Particles known as fermions (such as electrons) obey the Pauli exclusion principle, which decrees that only a single fermion can occupy a particular state, such as an orbital in an atom. But the second family of particles, bosons (such as helium atoms), have no corresponding restrictions on their occupation of states. This distinction in their individual properties also carries over to the collective properties of large numbers of fermions or bosons at low temperatures. The typical quantum state of many fermions is a Fermi liquid, formed, for example, by the valence electrons in all metals: the electrons can move from atom to atom in this state and conduct electricity across macroscopic distances, albeit with a finite resistance. In contrast, bosons typically form a Bose-Einstein condensate, which is the basis of superfluidity in helium and its ability to flow perpetually with negligible dissipation.
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