I saw this answer:
Is quantum entanglement functionally equivalent to a measurement?.
I had follow up questions that seem to have been addressed. Here are just a few similar topics:
Why does observation collapse the wave function?.
Does the observer or the camera collapse the wave function in the double slit experiment?.
What constitutes an observation/measurement in QM?.
After reading these, this is my question: The light from a laser in a 2-slit experiment must travel through a medium (air, water, glass, etc..). Why would a diffraction pattern would still be seen if the light interacts with particles just before entering the slit. These particles leak information about which slit the light went through into the environment. Why doesn't this information getting out cause decoherence?
Consider the case where just one particle sits in front of one of the slits. There is 2-slit experiment set up with a single particle emitted (particle A), and there is a second particle (particle B) already sitting and waiting in front of one of the slits. In order for particle A to go through this slit, it must pass by particle B which would B's state to change. Then particle A continues. It passes through the slit. On the other hand, if particle A passed through the other slit, then it does not interact with particle B, and no state change occurs for B. Particle A eventually lands on the landing detector. After particle A has landed, we can check particle B. If particle B's state has changed since we placed it in front of the slit before starting the experiment, then we know particle A went through that slit. Otherwise, A went through the other slit. Therefore, the final outcomes of A and B are correlated.
According to other answers, the wave function should collapse because particle B acts as a detector. Some say particle B measures particle A. So when A interacted with B, it's wave function collapsed, so there should not be an interference pattern.
This isn't my main question, but this leaves me wondering why there would also not be an interference pattern if A went through the other slit and did not become entangled with B. We know if B's state is unchanged, A went through the other slit. So how does B's mere presence cause A's wave function to collapse? No interaction occurred. If there was an interaction, then B's state would have changed. But how could B's state change if A went through the other slit? It's paradoxical.
However, this is my main question. I've seen a laser in a 2-slit setup produce an interference pattern in a classroom. My point is that particle B could have been any particle floating in front of the slit. Most photons would dodge the dust particles, but with the soup of all kinds of cosmic particles floating around, does the light from the laser really make it to the slits without interacting with anything along the way? If the light does in fact interact with other particles along the way, then does that mean the measurement problem depends on whether or not the information leaked out is discernable? In other words, these particles A interacts with carry information about which slit it went through. However, since we are unable to capture any of these witnessing particles, A goes on to create an interference pattern. It is only in a controlled setting where we can discern particle B's state that we can derive information about which slit A went through. How would such a system know in advance whether or not we would be able to discern such a thing after the event, thus deciding whether or not A should result in a interference pattern or not?
PS: If you think the laser is just dodging particles in the air, suppose we make the room very dusty. Or suppose we conduct the experiment underwater.