Now, nearly two decades later, atomic physicists at Goethe University led by Professor Reinhard Dörner have calculated a process that is shorter than femtoseconds for the first time ever: the measurement of how long it takes for a photon to cross a hydrogen molecule.

This is the shortest timespan that has ever been measured and amounts to about 247 zeptoseconds (a trillionth of a billionth of a second, or 10-21 seconds). To achieve this, the scientists irradiated a hydrogen molecule with X-rays from the X-ray laser source PETRA III at the Hamburg accelerator facility DESY.

They set it up so that one photon was sufficient to eject both electrons out of the hydrogen molecule. .