As the decades passed, however, it became clear that a better way of synchronising time was required. The timekeepers of Greenwich might have laid a claim to operate some of the world’s most accurate clocks, but they based their calculations on an unreliable reference: the time it took for Earth to spin through one rotation.

To deliver accurate time, all clocks require a periodic, repeating process – whether it’s a swinging pendulum or the electronic oscillations of a quartz crystal. The clocks at Greenwich were calibrated using the time it took for the Sun to reach the same position in the sky after a day. Their pendulum was therefore the Earth itself, spinning at a seemingly predictable rate. (This also applied to Universal Time, which replaced GMT in 1928.)

However, in the 20th Century scientists realised that our planet’s rotation speeds up and slows down over the years, due to gravitational effects from the Moon, Sun and other planets, geological shifts within the core and mantle, and even oceanic and climatic changes. In 1900, it was spinning almost 4 milliseconds slower, on average, than it was at the turn of the 21st Century. So while the world’s best timekeepers could claim greater accuracy than the average watch or grandfather clock, they themselves were wrong about the “true” time.

Atomic time

Around the same period, quantum physicists suggested that atoms might contain a far better way of keeping time than Earth’s rotation. Apply a specific frequency of electromagnetic radiation to an atom and its energy levels change. You can use an electronic counter to keep track of these transitions. Like a swinging pendulum, this makes for a stable periodic process on which to calculate a timescale. It would prove to be the basis for the “atomic clock”.

Atomic clocks keep time far more accurately than any clock based on Earth’s rotation – so accurate in fact, that if we fully based our world on them, eventually time would depart from night and day so that the Sun would rise at 18:00 in the evening. It’s why the world’s timekeepers add leap seconds every so often.

The hydrogen masers at NPL in London are some of the world’s most important atomic clocks. There are several hundred more of them around the world, operated by national metrology institutes, and they are the new arbiters of time for all of us. But it’s not quite as simple as reading out the time from them: no atomic clock is perfect, due to things like local gravitational effects or differences between their electronics.

Metrologists, therefore, need to iron out those imperfections. Here’s how that works: a lab like NPL records and refines timing information from its bank of atomic clocks – the hydrogen masers – applying the occasional correction if the clock appears to be drifting (metrologists call this “steering” and they do it using separate equipment used to define the length of a second…we’ll return to that later.)

NPL then sends that to the International Bureau of Weights and Measures (BIPM) in Paris. The timekeepers at the BIPM create an average of all those measurements, giving extra weight to the better-performing clocks. Further adjustments are made, and eventually, this process spits out what’s called International Atomic Time (TAI – Temps Atomique International).

Once a month, the BIPM sends out TAI in an extremely important document called “Circular-T” (You can download the latest one here.) This document allows national laboratories to steer their clocks again, and crucially, to distribute a precise time to the industries that need it. For the UK, that’s NPL’s job, but in the US it’s the National Institute of Standards and Technology and there are many more around the world. Circular-T essentially informs the modern-day equivalent of the dropping ball at Greenwich.

While most people don’t need to know the time down to the nanosecond, many industries and technologies do. “Satellite navigation is probably one of the most ubiquitous high accuracy requirements but there are others,” says metrologist Patrick Gill at NPL. “Communication synchronisation, energy distribution and financial trading all require high precision time.” New technologies also bring additional demands: the 5G network is built on precise synchronisation, for example, as is the navigation technology guiding autonomous vehicles.

The thing is, though, TAI is still a construct of a hypothetical “true” clock time: a measurement that the world is merely agreeing to keep to. It’s not just that it’s a weighted average of many different atomic clocks, each one giving slightly different readouts. There’s another reason, and it boils down to a fundamental question: what exactly is a second? Over the years, the definition of this SI unit has changed –  and therefore so has our definition of time. What’s more, it could change once again soon.

Redefining the second

It used to be that the second was defined as 1/86,400 of the mean solar day – the average time it takes for the Sun to reach the same point in the sky at midday, which takes approximately 24 hours. In other words, it was based on the Earth’s rotation, which we now know is irregular. The second, by this definition, would therefore have been longer in 1900 than it was in 1930 when the planet’s average rotation was faster. (Metrologists once had a similar problem with the kilogram: it was based on a block of metal held in a vault in Paris, but it would inexplicably change over time, and therefore so did everyone’s definition of the kilogram.)

Midway through the 20th Century, metrologists decided that this would not do. So, they created a new definition for time. In 1967, it was decided that the second should instead be based on a fixed numerical value of the unperturbed caesium ground-state hyperfine transition. “It’s a bit of a mouthful,” admits Gill. So, what does it mean?

Fundamentally, it’s just another periodic, repeating process – the basis for all timekeeping. If you bathe caesium atoms in microwaves, they release more electromagnetic radiation, with a specific frequency that depends on the energy levels within the atom. By measuring this frequency – like counting pendulum swings – you can measure time’s passage.

The scientists at NPL do this with what’s called a caesium fountain. “We use light to toss the atoms up in the air by about half a metre and they fall back under gravity. You can then interrogate that fountain using tunable microwaves,” explains Gill. The fountain setup is necessary, because “you want to make them as unperturbed as possible. If you’re holding the atoms by some other means, like electrically, or using light to hold them, that will change your frequency.”

This definition was chosen because caesium is reliable as an isotope – virtually all atoms in a sample will respond to electromagnetic radiation in the same way. Also, in the 20th Century, microwave frequencies could be more accurately and reliably measured than higher frequencies on the electromagnetic spectrum. It’s perhaps analogous to the way that you can measure your own heartbeat with a stopwatch, but you need more advanced technology to measure the frequency of a fly’s wings.

NPL is developing optical clocks based on neutral strontium atoms… with lasers (Credit: NPL)