Time feels steady and familiar in daily life. We experience it through routines, deadlines, memories, and the natural rhythm of day and night. Seconds tick by in an orderly fashion, minutes feel longer or shorter depending on our state of mind, and hours quietly shape how we work, rest, and connect with others. This lived experience of time feels solid and dependable, yet it represents only a tiny surface layer of how time actually behaves in the natural world.
Beneath this human perception lies a realm where time unfolds at speeds that defy intuition. At the level of atoms and subatomic particles, events happen so quickly that they blur into assumptions rather than observations. Scientists have long suspected that light, energy, and electrons do not act instantaneously, even across extremely small spaces. Now, for the first time, researchers have directly measured the shortest unit of time ever recorded, capturing a moment that lasts just 247 zeptoseconds. This fleeting interval represents the time it takes for a single particle of light to travel across one hydrogen molecule, revealing that even light follows a sequence rather than acting everywhere at once.
What Is a Zeptosecond and Why It Matters
A zeptosecond is a trillionth of a billionth of a second. Written numerically, it is a decimal point followed by twenty zeroes and then a one. This scale of time is so far removed from everyday experience that it can feel almost meaningless at first glance. Yet it is within these unimaginably brief moments that the most basic processes of nature take place, including the movement of electrons, the transfer of energy, and the behavior of light as it interacts with matter.
Before this discovery, scientists had already pushed time measurement into extremely small territory. In 2016, researchers reported measurements as short as 850 zeptoseconds, which at the time represented a major technical achievement. However, measuring even shorter intervals required new experimental strategies and greater precision. Moving from hundreds of zeptoseconds down to 247 zeptoseconds is not a minor adjustment but a substantial leap in resolution.
This matters because time defines how physical and chemical processes unfold. Chemical bonds break and form within femtoseconds, while the movement of light itself occurs on even shorter scales. By measuring time more precisely, scientists can better understand cause and effect at the most fundamental level. This knowledge improves theoretical models and supports progress in areas such as chemistry, materials science, and technologies that rely on light based interactions.
Image Credit: Sven Grundmann, Goethe University Frankfurt
How the Experiment Was Performed
To capture such a short interval, researchers deliberately chose the simplest possible molecular system. A hydrogen molecule contains just two protons and two electrons, making it an ideal candidate for observing basic physical interactions without added complexity. The experiment was carried out using intense X rays produced at the PETRA III particle accelerator at the Deutsches Elektronen Synchrotron in Hamburg.
Physicist Reinhard Dörner of Goethe University and his colleagues carefully adjusted the energy of the X rays so that a single photon would interact with the hydrogen molecule in a controlled way. As the photon passed through the molecule, it knocked one electron out first and then the second. This sequence produced two electron waves that spread out and overlapped, creating an interference pattern similar to ripples intersecting on water.
The team recorded this interaction using a Cold Target Recoil Ion Momentum Spectroscopy reaction microscope. This highly sensitive instrument allowed them to detect the interference pattern and track the precise position and orientation of the hydrogen molecule during the interaction. Because the researchers knew how the molecule was positioned in space, they could convert spatial data into an extremely precise measurement of time.
What Scientists Observed for the First Time
The measurements revealed something that had never been directly observed before. The hydrogen molecule did not respond to the incoming light all at once. Instead, there was a measurable delay between the photon interacting with one atom and then the other. As Reinhard Dörner explained, “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 the long standing assumption that light triggers an instant, uniform response across an entire molecule.
The delay occurs because information within the molecule spreads at the speed of light rather than instantaneously. Sven Grundmann, a coauthor of the study, described how the team calculated this timing by saying, “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.” This approach allowed the researchers to translate interference patterns into a direct measurement of time.
The final result showed that the photon took 247 zeptoseconds to travel from one atom to the other. There was slight variation depending on the distance between the hydrogen atoms at that exact moment, but the measurement clearly captured the speed of light operating within a single molecule. This marked the shortest time interval ever directly measured.
Why This Discovery Reaches Beyond Physics
Although this experiment sits firmly within fundamental physics, its implications extend far beyond the laboratory. Many modern technologies depend on precise interactions between light and matter. Medical imaging systems, radiation based therapies, and laser driven diagnostic tools all rely on understanding how photons behave when they encounter biological tissues.
In chemistry and pharmaceutical research, timing determines how molecules interact, bind, and change shape. More precise time measurements allow scientists to improve simulations used to study drug interactions and metabolic processes. Over time, this deeper understanding may support safer medications and more efficient research methods.
There are also implications for future technologies such as quantum computing and ultra fast electronics. As devices become smaller and operate at increasingly high speeds, controlling events at extremely short time scales becomes essential. Measurements at the zeptosecond level help define the physical limits within which these technologies must function.
A Wider Perspective on Time and Nature
This achievement reflects how rapidly scientific measurement has progressed over the past few decades. In 1999, the ability to measure femtoseconds was considered groundbreaking and earned a Nobel Prize. Today, scientists are working with time intervals thousands of times shorter, made possible by advances in instrumentation, experimental design, and theoretical insight.
Beyond its technical significance, this discovery offers a deeper perspective on how nature operates. Even within the smallest molecule, events unfold in a sequence governed by physical laws. Light, often described as instantaneous, still requires time to move and deliver information, even across atomic distances.
By continuing to study these fleeting moments, scientists gain a clearer picture of reality at its most fundamental level. Measuring the smallest slice of time ever recorded is not only a scientific milestone. It is a reminder that the universe still holds layers of detail that become visible only when we learn how to look more closely.
