Atomic Clocks: stories of a precision that changed the world
Atomic clocks are the most precise machines ever built by humankind. From Harold Lyons’s ammonia prototype in 1949 to strontium trapped in optical lattices, they marked the shift from observing the heavens to mastering quantum physics. Today they govern GPS, synchronise the world’s banking and test Einstein’s relativity. But they also hold some curious stories: a prototype with a wall clock on top, “failed” satellites that confirmed theories, and scientists who dared to redefine the second itself.
The world’s first atomic clock as it was presented on 6 January 1949, the day of the first Dr Harold Lyons (right), inventor of the ammonia absorption cell atomic clock, looks on A 1948 photograph showing the back of the first atomic clock and the electronic components
In 1949, the American physicist Harold Lyons, at the National Bureau of Standards (now NIST), unveiled the first working atomic clock, based on the ammonia molecule. The classic photograph of Lyons, with a wall clock placed on top of the prototype, became iconic: it was proof that this laboratory machine, which looked like a metal cabinet, measured time in an unprecedented way.
Louis Essen and the caesium that changed everything
The decisive leap came in 1955, when Louis Essen and Jack Parry, at the National Physical Laboratory (United Kingdom), built the first working caesium-133 clock. Essen already had experience in measuring the speed of light, and it was thanks to his rigour that caesium gained the status of a standard. It was this work that, in 1967, would lead to the redefinition of the second in the International System of Units.
The definition of the second and the role of institutions
Since 1967, the second has been the duration of 9,192,631,770 oscillations of caesium-133.
This decision, taken at Sèvres, moved the basic unit of time out of the astronomical realm (variations in the Earth’s rotation) and anchored it in quantum stability.
Since 1967, the second has been defined as the duration of 9,192,631,770 oscillations of the radiation of the caesium-133 atom, a decision taken at Sèvres by the General Conference on Weights and Measures. This redefinition marked the transition from the astronomical era to the quantum era: time no longer depended on the irregular rotation of the Earth, and its fundamental unit came to be based on an unchanging property of matter. Caesium-133 was chosen for its stability, its ease of handling in the laboratory and the reliable results already obtained with the first working atomic clock in 1955.
Accuracy and stability: a leading pair
Two concepts run through the whole story: accuracy, which measures closeness to the true value, and stability, which measures regularity over time. The physicist Jerrold Zacharias, of MIT, worked on caesium clocks as early as the 1950s and helped to clarify these concepts in practical applications.
From the laboratory to space: engineers and scientists behind GPS and Galileo
The GPS system was born of multidisciplinary teams in the United States, bringing together physicists, satellite engineers and the military. Caesium and rubidium clocks developed by groups such as that of James Jespersen ensured the system’s initial precision.
In Europe, the Galileo programme was driven by teams from ESA and industry, with contributions from hydrogen maser specialists such as John Vanier and engineers in Neuchâtel, Switzerland. In 2014, when two satellites ended up in the wrong orbits, researchers such as Pacôme Delva seized the opportunity to test general relativity with the clocks on board.
From caesium to light: the pioneers of optical clocks
The optical revolution has clear protagonists: Jun Ye, at JILA, who led the development of the world’s most precise strontium clocks; Hidetoshi Katori, in Tokyo, who created the “optical lattice” architecture for trapping strontium atoms with lasers; and Thorsten Tamm, at Germany’s PTB, who compared optical standards over long distances via optical fibre. These scientists paved the way for chronometric geodesy, a field that reveals differences in gravity by comparing clocks hundreds of kilometres apart. It is cutting-edge science built on the beat of atoms.
Three short stories to tell over a coffee break
1) A clock with another clock on top (1949). To show the public that his machine was a clock, Harold Lyons placed a wall clock on top of the ammonia prototype. The image became a symbol of the new era.
2) Louis Essen and the triumph of caesium (1955). Louis Essen faced criticism from several astronomers, who considered it risky to replace the observation of the stars with the regularity of an atom as the reference for measuring time. Decades later, his persistence was recognised: without him, the second would still be defined by the irregular motion of the Earth.
3) “Failed” satellites that confirmed Einstein (2014). Two Galileo satellites in the wrong orbits became a natural laboratory for testing general relativity. An engineering failure turned into one of the most elegant experiments in modern physics.
So what is all this precision for?
Satellite navigation: without atomic clocks and relativistic corrections, GPS and Galileo would become useless within minutes.
Telecommunications networks and banking: financial transactions and data flows are ordered to the nanosecond.
Fundamental science: optical clocks test fundamental constants, look for subtle variations over time and make chronometric geodesy possible.
And tomorrow?
Optical clocks already reach precisions of the order of 10⁻¹⁸, and standards based on strontium or ytterbium are now being discussed to replace caesium as the official definition of the second. When that day comes, the world will carry on as normal — but behind the internet, the satellites and cutting-edge science, the invisible protagonists will be the physicists and engineers who, from Harold Lyons and Louis Essen to Jun Ye and Hidetoshi Katori, transformed the way we measure time.
Take part
👉 You can answer Quiz 7 – Atomic Clocks here:
👉 And you can attend the lecture by Miguel Lino Diogo dos Santos at the Museu Medeiros e Almeida on 18/09/2025, from 6 pm to 8 pm. Admission is free with prior registration: Register for the lecture
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