Yes, I think that's more or less it. These diffraction patterns only occur if the slits have about the same width as the particle wavelength. The initial probability to find a particle is only significant in a region with the same order of magnitude as its wavelength. But the longer the beam, the more this uncertainty propagates, so a beam would widen. There was a small section on laser beams in that wiki article about diffraction.Ozymandyus said:I suppose that determining the precise position of the particle within its wavelength is the problem. At least thats the only thing I can conceive of as a problem. So if we know precisely how fast a particle is moving, we can only know that it is somewhere within its wave-length - i.e. for gamma rays for example could only be pinpointed to within 10^-5 nm?