If light falls on a solar sail with momentum $p$, is the pressure on the sail $\frac{2p}{A}$, saying $2p$ because it is completely reflected?

Considering the stage I'm at and with no extra information I was taken that as given, thanks.

Is it possible for a preparation to affect an experiment. Say if you prepared a spin or polarization, could you measure something different from measuring a particle with those properties without preparation? (aside from the fact that without preparation, you're probably going to measure a variety of properties)

@danielunderwood How do you imagine getting an initial state without "preparing" it in some way?

Yeah that would be a problem. The main thing that I was wondering if a particle "knows" that it was set up to be in a certain state. But I suppose that it doesn't matter how it went to a certain state, just that that was the state it was in

Yup

Is it correct that if you know the wavefunction of a system at one time and you know the Hamiltonian of the system, then you know the wavefunction at all times? Well provided there's no funny business going on in the Hamiltonian

@danielunderwood Yes

(unless you measure the system, then you have to start your prediction again with the result of the measurement as the new initial state)

@danielunderwood Thats, uh, "the measurement problem".

There's no single good answer for what happens during the measurement

The von Neumann measurement/decoherence/einselection answer would be closest to "temporarily changing the Hamiltonian"

But funny business like the delayed eraser is not so easily explained just in terms of that. It's a minefield, really.

Ahh alright I didn't know if the Hamiltonian changing was common of the attempts to solve the measurement problem or other ideas were completely different

Calling it a minefield makes me think of a quantum version of minesweeper. All the squares have a probability of having a mine, but it's not calculated until you click...sounds fun right?

On the bright side, I've never heard of the delayed eraser experiment, so it looks like I have some reading material

@vzn The absorber is for the muons (which you stop and allow to decay at rest because (a) you don't want them the detector and (b) you want to control the flavor and matterness of the beam). A few hundred meters (or kilometers) of dirt makes little difference to neutrinos at accelerator energies (a few to a few tens of GeV, mostly).

(For long baselines the matter effect is big enough to need to be included in your theoretical model, but the absorption is trivial.)