The observed elementary bosons are all gauge bosons: photons, W and Z bosons, gluons, except the Higgs boson which is a scalar boson. Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton, which is a boson of spin plus or minus two. Composite particles (such as hadrons, nuclei, and atoms) can be bosons or fermions depending on their constituents. More precisely, because of the relation between spin and statistics, a particle containing an even number of fermions is a boson, since it has integer spin. Examples include the following:  The number of bosons within a composite particle made up of simple particles bound with a potential has no effect on whether it is a boson or a fermion. Bose–Einstein statistics encourages identical bosons to crowd into one quantum state, but not any state is necessarily convenient for it. Aside of statistics, bosons can interact – for example, helium-4 atoms are repulsed by intermolecular force on a very close approach, and if one hypothesizes their condensation in a spatially-localized state, then gains from the statistics cannot overcome a prohibitive force potential. A spatially-delocalized state (i.e. with low ) is preferable: if the number density of the condensate is about the same as in ordinary liquid or solid state, then the repulsive potential for the "N"-particle condensate in such state can be no higher than for a liquid or a crystalline lattice of the same "N" particles described without quantum statistics. Thus, Bose–Einstein statistics for a material particle is not a mechanism to bypass physical restrictions on the density of the corresponding substance, and superfluid liquid helium has a density comparable to the density of ordinary liquid matter. Spatially-delocalized states also permit for a low momentum according to the uncertainty principle, hence for low kinetic energy; this is why superfluidity and superconductivity are usually observed in low temperatures. Photons do not interact with themselves and hence do not experience this difference in states where to crowd (see squeezed coherent state).