Eli5: Why can’t the wave-like behavior of photons through a slit be explained by the photon coming too close to the edge of the slit, being influenced by gravity or the strong nuclear force of the atoms in the slit material, thus causing it to change it’s trajectory and only appear wave-like?
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The short answer is that the four fundamental forces “affect” only their corresponding aspect of matter (mass, charge, etc.). Gravity doesn’t pull against charge, it only pulls against mass (photons don’t have mass), and electromagnetism doesn’t pull against mass it only pulls against electrical charge.
Changing the trajectory is not a problem, this can easily be explained by a particle model. In fact this is exactly what we see when you try to measure which slit a given photon travels through. What you can’t explain with a particle is the interference pattern, which only appears when you don’t try to measure at the slit.
Firstly, because gravity is a super weak force, so the “gravity” attributed to close proximity to the edge of a given slit wouldn’t be enough to affect the trajectory of any particle, even a larger one with mass. If this could explain the behavior, then tossing a baseball infinitely close to a wall’s edge should cause the ball’s flight to bend towards the wall as it passed, since the mass of a baseball is infinitely larger than that of a photon (zero mass).
Secondly, the strong nuclear force doesn’t act like gravity at all, and it doesn’t act on photons (at least not like this with photons passing near atoms); it holds an atom together, particularly the protons and neutrons. Keep in mind that protons all hold positive charge, so the strong force not only has to hold the protons next to neutrons with a much higher strength than gravity could, it also has to hold protons together against the coulomb force from the positive charges which repel each other.
Thirdly, the interference patterns of light on the opening side of the slits essentially “match” the interference patterns we see with sound waves.
So, basically, neither the strong force nor gravity could have any measurable effect on a photon, plus we see additional data demonstrating wave-like behavior beyond simple particle scattering. It’s the wave interference as sketched here that proves wave behavior:
https://en.m.wikipedia.org/wiki/Young%27s_interference_experiment#/media/File%3AYoung_Diffraction.png
Because the photon goes through both slits simultaneously. It can’t do that if it’s a particle.
You can put enough filters between the source and slits that only one photon at a time is going through. And you *still* get an interference pattern. That’s only possible if the photon is interfering with itself. And *that’s* only possible if it’s interacting with both slits at once, which a particle can’t do.
If you stick a detector on one of the slits so you *know* which slit it went through (essentially you force it to exhibit particle-like behavior) the interference pattern disappears.
Edit:typo
They key thing about the double-slit experiment isn’t that the particle changes directory. It’s that it interferes with itself.
When we pass a wave through a double-slit we get [a diffraction pattern](https://en.wikipedia.org/wiki/File:Single_slit_and_double_slit2.jpg). We get bright spots and dark spots. What we’ve done is split our single wave into two waves, the bright spots are where the waves are in phase and the dark spots where they are out of phase.
The weird thing about the double-slit experiment is that we [get the same sort of thing](https://en.wikipedia.org/wiki/File:Electron_buildup_movie_from_%22Controlled_double-slit_electron_diffraction%22_Roger_Bach_et_al_2013_New_J._Phys._15_033018.gif) when we do this with particles like electrons (definitely a particle, right?). There are places where you detect electrons, and there are places where you don’t get any. And you get this pattern even if you fire your electrons one at a time.
If the electron goes through one slit and is somehow influenced into changing directions, that’s all good but we wouldn’t get those “empty” spots. There is no reason why fewer electrons should have their path bent by those particular amounts. Each electron path gets bent randomly, except there are values we never get. And those values [match up with our wave pattern](https://en.wikipedia.org/wiki/File:Interference_electrons_double-slit_at_10cm.png).
Somehow our electron is partially going through both slits, and the part that goes through each slit interferes (in a wave-like manner) with the other part.
But maybe there is still some random factor we haven’t considered that means the path is getting bumped in a way where those paths aren’t valid? That’s when we start measuring which slit the electron goes through. When we do that we lose our diffraction pattern. The electron goes through just one slit and does its normal single-slit diffraction thingamy. No interference, just the normal random path-bending.
So what we find is a certain probability of getting an electron at a particular point if it goes through one slit. And a certain probability of it hitting the same point by going through the other slit. But when we let it do both the probability of it reaching that point *goes down*. Which is not how probability is supposed to work (if you have two ways of doing something the probability of getting it should go up)! But it is how waves work.
From the outside of the quantum system the only way to model what we observe is as it existing in a combination of the states where the electron goes through one slit and where it goes through the other, weighted with a certain phase (like a wave).
Of course from the individual electron’s point of view it does something perfectly reasonable; it goes through a slit and hits a spot on the screen that is normal for it. It is only when we repeat this with lots and lots of electrons that the large-scale pattern emerges.
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