If we add polarizers to the slits like DrChinese describes above, I believe there is a difference in predicted outcomes between quantum physics and classical physics. A difference I believe can be tested. Both quantum and classical physics predict that the interference pattern will disappear. Quantum physics tells us this happens because we have potential which-way information. In classical physics we have to analyze the split wave vectors to understand what happens. When the waves hit the screen in phase, like this, the superpositioned wave will be the same as the initial one (pre slits). When they're completely out of phase, the resulting wave vector will oscillate orthogonally to the initial one. In the minimas, where the phase shift is 1/4 or 3/4, the resulting wave vector will spin, ie the superpositioned wave will be circularly polarized, like this. All superpositions in between those will be some transition between the initial, orthogonal and circular oscillation. But in no case will there ever be destructive interference, hence no interference pattern. In the quantum scenario, if we fire photons through the slits one at a time, each photon will pass the slits and hit the screen polarized either horizontally or vertically. The possibility of obtaining this polarization is what destroys the interference pattern. In the classical scenario, if we fire extremely week light waves through the slits, each one will split in a horizontal and a vertical component, which will superposition at the screen more or less phase shifted. Depending on the phase shift, the vector oscillation will be more or less rotated / circularly polarized. Testing these two predictions should be easy. We add a polarizing filter in front of the slits to polarize the initial light diagonally. We then remove a strip in the center of the screen and we replace it with a second filter. We add a detector behind the screen to count photons / light waves. See illustration. Quantum physics predict that all photons hitting the center strip will be either horizontally or vertically polarized, so no matter what orientation we set on polarizer B, approximately half should pass through. Classical physics predict that all the superpositioned waves hitting the center strip will be mainly diagonally polarized. So if we orient polarizer B in line with polarizer A, a majority of the light will pass through. If we orient B orthogonally to A, only a minimum amount (if any) will pass through. Comments?