Key Takeaway:
The second, the fundamental unit of time, is being refined through advancements in timekeeping technology. Throughout history, humans have sought to track time with greater precision, starting with the Neolithic site of Newgrange in Ireland. In 1967, the International System of Units established that a second corresponds to 9,192,631,770 oscillations of radiation emitted during specific energy transitions in the caesium-133 atom. As technology advanced, scientists realized that even this could be improved by leveraging elements with higher frequencies, such as strontium and thorium. Nuclear clocks, which measure transitions within an atom’s nucleus, represent a quantum leap in timekeeping.
Time governs nearly every aspect of modern life, from the devices in our pockets to the satellites orbiting Earth. Critical systems like communication networks, power grids, and financial institutions rely on precision timing. At the heart of it all lies the second, the fundamental unit of time. Yet, even now, the definition of a second is being refined, driven by groundbreaking advancements in timekeeping technology.
Humanity’s Timeless Pursuit
Throughout history, humans have strived to track time with greater precision. One remarkable testament to this pursuit is the Neolithic site of Newgrange in Ireland. Constructed over 5,000 years ago, its passage tomb aligns perfectly with the winter solstice, showcasing an early understanding of solar cycles.
By 2,000 BC, water clocks—devices regulating the flow of water into or out of a vessel—emerged as one of the first mechanical attempts at time measurement. Aristotle, centuries later, suggested using the revolution of celestial spheres as a universal clock. These systems laid the foundation for modern mechanical clocks, which dominated timekeeping until the late 19th century.
Defining the Second: From Days to Atoms
For much of human history, the second was simply defined as 1/86,400 of a day, reflecting the division of time into hours, minutes, and seconds. In 1967, however, the definition shifted to something far more precise: the vibration of caesium atoms. The International System of Units established that a second corresponds to 9,192,631,770 oscillations of radiation emitted during specific energy transitions in the caesium-133 atom.
This transition frequency—a measure of how electrons in an atom absorb and release energy—became the standard for time. However, as technology advanced, scientists realized that even this could be improved by leveraging elements with higher frequencies, such as strontium and thorium.
Exploring the Limits of Atomic Clocks
Atomic clocks achieve their precision by measuring the frequency of atomic transitions. For caesium, this process is highly accurate, but transitions at higher frequencies—like those of visible light—offer even greater precision. In 2021, researchers demonstrated strontium-based atomic clocks, which utilize visible light frequencies, marking a significant step toward redefining the second.
The quest for precision continues with the development of nuclear clocks, which measure transitions within an atom’s nucleus rather than its electrons. This represents a quantum leap in timekeeping, as nuclear transitions occur at frequencies millions of times higher than those of caesium. Thorium-229, with transitions triggered by ultraviolet light, has emerged as a prime candidate for this groundbreaking technology.
The Role of Frequency Combs
To measure such precise transitions, scientists employ tools called frequency combs. These laser-based devices emit pulses of light at evenly spaced frequencies, resembling the teeth of a comb. By comparing an unknown frequency with a comb’s reference frequencies, researchers can achieve unmatched accuracy.
In September 2024, a team of U.S. researchers successfully developed a frequency comb capable of working in the ultraviolet spectrum, a critical step toward creating nuclear clocks. This advancement pushes the boundaries of what is possible in timekeeping, setting the stage for redefining the second by 2030.
Implications for Science and Technology
Precision timing has far-reaching implications, from improving GPS accuracy to enabling breakthroughs in fundamental physics. Nuclear clocks, for instance, could measure time to the 19th decimal place, allowing scientists to study phenomena like gravitational waves and test theories of general relativity at unprecedented scales.
With these advancements, the era of the caesium second may soon come to an end. However, this transition signals not just a technical achievement but also a deeper understanding of time itself. As humanity steps into this new frontier, the promise of nuclear clocks offers unparalleled precision—and perhaps a glimpse into the mysteries of the universe.
