Doing some kinematical calculations we say that particle possesses momentum p. It is definite, so we choose momentum representation to describe the state of particle. However I am curious in what frame of reference we measure particle momentum. We can not assign frame of reference to any physicial object due to the uncertainty relation. Surely we can introduce something averaged, imaginary point at which we fix the origin of our frame of reference. It will work almost perfectly for the case of laboratory frame of reference (uncertainty relation can be neglected for the macroscopic world) , I guess. But what if we have universe where there is only 3 or 4 electrons?