When textbooks introduce the de Broglie wavelength of a particle they always ask you to calculate the wavelength of a macroscopic object like a football or something. Then they conclude: "Since the wavelength of ordinary objects like footballs are only 0.00001 nm (or whatever) quantum mechanics doesn't manifest itself in the everyday world." Here is what I don't understand: Isn't the formula [tex]\lambda = h/p [/tex] only applicable to single particles, like electrons and protons. Can you just apply it to multi-particle systems like footballs?? I would think that the wavelength of a football must be some kind of superposition of the wavelengths of the single particles making up the football - which is too complicated to even consider. Thus I don't think it makes sense to talk about de Broglie wavelength for footballs, hats or cupcakes. I'm aware that the wave nature of particles isn't noticeable in our everyday, but I think this is a bad argument!