Erwin Hahn, a doctoral student at the University of Illinois at Urbana-Champaign, experiments in 1950 with radiofrequency pulses applied to a sample of nuclei in nuclear magnetic resonance (NMR), a then-recent technique developed by Felix Bloch and Edward Purcell (1952 Nobel Prize in Physics). Hahn is the first to use pulses of the right kind to observe the resonance's transient signals, instead of the usual continuous signal. One day, he observes on the oscilloscope a completely unexpected signal: after applying two radiofrequency pulses spaced in time, the nuclear spins — which had been dephasing relative to each other due to small differences in the local magnetic field each experiences — spontaneously realign, producing a second signal peak, an "echo", with no additional pulse applied at that moment. Hahn spends several weeks understanding the phenomenon and manages to predict it mathematically by solving Bloch's nuclear induction equations. He publishes the finding in "Spin Echoes" (Physical Review, 1950): the second radiofrequency pulse effectively reverses the direction of the spins' progressive dephasing, so that they momentarily realign again — a "time-reversal" phenomenon of each spin's individual precession. Hahn also publishes that same year, in a shorter paper, free induction decay (FID): the signal that naturally decays after a single radiofrequency pulse, with no need for a second pulse. Hahn himself would downplay this second result as "obvious", but it would over time become the basis of the modern pulsed approach to NMR, today dominant over the original continuous-wave method. The spin echo, for its part, allows precise measurement of nuclear spin relaxation times in the presence of magnetic-field inhomogeneities, and would become a fundamental technique not only in NMR but also, by analogy, in laser spectroscopy and neutron scattering. Its practical applications would prove decisive two decades later for the development of magnetic resonance imaging (MRI) by Paul Lauterbur and Peter Mansfield (see separate entry lauterbur-mansfield-mri-1973).