A Brief History of Timekeeping from Sticks in the Ground to Caesium Atomic Clocks, Humans Have Been Keeping Track of Time with Increasing Accuracy for Millennia
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physicsworld.com Measurement: Time iStock/Greyfebruary A brief history of timekeeping From sticks in the ground to caesium atomic clocks, humans have been keeping track of time with increasing accuracy for millennia. Helen Margolis looks at how we reached our current definition of the second, and where clock technology is going next On 1 November 2018, when this article is published, I placing a stick upright in the ground and keeping Helen Margolis is a will have been working at the UK’s National Physical track of its moving shadow as the day progressed. fellow in optical Laboratory (NPL) in Teddington for exactly 20 years This method evolved into the sundial, or shadow frequency standards and six days. The reason I know this is easy – I joined clock, with markers along the shadow’s path dividing and metrology at the on 26 October 1998 and, with the help of clocks and the day into segments. National Physical calendars, I can measure the time that’s passed. But However, sundials are useless unless the Sun is Laboratory in what did people do before clocks came about? How shining. That’s why mechanical devices – such as Teddington, UK, e-mail helen. did they measure time? water clocks, candle clocks and hourglasses – were [email protected] Over the millennia a myriad of devices has been developed. Then, in the 17th century, pendulum invented for timekeeping, but what they all have in clocks were developed, which were far more accu- common is that they depend on natural phenomena rate than any preceding timekeeping devices. Their with regular periods of oscillation. Timekeeping period of oscillation (in the lowest-order approxi- is simply a matter of counting these oscillations to mation) was determined by the acceleration due to mark the passage of time. gravity and the length of the pendulum. Because this For much of history, the chosen periodic phenom- period is far shorter than the daily rotation of the enon was the apparent motion of the Sun and stars Earth, time could be subdivided into much smaller across the sky, caused by the Earth spinning about intervals, making it possible to measure seconds, or its own axis. One of the earliest known timekeeping even fractions of a second. methods – dating back thousands of years – involved Nevertheless, the Earth’s rotation was still the Physics World November 2018 27 Measurement: Time physicsworld.com Standardizing time (see box on p30). Their device was not truly a clock as it did not run continuously, and was simply used to Solar time is not the same everywhere. In the UK, for example, Birmingham calibrate the frequency of an external quartz clock at is eight minutes behind London, and Liverpool is 12 minutes behind. While intervals of a few days. Nevertheless, by studying how communication and travel times between major centres of population were slow, the resonance frequency depended on environmen- this mattered little. But the situation changed dramatically with the construction tal conditions, Essen and Parry had shown convinc- of railways in the 19th century. Having different local times at each station ingly that transitions between discrete energy levels caused confusion and increasingly, as the network expanded, accidents and near in well-isolated caesium atoms could provide a much misses. A single standardized time was needed. more stable time-interval reference than any stand- The Great Western Railway led the way in 1840 and “railway time” was ard based on the motion of astronomical bodies. As gradually taken up by other railway companies over the subsequent few years. Essen later wrote: “We invited the director [of NPL] Timetables were standardized to Greenwich Mean Time (GMT), and by 1855 time to come and witness the death of the astronomical signals were being transmitted telegraphically from Greenwich across the British second and the birth of atomic time.” railway network. However, it was not until 1880 that the role of GMT as a unified But showing that the new standard was stable was standard time for the whole country was established in legislation. Four years insufficient to redefine the second. A new defini- later, at the International Meridian Conference held in Washington DC in the US, tion had to be consistent with the old one within the GMT was adopted as the reference standard for time zones around the globe and technical limit of measurement uncertainty. Essen the second was formally defined as a fraction (1/86 400) of the mean solar day. and Parry therefore proceeded to measure the fre- quency of their caesium standard relative to the astronomical timescale disseminated by the Royal “master clock” against which other clocks were cali- Greenwich Observatory. brated and adjusted on a regular basis. In the meantime, astronomers had switched to using ephemeris time, based on the