The smallest time interval ever directly measured in an experiment is 247 zeptoseconds, where one zeptosecond equals a trillionth of a billionth of a second. That measurement, achieved by tracking how long it takes a photon to cross a hydrogen molecule, sits at the frontier of what today’s instruments can resolve. But physics suggests there may be an even more fundamental limit: the Planck time, roughly 5.4 × 10⁻⁴⁴ seconds, below which our current understanding of time itself may break down. Between the zeptosecond measurements scientists have actually pulled off and the Planck time that theorists invoke, there is a vast and fascinating gap that reveals how much we still have to learn about the nature of time.
The Zeptosecond Record
In 2020, physicists at Goethe University Frankfurt set a new record by measuring a time interval of 247 zeptoseconds. To get a feel for how brief that is, consider that one zeptosecond is 10⁻²¹ seconds. If you stretched one second out to the age of the universe, a zeptosecond would still be vanishingly small by comparison. The team measured how long it took for a single photon of light to travel across a molecule of hydrogen, which has a bond length of less than an angstrom. By firing X-rays at the hydrogen and tracking the interference pattern of electrons ejected from each side of the molecule, they could determine the tiny time delay between the two emissions. That delay, averaging 247 zeptoseconds, corresponds to the photon’s travel time across the molecular bond.1PubMed. Zeptosecond birth time delay in molecular photoionization
This wasn’t just a stunt to claim a record. The measurement demonstrated that physicists can resolve events happening on timescales far shorter than the motion of atoms, opening up the possibility of watching electrons rearrange themselves in real time during chemical reactions. Before this result, the shortest measured events were in the attosecond range, a thousand times longer. The jump to zeptoseconds represented a genuine leap in experimental capability.
Attosecond Physics and Why It Matters
For most of the past two decades, the cutting edge of ultrafast science lived in the attosecond regime, where one attosecond equals 10⁻¹⁸ seconds. The 2023 Nobel Prize in Physics recognized three scientists for their work generating attosecond light pulses, which allow researchers to photograph the behavior of electrons inside atoms and molecules. At this timescale, you can watch an electron respond to a jolt of energy, see how charge redistributes across a molecular bond, or track the very first steps of a chemical reaction before atomic nuclei have time to move.
Realistic simulations have shown, for instance, that attosecond pump-probe techniques can map out ultrafast oscillation periods as short as about 2 femtoseconds for electron wave packets in excited states of helium atoms.2PubMed. Attosecond pump probe: exploring ultrafast electron motion inside an atom More recent work has pushed the technique further. In 2024, researchers used synchronized pairs of attosecond X-ray pulses from a free-electron laser to study what happens inside liquid water immediately after an electron is knocked out by ionization. The electronic response was confined entirely to timescales shorter than a femtosecond, confirming that the rearrangement happens before any hydrogen atoms have a chance to budge.3PubMed. Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water
These experiments represent a fundamentally different way of doing science. Rather than inferring what happened from the products of a reaction, attosecond and zeptosecond measurements let physicists watch the process unfold in something close to real time. The practical payoffs range from better understanding of radiation damage in biological tissue to designing faster electronic components.
Pushing Toward Even Shorter Pulses
The 247-zeptosecond measurement detected a natural time delay. But generating controllable light pulses that short is a separate challenge, and recent theoretical work suggests it is within reach. A 2025 proposal described a method for producing gamma-ray pulse bursts lasting roughly 850 zeptoseconds by combining the microbunching behavior of free-electron lasers with laser-Compton scattering. The resulting pulses would carry far more energy than attosecond optical pulses, with signal-to-noise ratios good enough for practical experiments.4PubMed. Zeptosecond γ-Ray Pulses Generation via FEL-Driven Microbunching and Laser-Compton Scattering
If these pulses can be realized in a lab, they would open the door to probing nuclear processes and the internal dynamics of protons and neutrons. At the scale of about 10⁻²⁴ seconds (yoctoseconds), the strong nuclear force governs interactions between quarks and gluons. The quark-gluon plasma that briefly existed after the Big Bang, and that physicists recreate in heavy-ion colliders, evolves on yoctosecond timescales. Having controllable zeptosecond pulses would bring experimentalists much closer to directly imaging those processes rather than reconstructing them from collision debris.
