4 ms·

This is a great thought experiment, thank you. I'm not totally clear how the machines could work without actually taking a measurement, though. It sounds like y

• lisper 5y ago
  > I don't get why reflection isn't an observation/interaction there either. I know this comment is going to get lost in the noise, but that is a really excellent point, one of the best that has been raised here so far. This is a point that is often glossed over, but it is actually really important, and quite challenging to explain without getting deep into the weeds. The answer is that passing through a half-silvered mirror is an observation/interaction, but it is special because it can be practically reversed by using additional mirrors so that you can get back to a state where you can no longer tell what the outcome of the "measurement" was. All measurements are reversible in principle, but some are irreversible in practice because the number of things you'd have to reverse is just too large. And in particular, by the time a measurement has affected the state of any macroscopic system (like a ball) it is absolutely impossible to reverse in practice, though not in principle. This process of becoming irreversible-in-practice is called "decoherence". See [1] for a more detailed explanation. [1] http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
  • thisrod 5y ago
    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.
  • GoblinSlayer 5y ago
    The reflection does affect the state of a macroscopic system - mirror, as momentum of the reflected photon changed and needs to conserve.
    • lisper 5y ago
      That's right. That's why when this experiment is actually done, the mirror is typically rigidly mounted to an optical bench, which is sitting on the surface of a planet. If the mirror were freely floating in zero G, the outcome would be different. It is a worthwhile exercise to calculate how small the mass of the mirror would have to be before you would actually notice a difference in the results.
• tsimionescu 5y ago
  > I'm not totally clear how the machines could work without actually taking a measurement, though. Here you're hitting on the heart of the Measurement Problem. In QM as it is understood today, unlike classical mechanics, there are two fundamentally different kinds of interactions between objects: quantum interactions and measurement. Quantum interactions are linear changes to the wave function, while measurements perform a non-linear update to the wave function (it becomes one for the measured value and 0 everywhere else). Unfortunately, we do not have any theory so far that explains what is the difference between a quantum interaction and a measurement. The experiment I described works with 'machines' that interact quantically with the 'balls', but does not reproduce if the machines measure the state of the balls. I will note that in the Many Worlds Interpretation, the measurement problem is somewhat different - it states that the state of the universe is always described by a wave function, but that parts of the wave which are sufficiently separated can no longer perceive each other somehow, usually called branching. Precisely when, why or how this happens are just as unknown, though decoherence seems to play a role
  • GoblinSlayer 5y ago
    They don't perceive each other when they evolve independently, i.e. when they are valid solutions of the schrodinger equation.
    • tsimionescu 5y ago
      Valid solutions to the Schrodinger equation give you the wave function amplitudes in multiple places; the particles in these places can interact with each other still, even if they are 'the same particle'. However, the wave function at different places interacts with the environment and start to shift in phase, eventually becoming unable to interfere with itself - this is called decoherence, and is a valid explanation about why and how we can't observe wave-like behaviors at large scales or in hot systems. On the other hand, we can only postulate, based on observations, that when a particle interacts with a measurement device, the measurement device will show a single value with a probability determined by the amplitude of the particle's wave function at that point. We can postulate that the wave function collapses, or we can postulate that the device branches out into different devices in different worlds (enough such devices&worlds to achieve the probability distribution through observer selection somehow), or many other ways of formulating the Born rule. But whichever way you put it, this rule must be added to your system to predict experimental results, it does not derive from the Schrodinger equation.