“All of the mystery of quantum mechanics” is contained within the double-slit experiment, Nobel laureate Richard Feynman famously said. In the experiment, first proposed in 1801 by British polymath Thomas Young, a beam of photons—particles of light—flies toward a wall with two slits cut in it. When the light reaches a screen behind the wall, it produces a telltale “interference pattern”: stripes of light interspersed with darkness. This pattern results only if the photons act like waves rather than like point particles, and the peaks and troughs of the waves coming through the two slits interfere with one another, sometimes adding light and sometimes canceling it out. When Young performed the experiment, using a modified setup, it seemed to establish that light was a wave and not a particle.