The Planck Time and the Theoretical Floor
Below the zeptosecond frontier lies an enormous desert of timescales that no experiment has touched. The theoretical floor is the Planck time, approximately 5.4 × 10⁻⁴⁴ seconds. It is derived from three fundamental constants: the speed of light, the gravitational constant, and the reduced Planck constant. The Planck time is not just an arbitrary combination of constants, though. It marks the scale at which quantum effects and gravitational effects become equally important, meaning our separate theories of quantum mechanics and general relativity can no longer be applied independently. We would need a complete theory of quantum gravity to describe what happens at or below this scale, and we do not have one yet.
Theoretical work in quantum gravity has explored what might happen near the Planck time. One study found that the standard quantum-mechanical evolution operator, which describes how a system changes from one moment to the next, behaves normally for time intervals longer than the Planck time but becomes fundamentally different for intervals shorter than it.5Journal of High Energy Physics. Probing the Planck scale: the modification of the time evolution operator due to the quantum structure of spacetime In plain terms, the math that physicists use to predict how things change over time stops working properly when you try to apply it to sub-Planck-time intervals. This does not necessarily mean time “doesn’t exist” below the Planck time, but it does mean that our current tools for describing time are inadequate there.
Other approaches within loop quantum gravity and related frameworks have explored whether spacetime itself might come in discrete chunks. Work in unimodular loop quantum cosmology has found that operators related to spacetime volume can have purely discrete spectra, meaning the allowed values are separated by gaps rather than forming a smooth continuum.6Classical and Quantum Gravity. Spacetime quanta?: the discrete spectrum of a quantum spacetime four-volume operator in unimodular loop quantum cosmology If spacetime really is quantized in this way, the Planck time might not just be a practical limit on measurement but an actual grain size of time itself.
The Chronon Hypothesis
The idea that time comes in indivisible chunks has a long history in physics, predating modern quantum gravity. The “chronon” is a hypothetical quantum of time, the smallest possible duration, analogous to how a photon is the smallest unit of light. Various proposals have assigned different values to the chronon, but a common suggestion places it near the Planck time. Recent theoretical work has developed the chronon concept further, introducing a hierarchy of temporal layers and exploring how a dual description of physical time, combining continuous and discrete aspects, might work.7Symmetry. Complementary Continuous-Discrete Time, Chronon Layering and Temporal Folding
The chronon remains firmly hypothetical. No experiment has detected a graininess to time, and there is no consensus among physicists that time must be discrete. The idea is appealing to some because it would resolve certain mathematical infinities that plague attempts to combine quantum mechanics with gravity. But it also raises philosophical puzzles: if time has a smallest unit, what does it mean for something to “happen” during a single chronon? Does the concept of “before and after” still apply within it? These questions sit at the intersection of physics and philosophy, and they do not have agreed-upon answers.
Quantum Speed Limits
Even setting aside whether time has a minimum grain, quantum mechanics places its own constraints on how fast things can change. The quantum speed limit is a fundamental bound on how quickly a quantum system can evolve from one state to a distinguishably different one. It arises from the time-energy uncertainty relation, a close cousin of the more famous position-momentum uncertainty principle. In essence, a quantum state cannot transform instantly; there is always a minimum time required, determined by the system’s energy.8PubMed Central. Quantum speed limit time in two-qubit system by dynamical decoupling method
These bounds matter beyond pure theory. In quantum computing, the speed at which a qubit can be flipped or entangled with another qubit is limited by the quantum speed limit. In quantum chemical dynamics, the rate at which molecules can rearrange is bounded the same way. Deriving tighter bounds on these speeds, including for systems that interact with their environment and lose quantum coherence, is an active area of research with direct implications for how fast quantum technologies can operate.9PubMed. Quantum speed limits in open system dynamics The quantum speed limit is not a smallest unit of time in the same sense as the Planck time, but it does mean that below certain durations, no physical process can produce a measurable change of state.
