In 1921, Albert Einstein received the Nobel Prize in physics for the discovery that light is quantized, interacting with matter as a stream of particles called photons. Since these early days of quantum mechanics, physicists have known that photons also possess momentum. The photon's ability to transfer momentum was used in a novel approach by scientists of the Max Born Institute, Uppsala University and the European X-ray Free-Electron Laser Facility to observe a fundamental process in the interaction of X-rays with atoms. The detailed experimental and theoretical results are reported in the journal Science.