Zeptosecond

A zeptosecond (symbol zs) is one sextillionth of a second — 10⁻²¹ seconds, a one followed by twenty-one zeroes worth of time. It is the shortest unit of time that physicists have ever actually measured something in: in 2020, a team in Frankfurt timed a photon crossing a hydrogen molecule at 247 zeptoseconds, setting the record for the fastest process ever observed. Below the zeptosecond lies only one shorter named SI interval — the yoctosecond (10⁻²⁴ s) — and beyond both, the Planck time (~5.4 × 10⁻⁴⁴ s), where physics itself may stop making sense.^[1]

The ladder of small time

Time gets small fast. Each named step is a thousand times shorter than the last:

 nanosecond  10⁻⁹  s — light travels ~30 cm (one foot)
 picosecond  10⁻¹² s — chemical bond vibrations
 femtosecond 10⁻¹⁵ s — fastest chemical reactions (Nobel 1999)
 attosecond  10⁻¹⁸ s — electron motion (Nobel 2023)
 zeptosecond 10⁻²¹ s — photon crossing a molecule; nuclear photons
 yoctosecond 10⁻²⁴ s — quark-gluon plasma flashes; hypothetical physics
 Planck time 10⁻⁴⁴ s — below this, "time" may not exist

To feel the scale: a zeptosecond is to one second as one second is to about 31.7 trillion years — roughly 2,300 times the current age of the universe. Light, the fastest thing there is, travels barely a third of a nanometre — about the width of a small virus — in one zeptosecond.^[2]

How do you even time 247 zeptoseconds?

No clock ticks that fast — the measurement is made with interference, not a stopwatch. In the 2020 experiment, Reinhard Dörner's group at Goethe University Frankfurt fired an X-ray photon at a hydrogen molecule and measured which of the two atoms the photon hit first by watching the interference pattern of the ejected electrons. The difference between the two arrival readings — how long the photon took to cross the molecule — came out to 247 zeptoseconds on average. It remains the shortest time interval ever measured.^[3]

The technique descends from attosecond science, the field that won the 2023 Nobel Prize in Physics — Anne L'Huillier, Pierre Agostini and Ferenc Krausz showed how to generate and use light pulses lasting attoseconds (10⁻¹⁸ s) to photograph electrons in motion. Zeptosecond measurements push the same idea three orders of magnitude further.^[4]

Where zeptoseconds live in nature

• Light crossing a small molecule — ~100–500 zs
• Fluorescence of a photon inside a nucleus — nuclear gamma emission times can reach the attosecond-to-zeptosecond range
• Quark-gluon plasma — the "soup" of free quarks produced in heavy-ion collisions lives and dies on timescales of ~10⁻²³–10⁻²⁴ s — deep yoctosecond territory
• The Zeptoverse — the newly opened realm of physics at 10⁻²¹ metres: a photon crosses a zeptometre in about three billionths of a zeptosecond, and the rare decays that probe that realm unfold over femtosecond-to-picosecond beauty-particle lifetimes

Units smaller than this

Only the yoctosecond (10⁻²⁴ s) is smaller among named SI units — invoked for the fleeting lifetime of quark-gluon plasma states. Below named units entirely sits the Planck time, ~5.4 × 10⁻⁴⁴ s — the time light takes to cross one Planck length — which many physicists suspect is not a small quantity so much as a boundary: the point where "before" and "after" may stop being meaningful concepts. Nothing shorter than a Planck time is measurable even in principle under current physics.^[5]

See also

• Zeptoverse — the realm at 10⁻²¹ metres
• Smallest things ever measured — the full gallery of tiny records
• Scientific notation
• History of physics

References

• Grundmann, Senftleben et al. — "Zeptosecond birth time delay in molecular photoionization," Science 370:339 (16 October 2020)
• Nobel Prize in Physics 2023 — attosecond light pulses (L'Huillier, Agostini, Krausz)
• BIPM — SI Brochure, decimal prefixes (zepto- and yocto- adopted 1991)
• ALICE collaboration — quark-gluon plasma lifetime measurements, CERN