Comment by lisper | Hacker News Reader

Comment by lisper | Hacker News Reader

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> 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-ar...

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.

> 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.

> 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.

> 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.

The reflection does affect the state of a macroscopic system - mirror, as momentum of the reflected photon changed and needs to conserve.

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.

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