Atomic Clocks and the Precision of Timekeeping
While the experiments above push toward shorter and shorter events, a parallel effort in physics focuses on measuring ordinary time intervals with extraordinary precision. The two goals are related but distinct. Attosecond physicists want to see fast events; atomic clock researchers want to count seconds with as little error as possible.
The current definition of the second is based on microwave transitions in cesium atoms, but optical atomic clocks, which use higher-frequency light to “tick” much faster, have long surpassed cesium in accuracy. Ytterbium optical lattice clocks, for instance, have been under development since the early 2000s as part of an international effort to redefine the second, potentially as soon as 2030.10150th anniversary of the Metre Convention — From Units to the Universe. Ytterbium Optical Lattice Clocks at KRISS for the Redefinition of the Second – Their Development and Comparison These clocks tick hundreds of trillions of times per second, making them sensitive enough to detect incredibly subtle effects.
How subtle? In 2022, physicists at JILA demonstrated that two atomic clocks separated by just one millimeter, roughly the width of a pencil tip, tick at measurably different rates because of Einstein’s general relativity. The higher clock, being slightly farther from Earth’s center of gravity, experiences time very slightly faster. After 90 hours of data collection, the team achieved a measurement precision 50 times better than any previous clock comparison, resolving a gravitational redshift at the level of 10⁻¹⁹.11National Institute of Standards and Technology (NIST). JILA Atomic Clocks Measure Einstein’s General Relativity at Millimeter Scale That is not a short duration in the same sense as a zeptosecond event, but it represents an ability to distinguish time differences so minute that they border on the surreal.
This kind of precision has practical consequences. Gravitational time dilation affects GPS satellites, which must correct for relativity to give accurate positions. As clocks get more precise, they become sensitive to smaller height differences, potentially enabling new forms of geodesy where clocks measure the shape of Earth’s gravitational field with centimeter-level resolution.
The Gap Between Measurement and Theory
It is worth pausing to appreciate the sheer scale of the gap between what has been measured and what theory predicts. The 247-zeptosecond record sits at 10⁻²¹ seconds. The Planck time is at 10⁻⁴⁴ seconds. That is a factor of about 10²³ between them, a gulf comparable to the ratio between the size of an atom and the size of the observable universe. Bridging it with direct measurement would require technologies that do not exist and may never exist, at least not in any form we can currently envision.
Some physicists have proposed indirect approaches. Certain quantum gravity models predict that the granularity of spacetime at the Planck scale would leave faint signatures in the light from distant astronomical sources, slightly blurring images or altering the arrival times of photons at different energies. Observations of gamma-ray bursts from billions of light-years away have been used to search for these effects, and so far, the results are consistent with spacetime being smooth down to very small scales. That does not rule out Planck-scale structure, but it constrains the more dramatic proposals.
The honest summary is that we have directly measured time intervals roughly a billion times shorter than a nanosecond, and we have theoretical reasons to think there is a floor roughly a billion-trillion times shorter still. Between those two landmarks, there is almost nothing but educated speculation and a lot of challenging math.
How Human Time Perception Compares
All of this sits in stark contrast to how humans actually experience time. The shortest interval you can consciously perceive, where you notice that two events did not happen simultaneously, is on the order of tens of milliseconds. That is around 10⁻² seconds, which is roughly 10¹⁹ times longer than the zeptosecond record. Your brain is working with a clock that ticks about ten quintillion times more slowly than the fastest events physicists can measure.
And even that subjective clock is unreliable. Research in neuroscience has documented a range of temporal illusions in which perceived durations are distorted by factors like eye movements, unexpected stimuli in a sequence, or the complexity and magnitude of what you are looking at. Time perception bends in measurable ways depending on attention, emotion, and sensory context, meaning the “now” you experience is already a rough average constructed by your brain after the fact. The precision gap between human perception and physical measurement is not just large; it is almost incomprehensibly so. Every scientific instrument that measures sub-nanosecond events is, in a real sense, extending our senses into a domain that evolution never needed us to access.