Observing a photon during flight When I was reading about the double-slit experiment in quantum mechanics, everything seems to make sense in terms of the waves and the interference pattern, but if thinking more about this observation, it brought up more questions on this experiment.
In basic terms, what does it mean for humans to observe an electron being fired from the gun and seeing it during flight? Our eyes observe electromagnetic radiation and photons are hitting our eyes and the senses in our eyes create a picture. 
As I understood it, when we see a tree outside, the sunlight/photons from the sun hit the tree and bounced back into our eyes (is this correct?). Does that mean sunlight/photon from a sun, hit the flying electron and bounced back to hit our eyes? If that is the case how does this work? It becomes way crazier if you think about this more. I actually took this "observing an electron during flight" for granted, but it seems like photon observing a photon or photon hitting a single photon and then creating an angle by bouncing back and hitting our eyes? If the photon hit the photon, does that change the flying photon that is about to go through the double slit, because it is hitting that photon?
Can someone describe to me how you understood this mechanics? Am I correct in how I described it or wrong? Here in quantum mechanics, there comes the time where "observation" "collapses the wave-function" if people believe in wave-function collapse by observation.
 A: If you had a laser you wouldn't see it unless it was aimed at your eye (ouch). Or if there is dust or such around for it to scatter off of.
And scattering is the key.  If you want to see something then it either has to get to your eye or it needs to deflect something towards your eye.
If you have a beam of electrons you could try to get something to scatter off it. Depending on the wavelength of the thing you scatter off the electron beam you might learn there is an electron beam in a general vicinity but not know exactly where.
If you want to know which way an electron goes then you need a smaller wavelength of thing to bounce off them.
Photons can interact but for many situations it is quite weak compared to interactions between charges and photons.
Collapse happens when you've separated the dynamics and so it is always about separating things or separating how they affected things.  Something that reacts the same to two thibgs will not separate them.  So again it will be hard for you to separate the split beams if you interact them with a large wavelength scattering becsuse it doesn't separate them.
A collapse happens when the thing and what you observe it with end up in states that are entangled. Or it happens after that, but that has to happen first. So the thing observing needs to have multiple possible states. And the thing being observed needs multiple possible states. And you need the final state to have particular states of A be associated with particular states of B. So that knowing the state of A tells you about the state of B. In the case of the large wavelength light the scattered state of light is associated with both states of the electron so you aren't going to get a collapse from that.
A: Photons and electrons are elementary particles and their behavior is predicted by quantum mechanical equations. Quantum mechanics predicts probabilities for a reaction to happen. Photon photon interactions have very very small probabilities of happening. Thus photons pass through each other for all measurable purposes at low energies ( light and below frequencies for sure).
Observation means interaction, and photons can interact with the electric field of atoms at higher probabilities. In elementary particle experiments one does not see a single photon or a single electron. One sees a point on a screen where they have interacted, like these single electron double slit slides. This is an interesting experiment on the subject showing how the boundary conditions for the problem influence the output results, interference or not.
Single photons are harder to experiment with but it has been done  and again the changes in the boundary conditions of the experiment in finding which way the photon went show the quantum mechanical probabilities at work..
A: As with most of the questions in this forum, the answer is right in the same section of the book where you read about the problem. Why don't you go back and find it?
First you talk about a photon hitting an electron, and then you suddenly change to a photon hitting a photon, a completely different problem (photons don't interact with other photons, only with charged particles, since a photon is the quantum of electromagnetic energy, and e/m forces only act between charged particles).
Any book on the slit experiment explains that to see an electron, you must bounce a photon off it, and this deflects the electron so that its motion is no longer predicted by the interference pattern. Since you now know which slit the electron took, there is no alternative route, and so no interference pattern. You get the cross section for a single slit.
