Physical reality of the existence of electron bubble in liquid $^4He$ (or $^3He$) renders a { \it clear experimental evidence} for a quantum particle (in an interacting environment as seen by electron in liquid helium) to occupy exclusively a space of size $\lambda/2$ that, obviously, depends on its energy/momentum. This unequivocally proves that { \it no particle} in a system of interacting bosons such as liquid $^4He$ has momentum $p=0$; in stead, { \it all particles} in the ground state of such a system are in the single quantum state of energy $\varepsilon_o = h^2/8md^2$ or momentum $p = h/2d$.
Categories: cond-mat.other