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Eugene Wigner and Gregory Breit connected the capture of slow neutrons with resonant excited states of a target atomic nucleus. A capture probability can change rapidly when the projectile energy crosses one of these states. Their treatment relates the peak to the coupling into the intermediate state and its possible decay channels. It therefore explains why a small interval of incident energies can contain pronounced structure, rather than a smooth response determined by the target's size alone.^[ref]

A physicist.^[ref]

Gregory Breit and Eugene Wigner explained strong variations in slow-neutron capture through resonant nuclear excitation. In modern notation, an isolated nuclear resonance has a contribution proportional to ${\Gamma }_n{\Gamma }_{\gamma }/[(E-E_r{)}^2+{\Gamma }^2/4]$, apart from spin and kinematic factors. Here ${\Gamma }_n$ and ${\Gamma }_{\gamma }$ describe the entrance and radiative channels, while $\Gamma$ is the total decay width. With slowly varying prefactors, moving $\Gamma /2$ to either side of $E_r$ halves the peak, connecting its observed width with the unstable intermediate state.^[ref]

A thought experiment placing a friend's physical measurement record inside a larger isolated quantum laboratory. Conditioned on a record the friend has a definite outcome, while an external observer using universal unitary evolution can assign an entangled laboratory state. Its contrast with a collapsed mixture is an instance of the quantum measurement problem; it does not require giving consciousness a special dynamical law. Eugene Wigner used the observer-within-the-system construction to examine such tensions.