The Unseen Race Against Time: From Atomic Clocks to Nuclear Clocks
At the ticket gate of Shanghai Hongqiao Railway Station, you stare at the "estimated departure time" on your phone, every tick of the minute hand tugging at your nerves—a testament to modern society's relentless pursuit of punctuality. As you finally dash onto the high-speed train just as the doors close, you might not realize that behind this race against time lies a meticulously crafted timekeeping system, built over a century of human effort, ensuring every second counts.
From atomic clocks to nuclear clocks, from Earth's rotation cycles to nuclear energy level transitions, humanity's measurement of time is breaking limits at a rate where precision increases by an order of magnitude every seven years. This silent revolution has long transcended the goal of "catching a train on time" and has become a key to unlocking the universe's mysteries.
01: From Sundials to Atomic Clocks: The Quantum Leap in Timekeeping
Humanity's perception of time began with observing celestial movements. Ancient Egypt's solar calendar and China's gnomon measurements attempted to translate the trajectories of the sun and moon into measurable time increments. However, the irregularity of Earth's rotation meant that this celestial-based "Universal Time" (UT) always contained errors—in the 19th century, marine chronometers could be off by several seconds per day, a potential disaster for transoceanic navigation relying on longitude calculation. It wasn't until the early 20th century, with the invention of the quartz clock, that humans first achieved stable measurement of the "second," though millimeter-scale mechanical vibrations still couldn't meet the demands of precision science.
The real breakthrough happened in the microscopic world. In 1949, Essen and Parry at the U.S. National Bureau of Standards created the world's first atomic clock by utilizing the energy level transition of cesium-133 atoms. This behemoth, occupying an entire room, used microwaves to excite outer electrons to transition between ground and excited states, generating a stable oscillation frequency of 9,192,631,770 cycles per second, providing a quantum-scale benchmark for the definition of the "second."
In 1967, the 13th General Conference on Weights and Measures officially defined one second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom," marking humanity's transition from "macroscopic phenomena" to "quantum resonance" in timekeeping.
Chinese scientists have made remarkable strides in this field. In 2024, the team led by Pan Jianwei at the University of Science and Technology of China developed a strontium atomic optical lattice clock achieving a staggering accuracy with both stability and uncertainty better than 5×10⁻¹⁸ over 10,000 seconds—meaning this clock would be off by less than one second in 60 million years, surpassing the accuracy of the U.S. NIST-F1 cesium fountain clock (3×10⁻¹⁶) and becoming the most accurate atomic clock in Asia.
This type of optical clock, based on laser cooling technology, traps strontium atoms in an optical lattice, greatly reducing errors caused by atomic thermal motion, providing a "mobile time reference" for space science experiments.
02: Atomic Clocks: The Cornerstone of Modern Civilization's Time
In the GPS satellite navigation system, each satellite carries four rubidium atomic clocks. These devices, smaller than 30 cm in length and width, provide positioning for global users with a precision of parts per ten billion of a second. But the value of atomic clocks goes far beyond this: in the LIGO gravitational wave detector, two laser interferometers 4,000 kilometers apart rely on atomic clocks synchronized to the nanosecond to capture the ripples in spacetime produced by merging black holes; at CERN's Large Hadron Collider, the data from trillions of particle collisions per second require picosecond-level timing accuracy to accurately reconstruct particle trajectories.
The core advantage of atomic clocks lies in converting time measurement into frequency counting. When cesium atoms resonate in a microwave field, the stability of their energy level transition frequency reaches the order of 10⁻¹⁴—equivalent to a clock that, if started at the Big Bang, would be off by less than one second today. The advent of optical clocks has pushed this precision to 10⁻¹⁸. By using lasers to excite optical transitions in strontium atoms (frequency about 430 trillion cycles per second), their stability is 1,000 times higher than microwave atomic clocks.
This "microscope of time" allows scientists to detect subtle changes in Earth's gravitational field. In 2023, Chinese scientists used an optical clock network to observe, for the first time, changes in gravitational acceleration in the Mount Everest region due to snowmelt, with a precision of 10⁻¹¹ g—equivalent to detecting a mass difference of 1 microgram in a 1-ton object.
A more profound impact lies in the construction of Coordinated Universal Time (UTC). This time scale, blending the uniformity of atomic time with the astronomical significance of Universal Time, is kept synchronized with Earth's rotation through leap second adjustments, serving as the unified time standard for global communications, finance, and power systems. At the Shanghai Stock Exchange, each transaction timestamp must be precise to the microsecond to avoid order matching errors caused by nanosecond-level discrepancies; in the national power grid's smart grid, the timing synchronization accuracy of the entire system must be better than 1 microsecond, otherwise the dispatch of distributed energy resources would fall into chaos.
