The dynamics of electrons change ever so slightly on each interaction with a photon. Physicists at ETH Zurich have now measured such interplay in its arguably purest form—by recording the attosecond-scale time delays associated with one-photon transitions in an unbound electron. The photoelectric effect, whereby photons impinging on matter cause the emission of electrons, is one of the quintessential effects of quantum mechanics. Einstein famously explained the key mechanism underlying the phenomenon in 1905, earning him the 1921 Nobel Prize in Physics. He built on a concept introduced five years earlier by Max Planck: Electromagnetic energy is absorbed and emitted...