The time it takes for a particle of light – a photon – to pass through a hydrogen molecule is now known as 247 zeptosecond, and scientists have for the first time been able to measure its duration.

Zeptosecond are a billionth of a second, or 10-21 seconds, which is an incredibly short time.

In 2016, researchers reported in the journal Nature Physics that using lasers, they were able to measure time up to 850 zeptosecond.

The measurement of zeppelins is a real scientific achievement since in 1999, scientists measured the time interval (and as a result received the well-deserved Nobel Prize), which is known as femtosman and is a billionth of a millionth of a second, or 10-15 seconds. It is in femtoseconds that the time required to form chemical bonds is measured. Even for light to pass through the molecule of hydrogen (H2), zeptotips are enough.

To measure this shortest distance, in Hamburg, physicist Reinard Darner of Goethe University and his colleagues used X-rays from a particle accelerator from PETRA III.

The researchers gave the X-rays enough energy to separate electrons from a hydrogen molecule for a single photon. (A hydrogen molecule consists of two protons and two electrons). The photon emitted one electron first from H2 and then another. This process is almost like jumping on a surface of water.

By interacting with a photon molecule, a wave called an interference wave was created. Darner and his colleagues measured this wave with a device called a COLTRIMS reaction ion moment microscope (COLTRIMS). It is a very sensitive particle detector and it can detect very fast atomic and molecular reactions.

The COLTRIMS microscope recorded both the interference wave and the location of the hydrogen molecule during the interaction.

“Because we already knew the spatial orientation of the hydrogen atom, we used two-electron wave interference to accurately calculate when a photon would reach the first and second hydrogen atoms,” said Sven Grandman, co-author of the study at the University of Rostock in Germany.

This time turned out to be 247 zeptoseconds with small errors, depending on the distance between the hydrogen atoms in the molecule and the time at which the photon is emitted. Finally, with these measurements we can determine the speed of light in a molecule.