Cosmos Editorial
German physicists say they have measured the shortest unit of time ever – a mere 247 zeptoseconds.
That’s 247 trillionths of a billionth of a second – think 20 zeroes after the decimal point – and it’s the time it takes for a photon to cross a hydrogen molecule, the team from Frankfurt’s Goethe University writes in a paper in the journal Science. On average, anyway.
It’s not the first time we’ve been in this rarefied neighbourhood. In 2016, researchers led by Germany’s Max Planck Institute revealed that the electron emission from helium atoms takes 856 zeptoseconds.
Reinhard Dörner’s team has raised the bar, however, working in collaboration with the accelerator facility DESY in Hamburg and the Fritz-Haber-Institute in Berlin.
They carried out the time measurement on a hydrogen molecule which they irradiated with X-rays from the synchrotron lightsource PETRA III at DESY. The explanation goes a bit like this.
They set the energy of the X-rays so that one photon was sufficient to eject both electrons out of the hydrogen molecule. As electrons behave like particles and waves simultaneously, the ejection of the first electron resulted in electron waves launched first in the one, and then in the second hydrogen molecule atom in quick succession, with the waves merging.
The photon behaved here much like a flat pebble that is skimmed twice across the water: when a wave trough meets a wave crest, the waves of the first and second water contact cancel each other, resulting in what is called an interference pattern.
The team then measured the interference pattern of the first ejected electron using the COLTRIMS reaction microscope, which makes ultrafast reaction processes in atoms and molecules visible.
Simultaneously with the interference pattern, COLTRIMS allowed the determination of the orientation of the hydrogen molecule. The researchers here took advantage of the fact that the second electron also left the hydrogen molecule, so that the remaining hydrogen nuclei flew apart and were detected.
“Since we knew the spatial orientation of the hydrogen molecule, we used the interference of the two electron waves to precisely calculate when the photon reached the first and when it reached the second hydrogen atom,” says co-author Sven Grundmann
“And this is up to 247 zeptoseconds, depending on how far apart in the molecule the two atoms were from the perspective of light.”