By Pauli's exclusion principle, the ground state Ψ Ψ 0 of a Fermi gas consists of fermions occupying all momentum states corresponding to momentum p < p F with all higher momentum states unoccupied. As the interaction is turned on, the spin, charge and momentum of the fermions corresponding to the occupied states remain unchanged, while their dynamical properties, such as their mass, magnetic moment etc. are renormalized to new values. Thus, there is a one-to-one correspondence between the elementary excitations of a Fermi gas system and a Fermi liquid system. In the context of Fermi liquids, these excitations are called " quasiparticles ".