1945 Nobel Prize in Physics
Reason for Award
for the discovery of the Exclusion Principle, also called the Pauli Principle (original paper: Zeitschrift für Physik 31 (1925) 765-783)
Laureates
United States of America
Explanation
Every electron is like a tiny ball that does not want to share the exact same seat with another electron. Pauli discovered a rule that says “only one electron per seat.” Because of this rule, electrons line up in order inside atoms, creating all the different elements we know. Iron, gold, and even water molecules keep their shapes because the rule is obeyed. Without it, matter would collapse into one spot and the world around us would look completely different.
Related Keywords
Pauli Exclusion Principle
The principle that forbids identical fermions from occupying the same quantum state, underpinning atomic structure and matter stability. It stems from the antisymmetry of the many-body wavefunction and is almost axiomatic within quantum mechanics. By generating electronic periodicity, it explains the periodic table and chemical behavior. On cosmic scales it produces the degeneracy pressure that supports white dwarfs and neutron stars against gravity. In modern quantum information and ultracold Fermi-gas research it appears as forbidden transitions and Pauli blocking.
quantum number
Integers or half-integers that label quantum states, such as principal, orbital, magnetic, and spin quantum numbers. The exclusion principle forbids two fermions with all identical quantum numbers from coexisting. Quantum numbers set energy levels and angular momentum, determining spectral multiplets. They also describe internal degrees of freedom in nuclei and elementary particles, closely linked to symmetry groups like SU(2) and SU(3). In experiments, quantum-number conservation provides selection rules and guides searches for new particles.
spin
An intrinsic form of angular momentum; electrons and protons carry spin-½. Spin precesses in a magnetic field, forming the basis of NMR and MRI. Under the exclusion principle two electrons can occupy the same spatial orbital only if their spins are opposite. Spin manipulation serves as quantum bits in quantum computing. In condensed-matter physics it drives spintronics and magnetic-material design, where maintaining coherence in solids is a key challenge.
fermion
A particle with half-integer spin obeying the Pauli Exclusion Principle; electrons, protons, neutrons, quarks, and leptons are examples. Fermions possess antisymmetric wavefunctions and follow Fermi–Dirac statistics in many-body systems. In metals they form a Fermi surface, giving characteristic temperature dependences to heat capacity and conductivity. In astrophysics they generate degeneracy pressure supporting white dwarfs and neutron stars. Research on Majorana fermions and Dirac semimetals now targets quantum-information and high-energy simulations.
atomic shell model
A model in which electrons occupy hierarchical shells (K, L, M…) and orbitals, with seating determined by the exclusion principle and quantum numbers. Rows of the periodic table correspond to completed shells, and noble-gas stability stems from filled outer shells. Chemical reactions involve rearranging shell electrons to form bonds, explaining periodic ionization energies and atomic radii. X-ray absorption and photoelectron spectroscopy directly probe shell structure and are widely used in materials analysis. Nuclear physics uses an analogous shell model for neutrons and protons, explaining magic numbers.
Fermi–Dirac statistics
A statistical model giving the occupancy probability of energy levels by fermions, with the exclusion principle built in. Near absolute zero all particles fill states up to the Fermi energy, creating the Fermi surface. It is essential for carrier-density calculations in semiconductors and for analyzing thermal properties of metals. The distribution has the denominator 1+e^{(ε−μ)/kT}, contrasting with the Bose distribution. Cosmology uses it to estimate the energy spectrum of the neutrino background radiation.
degeneracy pressure
A quantum-mechanical repulsive pressure arising when fermions are compressed, enforced by the exclusion principle. Electron degeneracy pressure supports white dwarfs, while baryonic degeneracy supports neutron stars; neutrino degeneracy influences the early universe. Unlike classical ideal-gas pressure, degeneracy pressure persists even at low temperature, dictating the temperature–pressure relation of ultradense objects. It is vital for deriving the Chandrasekhar mass limit and for core-collapse supernova scenarios. Degeneracy effects in low-dimensional and strongly correlated electron systems are now major topics in condensed-matter research.
spin-statistics theorem
A theorem in relativistic quantum field theory proving that integer-spin particles obey Bose statistics while half-integer-spin particles obey Fermi statistics. The exclusion principle therefore follows inevitably for spin-½ and higher fermions. The theorem assumes CPT symmetry and Lorentz invariance, refining Pauli’s early argument through work by Wigner and Schwinger. In topological quantum field theory, anyonic statistics appear as exceptions in two dimensions, underpinning the quantum Hall effect and topological quantum computation. Everyday verification comes from the fact that photons can pile up in a single state whereas electrons cannot.