Writing in the Proceedings of the National Academy of Sciences, Canton et al.1 report the direct observation of interference patterns in the spectra of electrons produced when diatomic molecules are irradiated with ultraviolet light. The patterns provide the first unambiguous proof that such molecules can behave as two-centre emitters of electron waves.
The question of whether light consists of particles or waves has been debated for centuries. Although Christiaan Huygens proposed in 1678 that light consists of waves, the photon was generally considered to be a particle until Thomas Young reported his classic double-slit experiment in 1803. Young illuminated a panel containing two parallel slits with a point source of light, and observed that the light passing through the slits formed an interference pattern — a series of light and dark bands — on a screen behind the panel. This unambiguously proved the wave character of light. Imagine the confusion, then, when Arthur Compton also unambiguously proved the particulate nature of light in 1923, in studies of the scattering of high-energy photons.