The proton may be significantly lighter than currently believed—1.007276466583 amu compared with 1.007276466879 amu—reports a group led by physicist Sven Sturm of the Max Planck Institute for Nuclear Physics (Phys. Rev. Lett. 2017, DOI: 10.1103/PhysRevLett.119.033001). A lower proton mass could affect the values of physical constants such as the Rydberg constant, which represents the lowest-energy photon that can ionize a hydrogen atom from its ground state. The scientists used a Penning trap, which holds a proton in a combination of magnetic and electric fields. The frequency at which the proton orbits in the magnetic field is proportional to its charge-to-mass ratio.