do that? You haven't provided and references and your profile is empty. > in principle, physical processes are all reversible Correct. This is a straightforward mathematical property of the Schroedinger equation. > it's impossible in principle for someone to learn the result of a measurement in the morning, then unlearn it when the measurement is reversed during the afternoon That's right. But that's not because it's impossible to reverse the measurement. It's because when you reverse a measurement you don't just "unlearn" the result. You really should read the reference I gave you.