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Meet the zeptosecond, the shortest unit of time ever measured

Scientists have measured the shortest unit of time ever: the time it takes for a light particle to travel through 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 zeros and a 1. Previously, researchers had plunged into the realm of zeptoseconds; in 2016, the researchers reported in the journal Physics of nature used lasers to measure time in increments of up to 850 zeptoseconds. This precision is a huge leap forward from the 1999 Nobel Prize-winning work which first measured time in femtoseconds, which are millionths of billionths of a second.

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

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The researchers set the X-ray energy so that a single photon, or particle of light, ejects the two electrons from the hydrogen molecule. (A hydrogen molecule is made up of two protons and two electrons.) The photon bounced one electron off the molecule, and then the other, a bit like a pebble jumping on top of a pond. These interactions created a wave pattern called the interference model, which Dörner and his colleagues could measure with an instrument called a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) reaction microscope. This instrument is essentially a very sensitive particle detector capable of recording extremely fast atomic and molecular reactions. The COLTRIMS microscope recorded both the interference pattern and the position of the hydrogen molecule during the interaction.

“Because we knew the spatial orientation of the hydrogen molecule, we used the interference of the two electronic waves to accurately calculate when the photon reached the first and when it reached the second hydrogen atom,”

; said Sven Grundmann, co-author of the study at the University of Rostock in Germany, he said in a statement.

That time? Two hundred forty-seven zeptoseconds, with some leeway depending on the distance between the hydrogen atoms inside the molecule at the precise moment the photon passed winged. The measurement is essentially capturing the speed of light within the molecule.

“We observed for the first time that the electron shell in a molecule does not react to light anywhere at the same time,” Dörner said in the statement. “The delay occurs because information within the molecule only spreads at the speed of light.”

The findings were detailed Oct. 16 in the journal Science.

Originally published in Live Science.

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