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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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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