We thank R. Lofstedt for bringing to our attention the importance of ref. 14; C. Regan for advice; B. Naranjo for many insights and for use of his uniquely designed liquid scintillator detection system; B. Kappus and S. Khalid for experimental assistance; J. Cambier for valuable discussions, E. Adams for archival assistance; and A. Bass for an independent translation of ref. 2. We thank Hamamatsu Corporation for lending us X-ray cameras. Various stages of this research were supported by the Office of Naval Research and the Defense Advanced Research Projects Agency (Microsystems Technologies Office and Defense Sciences Office) . J.E. thanks the Fulbright–Garcia Robles Scholarship Program and UC-MEXUS-CONACYT for support.

Author Contributions J.H. was instrumental in motivating this research. J.E. pinned down the correlation between the force and X-ray emission. C.C. obtained the GeV pulses, and S.P. is the principal investigator.