For centuries, time was understood only through repetition and rhythm. Sunrises, heartbeats, pendulums, and planetary orbits gave humanity a sense of duration, but none of these revealed what time actually is at its most fundamental level. Even the invention of mechanical clocks and atomic clocks only refined our ability to count time, not to understand it. Modern physics, however, has been steadily pushing deeper, shrinking the smallest measurable moments and revealing that time is not smooth or continuous, but layered and granular. Each improvement in measurement peels back another layer of reality and exposes processes that were once assumed to happen instantaneously.

That pursuit has now reached an unprecedented milestone. Scientists have measured the shortest unit of time ever recorded, capturing the duration it takes for a single particle of light to cross a hydrogen molecule. That time is 247 zeptoseconds. A zeptosecond is a trillionth of a billionth of a second, represented as a decimal point followed by twenty zeros and a one. At this scale, time is no longer connected to clocks, motion, or even chemistry in the familiar sense. It is tied instead to the movement of light itself, the fastest signal the universe allows. This measurement is not simply about speed or precision, but about revealing how reality updates itself at the smallest known scales.

Image Credit:  Sven Grundmann, Goethe University Frankfurt

From Femtoseconds to Zeptoseconds

The journey toward measuring time at this level did not happen overnight. In 1999, scientists achieved a major breakthrough by measuring time in femtoseconds, which are millionths of a billionth of a second. This advance was significant enough to earn a Nobel Prize because it allowed researchers to directly observe chemical reactions as they happened. For the first time, scientists could watch molecular bonds break and form, revealing chemistry as a dynamic process rather than a static before and after snapshot. Femtosecond measurements transformed physics, chemistry, and materials science by opening a window into ultrafast processes.

Over time, experimental techniques improved, and researchers began probing even shorter intervals. By 2016, scientists reporting in the journal Nature Physics succeeded in measuring time in increments down to 850 zeptoseconds using advanced laser systems. This achievement pushed time measurement beyond electron motion and into the realm where light itself becomes the subject of observation. At these scales, traditional assumptions about simultaneity and instantaneous interaction begin to fail, forcing physicists to rethink how energy and information propagate through matter.

The new measurement of 247 zeptoseconds represents a significant leap beyond previous records. It is not simply a matter of improved resolution but a qualitative shift in what can be studied. Instead of observing how electrons behave over time, scientists are now directly measuring how long it takes light to move within a molecule. This brings experimental physics closer than ever to the fundamental speed limit of the universe and provides direct evidence that even at atomic scales, events unfold in sequence rather than all at once.

Measuring Light Inside a Hydrogen Molecule

The experiment that achieved this record was led by physicist Reinhard Dörner and his colleagues at Goethe University Frankfurt. The team focused on hydrogen, the simplest molecule in existence, composed of two protons and two electrons. Because of its simplicity, hydrogen provides an ideal testing ground for studying fundamental physical processes without the added complexity of larger molecules. The goal was to measure how long it takes a photon to travel from one hydrogen atom to the other within the molecule.

To accomplish this, the researchers used powerful X rays generated by PETRA III at DESY, a particle accelerator facility in Hamburg. The energy of the X rays was carefully tuned so that a single photon would knock two electrons out of the hydrogen molecule. As described by the researchers, the photon hit one electron first and then the second, in a process similar to a pebble skipping across the surface of water. This sequential interaction created overlapping electron waves.

These overlapping waves formed what is known as an interference pattern, which could be detected and analyzed using a Cold Target Recoil Ion Momentum Spectroscopy reaction microscope, or COLTRIMS. This highly sensitive instrument allowed the team to record both the interference pattern and the precise spatial orientation of the hydrogen molecule during the interaction. By combining this information, the researchers could reconstruct the timing of the photon’s interaction with each atom in the molecule.

Capturing a 247 Zeptosecond Interval

The key to determining the time delay lay in understanding how the interference pattern was skewed. Because the scientists knew the exact orientation of the hydrogen molecule, they could use the interference of the two electron waves to calculate when the photon reached the first hydrogen atom and when it reached the second. As Sven Grundmann, a study coauthor at the University of Rostock, explained, “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.”

The result of this analysis was a time interval of 247 zeptoseconds, with slight variation depending on the distance between the hydrogen atoms at the exact moment the photon passed through. This measurement effectively captures the speed of light within the molecule itself. Rather than assuming light interacts with the entire molecule instantaneously, the experiment shows that there is a measurable delay, even at this minuscule scale.

This finding represents the shortest time interval ever directly measured. It confirms that the propagation of light, and therefore information, is not instantaneous even within a single molecule. The implication is that all physical systems, no matter how small, are governed by finite transmission times that can now be experimentally observed.

What the Experiment Reveals About Time

Beyond setting a record, the experiment revealed something fundamentally important about how matter responds to light. As Reinhard Dörner stated, “We observed for the first time that the electron shell in a molecule does not react to light everywhere at the same time.” This observation challenges long held assumptions that interactions at atomic scales occur instantaneously. Instead, the response of the molecule unfolds over time, governed by the speed at which information can travel.

Dörner further explained the reason for this delay by stating, “The time delay occurs because information within the molecule only spreads at the speed of light.” This simple statement carries profound implications. It means that even the internal structure of a molecule experiences a sequence of cause and effect. There is no absolute simultaneity, only propagation.

This insight connects directly to broader questions in physics about the nature of time itself. If even electrons within a molecule respond sequentially, then time is deeply woven into the fabric of physical interactions. It is not merely a background parameter but an active feature of how reality operates.

Why This Discovery Matters

Measuring time at the zeptosecond scale opens new possibilities for science and technology. It allows researchers to observe and model processes that were previously too fast to detect, including the earliest stages of light matter interaction. This could lead to improved understanding of radiation damage in biological systems, more precise control of chemical reactions, and advances in quantum technologies that rely on ultrafast timing.

The discovery also pushes physics closer to the limits imposed by nature itself. As measurements approach smaller and smaller intervals, scientists begin to encounter fundamental constraints related to quantum mechanics and relativity. These constraints suggest that there may be a lower bound to meaningful time measurement, beyond which the concept of time may no longer apply in the way we understand it.

Ultimately, this achievement represents more than a technical milestone. It demonstrates that time, even at the smallest scales we can access, is structured, measurable, and finite. By capturing a moment as brief as 247 zeptoseconds, scientists have shown that the universe operates through an ordered sequence of events, no matter how fast those events unfold. In doing so, they have brought us one step closer to understanding the true rhythm of reality itself.

Loading...