Scientists Clocked the Shortest Time Period Ever: 247 'Zeptoseconds'

Scientists have clocked the shortest time measurement ever, expressed as 247 “zeptoseconds,” during an experiment that probed one of the most basic encounters between light and matter.

A zeptosecond is equal to a trillionth of a billionth of a second, making it one of the tiniest units of time known to science. Even shorter timescales exist in theory, such as the cosmic constant called Planck time, but these units are so exotic that it is not yet possible to measure them in a laboratory environment.

Scientists led by Sven Grundmann, a nuclear physicist at Goethe University in Frankfurt, Germany, snagged the record-breaking measurement by observing the time it took for a photon (a particle of light) to travel across a hydrogen molecule. 

“The length of the hydrogen molecule was known already, as is the speed of light,” Grundmann said in an email. “Hence, one could simply calculate that it takes a photon 247 zeptoseconds to pass through the molecule.”

“However, this time has never been measured before because there was no suitable clock,” he added. “Our measurement clocked the shortest period of time ever (so far).”

The measurement emerged from the team’s research into photoionization, described as “one of the most fundamental processes caused by the light-matter interaction” in a paper published on Thursday in Science. Photoionization occurs when photons are absorbed by an atom or molecule, dislodging one or more of its electrons in the process, which turns the atom or molecule into an ion.

“There is currently considerable interest in experimental studies of various ultrafast processes,” Grundmann and his colleagues note in the study. “Of particular interest are the real-time dynamics of photoionization,” the team added, because “the time scale of this process poses many open questions.”

One of those open questions concerns the nature of the so-called “birth time delay,” which is the time gap between the emission of different electrons within the same molecule due to the absorption of a photon. 

To measure this extremely fast period, Grundmann and his colleagues blasted molecular hydrogen with pulses of X-ray laser light using the PETRA III facility at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. 

A hydrogen molecule is composed of two hydrogen atoms, each with one proton and one electron, that bond together in a compound. As the high-energy light moved through the molecular hydrogen, it kicked out both electrons from their orbitals in quick succession. 

“We did not try to break any record on purpose,” Grundmann said. “From our perspective, the most interesting finding was that the electronic shell of the molecule did not react as one single unit upon being hit by the photon.”

Grundmann’s team was able to pick up the distinct wave patterns emitted by the electrons as they were ejected, thanks to a sophisticated reaction microscope called COLTRIMS. The researchers compared the electron waves to the intersecting ripples created by two skips of a stone on a pond. 

The unprecedented real-time look at photoionization enabled the scientists to clock the birth time delay, or the “the travel time of the photon from one molecular center to the other” at 247 zeptoseconds, the study said.  

This ultrafast timescale beats out the next fastest event on record, which lasted 850 zeptoseconds and was also the result of a photoionization experiment, according to a 2016 Nature Physics study.

It’s possible that future experiments along these lines will manage to capture even smaller measurements of time, while also revealing the weird physics that occurs in and around atoms and molecules. Grundmann pointed out that some elementary particles have estimated lifespans that are measured in yoctoseconds, which are 1,000 times shorter than zeptoseconds.

“Time is an obscure concept in the quantum world,” Grundmann noted. “I hope that this study will contribute to the discussion about time and eventually help us to get a better understanding of it.” 

“My team and I will further contribute experimentally,” he said, “We have some other gaseous molecules, liquids, and solids in mind where we will try to watch the propagation of light in matter.”