Einstein received the Nobel Prize for explaining the photoelectric effect — that, simply put, a single atom is irradiated with light. Einstein believed that light consists of photons that transfer only quantized energy to the electron of an atom. If the photon’s energy is sufficient, it knocks the electrons out of the atom. But what happens to the photon’s momentum in this process? To answer the question, physicists at Goethe University constructed a new, high-resolution spectrometer called super COLTRIMS because it further advances the COLTRIMS principle invented at the university. COLTRIMS, an acronym for COLd Target Recoil Ion Momentum Spectroscopy, consists of ionizing individual atoms and then precisely determining the momentum of the particles. Inside the COLTRIMS spectrometer, photons collide with individual argon atoms, thereby removing one electron from each of the atoms. When photons from a laser pulse bombard an argon atom, they ionize it. Breaking up the atom partially consumes the photon’s energy. The remaining energy is transferred to the released electron. The transfer of the photon momentum to electrons is so tiny that it was previously not possible to measure it. The momentum of these electrons is now measured with extreme precision by the COTRIMS spectrometer. The Goethe physicists investigated which reaction partner — electron or atom nucleus — conserves the momentum of the photon. “The simplest idea is this: As long as the electron is attached to the nucleus, the momentum is transferred to the heavier particle, that is, the atom nucleus. As soon as it breaks free, the photon momentum is transferred to the electron,” professor Reinhard Dörner said. This would be analogous to wind transferring its momentum to the sail of a boat. As long as the sail is firmly attached, the wind’s momentum propels the boat forward. The instant the ropes tear, however, the wind’s momentum is transferred to the sail alone. However, researcher Alexander Hartung discovered that the electron not only receives the expected momentum, but additionally it receives one-third of the photon momentum that “should” have gone to the atom nucleus. The sail of the boat therefore “knows” of the impending accident before the ropes tear and steals a bit of the boat’s momentum. To explain the result more precisely, Hartung uses the concept of light as an electromagnetic wave: “We know that the electrons tunnel through a small energy barrier. In doing so, they are pulled away from the nucleus by the strong electric field of the laser, while the magnetic field transfers this additional momentum to the electrons.” The COLTRIMS spectrometer built by Alexander Hartung as part of his doctoral research in the experiment hall of the Faculty of Physics, Goethe University. Courtesy of Alexander Hartung. To ensure that the additional momentum of the electron was not caused accidentally by an asymmetry in the apparatus, in his experiment Hartung had the laser pulse hit the argon gas from two sides: either from the right or the left, and then from both directions simultaneously. This new method of precision measurement promises to lead to a deeper understanding of the role of the magnetic components of laser light in atomic physics. The research was published in Nature Physics (https://doi.org/10.1038/s41567-019-0653-y).