Spiros Michalakis, manager of outreach and staff researcher at Caltech’s Institute for Quantum Information and Matter (IQIM), and Matthew Hastings, a researcher at Microsoft, have solved one of the world’s most challenging open problems in the field of mathematical physics. The problem, related to the “quantum Hall effect,” was first proposed in 1999 as one of 13 significant unsolved problems to be included on a list maintained by Michael Aizenman, a professor of physics and mathematics at Princeton University and the former president of the International Association of Mathematical Physics.