In '99, Markus Arndt published his paper in which single buckyballs (primarily C60 molecules) were fired at a diffraction grating and quantum interference was found. Arndt's experiment used single molecules that had a velocity of roughly 220 meters per second. The molecules traveled roughly 1 meter to the target. Now imagine that the rate at which these molecules were fired at the diffraction grating was slowly increased such that instead of just one molecule being in the apparatus at a time, 2, then 3, then many more would actually be moving through the apparatus at any moment. For example, if the molecules were shot out at a rate of 220,000 moelcules per second, the molecules would be 1 mm apart and there would be roughly 1000 molecules transiting through the apparatus at once. One could then imaging even closer spacing between molecules, spacing so close, the molecules might even begin interacting. At some point, I would think the molecules would be so close that they would interact with each other and/or you might even be able to see a thin stream of them. As the molecules got closer and closer together, is there a possibility that they would stop behaving as quantum particles and start acting like classical particles (ie: the diffraction pattern would disappear)?