03: Nuclear Clocks: A Paradigm Shift from Electron to Nuclear Transitions
As scientists turn their attention from outer electrons to the atomic nucleus, an even more stable timekeeping world is opening up. In September 2024, the team led by Ye Jun published nuclear clock research in Nature, reporting the first measurement of a nuclear energy level transition in thorium-229m. Unlike atomic clocks, which rely on electromagnetic interactions of electron clouds, nuclear clocks utilize quantum state transitions of protons and neutrons within the nucleus. The gamma-ray frequency corresponding to the nuclear energy level difference is four orders of magnitude higher than microwaves, theoretically capable of pushing timing precision to 10⁻²¹—meaning a clock that, if started at the birth of the universe 13.8 billion years ago, would be off by less than 0.1 seconds today.
The advantage of nuclear clocks stems from the nucleus's "resistance to interference." Electrons are highly susceptible to external electromagnetic fields, whereas the nucleus, bound by the strong nuclear force, is over 1,000 times less sensitive to electric fields. Ye Jun's team successfully measured the metastable state lifetime (about 10,000 years) of the 229mTh isotope and observed its 3.5 keV energy level transition for the first time by coupling a strontium atomic clock with a thorium-doped crystal.
Although the current prototype's accuracy is still slightly lower than top optical clocks, its technical path shows remarkable potential: by exciting the thorium nuclear transition with ultraviolet light (instead of the X-rays required for traditional nuclear clocks), the experimental setup is significantly simplified, clearing obstacles for space applications.
This technological breakthrough is reshaping the paradigm of fundamental physics research. In dark matter detection, the ultra-high stability of nuclear clocks could be used to search for primordial spacetime ripples; in measuring the fine-structure constant, frequency drift in nuclear transitions could reveal variations of fundamental physical constants over spacetime—crucial for testing Einstein's theory of general relativity. Even more imaginative is the "space nuclear clock plan": if a nuclear clock were placed on a satellite, away from Earth's gravitational field disturbances, its precision could potentially break the 10⁻²² barrier, becoming the ultimate tool for detecting gravitational redshift and verifying the equivalence principle.
04: The Ultimate Frontier of Spacetime Measurement: From Lab to Cosmos
At the Jiuquan Satellite Launch Center, successors to the "Mozi" quantum satellite await launch, carrying domestically developed optical clocks into space. This planned "Time and Frequency Science Experiment Satellite" will operate a cold atom clock in the microgravity environment of space for the first time, aiming for long-term stability on the order of 10⁻¹⁸. In the international arena, the U.S. "Cold Atom Space Clock" (ACES) has been operating on the International Space Station for years, with its combination of microwave and optical clocks verifying the "time dilation" effect predicted by general relativity—at 400 km altitude, clocks run about 7 microseconds faster per day than on Earth, an error precisely measured, making it one of the most stringent experimental validations of relativity.
Advances in precise timekeeping are also redrawing the boundaries of engineering. The "Ultra-Reliable Low-Latency Communication" (URLLC) scenarios in 5G require time synchronization accuracy between base stations of less than 100 nanoseconds, a specialty of rubidium atomic clocks. In autonomous driving, the construction of high-precision maps relies on lidar synchronized to the nanosecond, and the miniaturization of atomic clocks (like chip-scale atomic clocks, with a volume of only 1 cm³) is bringing this precise timekeeping into everyday life.
Even more forward-looking is the concept of a "spacetime reference network": using optical and nuclear clocks distributed globally to create a time coordinate system with errors not exceeding 10⁻¹⁸, providing a unified spacetime reference for future interstellar navigation and deep space exploration.
Epilogue
In the quantum information laboratory in Anhui, Pan Jianwei team's optical lattice clock is undergoing its 100,000th transition measurement. The lattice woven by laser beams in the vacuum chamber cools strontium atoms to near absolute zero. These precision instruments, seemingly distant from daily life, are in fact the "temporal nerves" of human civilization—from high-speed rail scheduling to gravitational wave detection, from smartphone navigation to satellite communication, behind every precise "tick" lies the stable resonance of the quantum world.
From nuclear energy level transitions to cosmic spacetime ripples, humanity's exploration of time never ceases. As nuclear clocks' UV lasers excite thorium nuclei's metastable states for the first time, and as optical clocks enter the microgravity environment of space aboard satellites, we are witnessing not just a breakthrough in timing precision, but a leap in our understanding of the laws of nature. Perhaps, as Ye Jun remarked in the paper, "When time measurement precision surpasses 10⁻²¹, we will ultimately discover that the essence of time is hidden within the quantum oscillations of the atomic nucleus."
This is a grand journey that began with the pursuit of punctuality to catch a high-speed train, but leads to the ultimate mysteries of the universe. In this process, humanity finally understands that the most accurate clock is not meant to chase time, but to understand time itself.
