Can someone describe the physical processes which distinguish between separate and single particles when dealing with a collection of particles in the context of the De Broglie wavelength? The De Broglie wavelength is inversely proportional to the momentum of a "particle". Assume "separate" particles A and B (e.g. separated but moving with the same speed and direction) and having momentum a and b respectively, thus each have wavelengths proportional to 1/a and 1/b. Loosely speaking one can look at the system of particles A and B, and it has a momentum a+b, which implies that the "system" (if it can be seen as a "compound' particle) has a wavelength proportional to 1/(a+b). The interference outcomes based on these two different wavelengths contradict one another. So given any context it would seem physically speaking there is a mechanism which physically differentiates between "separate particles" which have De Broglie wavelengths based on the particles separately and "compound" particles which have De Broglie wavelengths based on the sum of momenta of the individual particles. When do separate particles of a system physically become "one particle" (according to De Broglie) and why? What physical mechanism makes this distinction in reality? Nuclear forces? Covalent bonds? Are ionic bonds enough? Does the size of the compound particle matter or only the magnitude of the binding forces?