orbital period of From crystal to atomic the Earth around the Sun. Their rationale was that As technology progressed, the need for higher-reso- it is more stable than the Earth’s rotation, but unfor- lution timing increased. Pendulum clocks were grad- tunately for most practical measurement purposes it ually overtaken by quartz clocks, the first of which is impractically long. Nevertheless, the International was built in 1927 by Warren Marrison and Joseph Committee for Weights and Measures followed their Horton at the then Bell Telephone Laboratories in lead, and in 1956 selected the ephemeris second to the US. In these devices, an electric current causes a be the base unit of time in the International System quartz crystal to resonate at a specific frequency that of Units. As Essen put it: “Even scientific bodies can is far higher than a pendulum’s oscillations. make ridiculous decisions.” The frequency of such clocks is less sensitive to But ridiculous or not, he needed to relate the environmental perturbations than older timekeep- caesium frequency to the ephemeris second, a task ing devices, making them more accurate. Even so, he accomplished in collaboration with William quartz clocks rely on a mechanical vibration whose Markowitz from the United States Naval Observa- frequency depends on the size, shape and tempera- tory. Finally, in 1967 the General Conference on ture of the crystal. No two crystals are exactly alike, Weights and Measures decided that the time had so they have to be calibrated against another refer- come to redefine the second as “the duration of ence – this was the Earth’s period of rotation, with 9 192 631 770 periods of the radiation corresponding the second being defined as a 1/86 400th of the mean to the transition between the two hyperfine levels of solar day (see box above). the ground state of the caesium-133 atom”. There are problems with this definition of the sec- ond, however. As our ability to measure this unit The next generation of time improved, it became clear that the Earth’s More compact and less costly – albeit less accu- period of rotation is not constant. The period is not rate – versions of caesium atomic clocks have also only gradually slowing down due to tidal friction, but been developed, and applications have flourished. also varies with the season and, even worse, fluctu- We may not always realize it, but precision timing ates in unpredictable ways. underpins many features of our daily lives. Mobile In 1955 NPL set in motion a revolution in time- phones, financial transactions, the Internet, electric keeping when Louis Essen and Jack Parry produced power and global navigation satellite systems all rely the first practical caesium atomic frequency standard on time and frequency standards. But although the caesium transition has proved an enduring basis for the definition of the second, cae- Although the caesium transition has sium atomic clocks may now be reaching the limit of their accuracy and improvements may open up proved an enduring basis for the new applications. In response, a new generation of atomic clocks is emerging based on optical, rather definition of the second, caesium than microwave, transitions. These new clocks get their improved precision from their much higher atomic clocks may now be reaching operating frequencies. All other things being equal, the stability of an atomic clock is proportional to its operating frequency and inversely proportional to the limit of their accuracy the width of the electronic transition. In practice, 28 Physics World November 2018 physicsworld.com Measurement: Time NPL NPL though, the stability also depends on the signal-to- recently demonstrated a stability of one part in 1018 Then and now noise ratio of the atomic absorption feature. for averaging times of a few thousand seconds. How- Jack Parry and Louis In an optical atomic clock, an ultra-stable laser is ever, trapped-ion optical clocks have also demon- Essen developed locked to a spectrally narrow electronic transition strated stabilities well below those of caesium atomic their caesium in the optical region of the spectrum – the so-called clocks, and both types have now reached estimated frequency standard “clock transition”. The optical clocks being studied systematic uncertainties at the low parts in 1018 level. in 1955 (left) but scientists today are today fall into two categories: some are based on sin- This far surpasses the accuracy of caesium primary focusing on optical gle laser-cooled trapped ions and others are based standards and raises an obvious question: is it time clocks (right). on ensembles of laser-cooled atoms trapped in an to redefine the second once again? optical lattice. The former, a single laser-cooled ion in a radio- The future of time frequency electromagnetic trap, comes close to the The frequency of the selected optical standard would, spectroscopic ideal of an absorbing particle at rest of course, need to be accurately determined in terms in a perturbation-free environment.