Meet the zeptosecond, the shortest unit of time ever measured

A particle of light, called a photon (yellow arrow), produces electron waves out of an electron cloud (grey) of a hydrogen molecule (red: nucleus). The result of those interactions is what’s called an interference pattern (violet-white). The interference pattern is slightly skewed to the right, allowing researchers to calculate the time for the photon to get from one atom to the next
(Image: © Sven Grundmann/Goethe University Frankfurt)

Scientists have measured the shortest unit of time ever: the time it takes a light particle to cross a hydrogen molecule.

That time, for the record, is 247 zeptoseconds. A zeptosecond is a trillionth of a billionth of a second, or a decimal point followed by 21 zeroes and a 1. Previously, researchers had dipped into the realm of zeptoseconds; in 2016, researchers reporting in the journal Nature Physics used lasers to measure time in increments down to 850 zeptoseconds. This accuracy is a huge leap from the 1999 Nobel Prize-winning work that first measured time in femtoseconds, which are millionths of a billionths of seconds.

It takes femtoseconds for chemical bonds to break and form, but it takes zeptoseconds for light to travel across a single hydrogen molecule (H2). To measure this very short trip, physicist Reinhard Dörner of Goethe University in Germany and his colleagues shot X-rays from the PETRA III at Deutsches Elektronen-Synchrotron (DESY), a particle accelerator in Hamburg.

The researchers set the energy of the X-rays so that a single photon, or particle of light, knocked the two electrons out of the hydrogen molecule. (A hydrogen molecule consists of two protons and two electrons.) The photon bounced one electron out of the molecule, and then the other, a bit like a pebble skipping over the top of a pond. These interactions created a wave pattern called an interference pattern, which Dörner and his colleagues could measure with a tool called a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) reaction microscope. This tool is essentially a very sensitive particle detector that can record extremely fast atomic and molecular reactions. The COLTRIMS microscope recorded both the interference pattern and the position of the hydrogen molecule throughout the interaction.

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