ExclusorPauli ExclusionMug

Exclusor works one door, and he does not check names. He checks states.

His line is NOT IN MY STATE. A quantum state for an electron means everything about it: which orbital it occupies and which way its spin points. Two electrons can share one orbital, one spin-up and one spin-down. What they cannot share is the whole state.

Wolfgang Pauli stated the rule in 1925, to account for atomic spectra. He needed a fourth quantum number with only two values and ruled that no two electrons in an atom could share all four. Later that year George Uhlenbeck and Samuel Goudsmit identified it as electron spin. Pauli received the 1945 Nobel Prize.

The modern form is the first four lines on this mug. Swap two identical electrons and their joint wavefunction changes sign. Put both in the same state, a equal to b, and the two terms in the first line are identical and cancel: the wavefunction is zero. That state does not exist. Markus Fierz in 1939 and Pauli in 1940 showed that relativistic quantum theory makes every particle of half-integer spin obey it.

That is why electrons fill atoms shell by shell instead of piling into the lowest one, and why chemistry has a periodic table. Scale it up and the same rule pushes back: in 1926 Ralph Fowler showed that tightly packed electrons exert degeneracy pressure, the last formula. It holds up white dwarfs, up to Chandrasekhar's limit of about 1.4 solar masses. Exclusion is not a force, but together with electric attraction it keeps bulk matter from collapsing, as Dyson and Lenard proved in 1967, and Lieb and Thirring, far more simply, in 1975.

Exclusor never shoves. He just knows who is already inside.