The original Wigner's friend scenarios

The Wigner's friend paradox, like the Schrödinger's cat paradox, challenges our understanding of the foundations of quantum theory. The paradox is best illustrated by a brief scenario featuring two observers who draw different conclusions. We describe the Wigner's friend scenario as told by E. P. Wigner, and then an earlier version as told by Hugh Everett. Wigner and Everett offer different resolutions of essentially the same paradox. Decoherence theory provides a third resolution. Despite different interpretations (or their absence), these three stories fit together to form a consistent picture without a paradox.