One of the deep unsolved problems of physics is the measurement problem of quantum mechanics. How is it that a system described by probability function becomes suddenly a system in a definite, well-defined state whenever a measurement is made?

Another deep, unsolved problem of physics is the arrow of time. Since all the fundamental laws of physics work equally well forward or backward in time, why is the past a fixed history that we can learn about, while the future is an open-ended set of possibilities about which we can only guess?

Freeman Dyson had one of the most independent and creative minds in science. Twenty years ago, at an all-star gathering of some of the world’s greatest physicists, Dyson proposed a solution to both these problems at once. Essentially, he proposes adding a rule to the usual formulation of quantum mechanics that says, “Propagate the wave function always forward in time. You may use what you know to compute quantum probabilities for the future, but never the past.”