Louis Essen: Time lord of precision atomic clocks

01 March 2023
Jeff Hecht
Louis Essen (right) and Jack Parry standing next to the world’s first cesium atomic clock
Louis Essen (right) and Jack Parry standing next to the world’s first cesium atomic clock, developed at the UK National Physical Laboratory in 1955.

Louis Essen’s 1955 invention of the first practical atomic clock changed the basis of timekeeping from the periodic motion of the Earth, as recorded by astronomers, to the periodic motion of electrons in cesium atoms as measured by physicists.

That is not just an esoteric exercise in fussy academic physics. The fast pace of our tech-driven lives depends on precision timekeeping. Essen’s clock has everything to do with the next time you don’t get lost—GPS relies on exceedingly accurate time measurements. We also can credit Essen with pinning down the speed of light to nine-digit accuracy, without which we couldn’t dream of robotic cars or spacecraft steering themselves with pinpoint precision.

The significance wasn’t lost on the science and people of Essen’s time, either. Soon after the debut of Essen’s clock, astronomer Gerald Clemence suggested naming the atomic second the “essen.”

But Essen would have none of it, writes his son-in-law Ray Essen in Revolutions in Time, a biography based on the elder Essen’s unpublished memoir, Time for Reflection. “Rather than being flattered, he felt it would duplicate one of the most fundamental units of measurement [...leading] to widespread confusion.”

Such practical humility was characteristic of Essen, who had grown up poor in a house that lacked indoor plumbing. By the time of his death in 1997 at age 89, however, he was a Fellow of Britain’s Royal Society, a reflection of his long and distinguished science career. In one obituary, Royal Society colleague Sir Alan Cook called Essen “a modest and somewhat self-effacing man devoted to the pursuit of the highest precision in electrical measurements.” The Guardian newspaper was more succinct, dubbing Essen a “time lord.”

 Indeed, poverty in childhood didn’t hold Essen back. In 1928, the 19-year-old graduated with first-class honors from University College Nottingham in the UK. With a scholarship, he was able to continue postgraduate study there in physics. But he had also grown tired of living with his cash-strapped parents in Nottingham. After hearing of a plum position—junior scientific assistant—at the National Physics Laboratory (NPL) near London, he applied, and was pleased to start in 1929 at an annual salary of £175.

Essen’s job was to assist David Dye, who directed NPL’s division of electrical standards and measurements. They got along well, and Dye’s quest for accurate measurement rubbed off on Essen. They were working on a 20-kHz quartz ring oscillator to measure radio frequencies in 1932 when a bout with pneumonia killed Dye at the young age of 44. NPL asked Essen to finish the oscillator, and his improvements made it a laboratory standard.

Then he built a better one. Essen had mastered the technology so well he was able to design and build a new clock resonating at 100 kHz, a frequency much more useful for the young radio industry.

In 1936, NPL made the 100-kHz clock its primary standard of time and frequency. Essen built five copies for official timekeeping, which served accurately and reliably for more than 20 years at the US Naval Observatory, the Royal Observatory in Greenwich, UK, and elsewhere. Essen also wrote a thesis on the new clock, which earned him a doctorate.

The onset of World War II shifted Essen’s work from precision clocks to microwave electronics for military systems. NPL designated him as its expert on the speed of light to answer questions from industry and government agencies, which needed to know it very precisely for military targeting. After the war ended, he measured the speed of light with record nine-digit precision.

Essen also realized that the gigahertz frequencies of microwave electronics could offer the next big advance in time measurement. They could measure time down to billionths of a second and be locked to natural resonances of atoms and molecules in the gigahertz range to create atomic clocks stabilized to very high precision. However, postwar Britain lacked the needed money.

The US had money to build an atomic clock and, in 1947, the US National Bureau of Standards (NBS), now the National Institute of Standards and Technology, put physicist Harold Lyons in charge. Lyons based his atomic clock on the 24-GHz resonance of nitrogen molecules. He spiffed up his equipment for public display by topping it with an ordinary electrical wall clock, and a gold-plated loop filled with ammonia.

The US clock made headlines, but it did not impress Essen. Its frequency was unstable, and it could only run for a few hours at a time. He sketched out a two-page proposal for NPL to make a more-practical atomic clock using cesium atoms.

Cesium is a soft alkali metal that ignites in air and water, so it can be troublesome. But Essen thought cesium had important advantages for an atomic clock. It has one outer electron, a single stable isotope, a simple and narrow spectrum, and a resonance at 9.192631770 GHz that microwave sources could match.

Essen received no response to his proposal until 1950 when a new NPL director said he liked the idea. But money was still tight. When Essen visited the US in 1953, he learned NBS was working on a cesium clock, but its performance couldn’t match even Essen’s quartz clocks. When he returned, Essen thought the US was ahead. Although he expected the US would make the first atomic clock, he knew a second independent clock would be needed for comparison. He persuaded his boss they could launch a program to build one without hiring new staff, and they charged ahead.

Their key goal was an atomic clock able to operate for a long time with atomic accuracy. Essen decided he could achieve that by using an atomic beam device to control his quartz oscillators. He and microwave expert Jack Parry spent long hours measuring electromagnetic fields and installing high-vacuum equipment to contain the cesium. It worked for the first time on 24 May 1955, and within months, it was recognized as the first fully operational atomic clock.

The cesium clock heated up a simmering dispute between astronomers and physicists over time standards. The length of the second had long been based on the length of the day, but Essen’s quartz clocks had detected fluctuations in length of the day. In 1956, astronomers redefined the second in terms of the length of the year. By then, however, cesium clocks had proved better timekeepers than the length of the year, leading Essen and other physicists to urge redefining the second in atomic time.

Trying to save the astronomical second, Clemence, who was head of the US Nautical Almanac Office, wrote to Nature proposing naming the atomic second after Essen. But the idea went nowhere. In 1967, the General Conference on Weights and Measures defined a second as the duration of 9,192,631,770 waves of a cesium clock, a measurement that remains the global standard today.

Essen and others suggested keeping astronomical time in phase with the more accurate atomic time by adding an occasional leap second. Physicists and astronomers accepted the compromise in 1972 but it is scheduled to be eliminated in 2035.

Near the peak of his career in the 1950s, Essen got himself into trouble by publicly challenging time dilation in Einstein’s special theory of relativity. One NPL colleague called it “scientific blasphemy coming from a Fellow of the Royal Society.” Others were simply bemused. But when Essen continued the argument, he was pushed to retire in 1972.

Unfortunately, Essen’s misadventures in relativity may have faded his name, but his remarkable success in precision clocks and measurement is the legacy of a Time Lord that lives on in 21st century technologies.

Jeff Hecht is an SPIE Member and freelancer who writes about science
and technology.


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