How long does it take a light particle to cross a hydrogen molecule? Turns out, it's 247 zeptoseconds. The shortest unit of time ever, a zeptosecond is a trillionth of a billionth of a second -- or, in other "words," 0.000000000000000000001 seconds. To measure this journey, physicist Reinhard Dörner of Goethe University in Germany, and his colleagues shot x-rays from the PETRA III at Deutsches Elektronen-Synchrotron, a particle accelerator in Hamburg. A single photon (i.e., a particle of light) knocked the two electrons out of the hydrogen molecule one after the other, creating a wave pattern called an "interference pattern," which was measured with a tool called Cold Target Recoil Ion Momentum Spectroscopy reaction microscope.