Question about quantum interaction Here are some of my doubts: (there's quite a number which I couldn't find satisfying ans to, you can choose any qn to ans, thanks in advance)


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*How close must two wave functions be to be considered an interaction? 

*Does a wavefunction have a definite domain or does it extend to infinity? 

*In an electron diffraction experiment setup, there's a big tranparent bulb containing vacuum and fluorescent screen. Why is it that the wavefucntion stays even when light is allowed to passed through it? Wouldn't it collapse midway due to interaction with photons?

*if I put detectors on both slit in a double slit experiment and not look at the result but look at the fluorescent screen do I still see double slit interference pattern or 2 bright lines?

*if the reason that a macro object doesn't exhibit quantum property is decoherence, does it mean that all the atomic and subatomic particles in the body has collapsed to one state?

*It's impossible to get rid of interaction with the environment, so why is it that everything in the universe not undergo decoherence since decoherence occurs upon interaction?
 A: Please ask only one question per post, but here I can give at least brief answers to everything.


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*There is always just one wave function for a composite system and all of its part are related to each other.

*A wavefunction might have compact support but in general, you should expect (at least mathematically) that is extends to infinity, but of course it will drop fast enough for large distances.

*Do not think of the wave function as an object. Especially not an object in position space, since the wave function is a map $\psi: \mathbb{R}^{3N} \to \mathbb{C}$, it lives on configuration space.

*Where you look is not the decisive thing in quantum mechanics, but the question is where apparatuses are put that through their interaction disturb the wave function evolution and effectively lead to a collapse. So if you put detectors that strongly interact, you destroy the interference pattern. A different thing are weakly interacting detectors like in delayed choice quantum experiments.

*The reason cannot be decoherence alone, which was already seen by Schrödinger in his famous cat example. Decoherence gives you no collapse, the Schrödinger equation is still linear. This is the so-called measurement problem and you may look up solutions for it.

*I don't have a definitve answer here, but why do you think that not everything undergoes decoherence sooner or later in the universe? 
