Researchers have recorded the briefest interval of time ever measured: 247 zeptoseconds—the duration for a photon of light to traverse a hydrogen molecule. That's 0.000000000000000000247 seconds.A zeptosecond equals one trillionth of a billionth of a second, a realm where light, the universe's speed champion, advances mere fractions of an atomic diameter. For scale, a single second contains as many zeptoseconds as there are seconds in 31.7 trillion years—vastly exceeding the age of the cosmos. Physicist Reinhard Dörner and colleagues at Goethe University Frankfurt achieved this using intense X-rays from Hamburg's PETRA III accelerator. They aimed at hydrogen molecules—the simplest in existence, comprising two protons and two electrons. An incoming photon struck both electrons in rapid sequence, akin to a stone skipping across water. To resolve this fleeting event, the team employed a COLTRIMS reaction microscope, an ultra-precise instrument that tracks particle positions and momenta. By examining the interference patterns from the two expelled electrons, they pinpointed the precise lag between the photon's impact on the first electron and the second.The finding: 247 zeptoseconds. This demonstrates that light does not illuminate a molecule instantaneously, even at this tiny scale; the delay stems from light's finite velocity of roughly 186,000 miles per second (300,000 km/s). It represents the first direct observation of light propagating inside a molecule. By contrast, chemical reactions unfold over femtoseconds—a thousandfold longer. Zeptosecond precision opens a window into quantum timescales, where electron and photon dynamics govern matter's core behaviors.