How Do We Know What Time It Is?

Credit: NIST

Time is probably the most measured quantity on Earth. It tells us when to wake and when to sleep; when to eat, work and play; when buses, trains and planes will depart and arrive. It helps organize and coordinate our lives. Scientists use time to measure and understand countless features of our world. 

Yet we cannot measure time directly. We cannot see it, hear it, taste it, touch it or smell it. Instead, we measure time intervals — the durations separating two events. “Time” is the accumulation of these intervals.

At first glance, telling time seems simple: You glance at your watch or phone, and there’s the time. These days, cellphones, computers and smartwatches provide highly accurate time and appear to set themselves. 

But behind this veneer of simplicity is an intricate global timekeeping effort involving hundreds of sophisticated atomic clocks operated by scientists located around the world. Each of us depends on a global network of atomic clocks that are continuously being measured, compared and synced to each other, and that are tuned to even purer and more precise timing tones produced by some of the best clocks ever made.

From early sundials and other rudimentary clocks, timekeeping has evolved into an exquisitely orchestrated global symphony that plays 24 hours a day, 365 days a year, literally never missing a beat. This symphony of time may be one of humanity’s most complex and important — and, perhaps, even beautiful — achievements.

Ticking Off the Nation’s Seconds

So how does it all work? Let’s start with the atomic clocks housed at NIST’s Boulder, Colorado, campus, one of the roughly 90 national labs around the world devoted to measurement science, or metrology.

Inside repurposed chicken incubators augmented with off-the-shelf humidifiers sit a dozen or so atomic clocks known as hydrogen masers. These masers are not as accurate as state-of-the-art atomic clocks, but they are very stable, meaning their ticking rate stays steady over time. 

Alongside the masers are about a half-dozen commercial cesium beam clocks, which are more accurate but less stable. Together, the masers and cesium clocks form the beating heart of U.S. civilian timekeeping. These “workhorse” clocks run day and night and never miss a beat. A failure would be disastrous, because we would no longer know what time it is.  

Keeping these workhorse clocks running is serious business. They are distributed among seven different rooms in case of a power failure and are constantly monitored. At least one NIST staff member is always on call, like an attentive parent, to fix any problems — even in the middle of the night.

Electronic counters read these clocks’ microwave-frequency time signals and send them to a central computer. That computer runs an algorithm to calculate a weighted average of the clock data and produces a single repeating signal that oscillates 5 million times per second. The more stable a clock is, the more weight the algorithm gives its time signals. 

Each five millionth “tick” of the averaged signal, a green light appears on a console deep at the heart of NIST’s time laboratory. That flashing light represents the ticking of seconds in the United States. From that moment, it’s a race to distribute the time to people around the country and beyond.

But this is only part of the story. The masers and beam clocks that drive the green flashing light are very good clocks, but they are neither perfectly stable nor perfectly accurate. To produce the pristine seconds that modern society demands, even better clocks are needed.

Exquisite Fountains

That’s where a device called a cesium fountain clock comes in. Fountain clocks are the most accurate and stable operational clocks the world has ever seen. The best are so good that if one had been running since the time of the dinosaurs, it would have lost or gained less than one second. These are the clocks used to realize the official second as defined by the International System of Units, or SI, also known as the metric system.

Fountain clocks must be custom-built, and they are so expensive and technologically advanced that only a few countries have them. And unlike masers and beam clocks, cesium fountains cannot reliably run day in, day out. Still, they play a crucial role in timekeeping: They’re the tuning forks that the world tunes its clocks to.

At NIST’s Boulder lab, scientists run two cesium fountain clocks, called NIST-F3 and NIST-F4, for days or weeks at a time. Their signals are used to calibrate and correct the NIST time scale, keeping it tightly tuned to the official international second. 

But even that is just part of the story.

A Global Timekeeper for a Globalized World

In today’s globalized world, nobody keeps time alone. In 1960, the nations of the world began jointly producing a time scale called Coordinated Universal Time, or UTC. (A time scale is an agreed-upon system for keeping time using data from clocks around the world.) Since the 1980s, an organization called the International Bureau of Weights and Measures, often referred to by its French-language acronym, BIPM, has overseen UTC.

NIST’s timekeepers send output from their clocks via satellite to BIPM, located in international territory outside of Paris. Some 90 other labs around the world send data from around 450 clocks to BIPM as well. 

NIST is not the only official timekeeper even in the United States. The U.S. Naval Observatory operates more than 100 masers, beam clocks and rubidium fountain clocks at labs in Washington, D.C., and Colorado Springs, making it the largest single contributor to international time.

NIST also provides data from its fountain clock NIST-F4, as do nine other national metrology labs that operate such clocks. A few labs, including NIST, also send data from optical clocks that are even more stable and accurate than fountain clocks, but are not considered primary frequency standards because the second is currently defined in terms of cesium.

To create UTC, BIPM’s scientists assign each contributing clock a weight based on how stable its output is, then calculate a weighted average of all the data. BIPM uses cesium fountain and optical clock data to calibrate the signal produced by averaging the other clocks.  

Each month, BIPM sends out a bulletin telling every national timekeeping institute how far its clocks are off from UTC. The institutes then correct and update their time scales. BIPM also sends weekly estimates that can be used to make provisional corrections in between the official monthly bulletins.

UTC itself is not available to us directly. Think of it not as a clock but as a global metronome, keeping the world’s timekeeping orchestra on the beat. We are interested not in the metronome beat itself, but in the symphony that it conducts.

From Atoms to You and Me

There remains one more problem to solve: how to send time into the world, from national time labs to the billions of people and machines that need it. 

In the U.S., NIST broadcasts its time scale, UTC(NIST), to the country and the world via radio stations in Fort Collins, Colorado, and the island of Kaua‘i in Hawai‘i. Clocks with radio receivers tuned to the 60 kilohertz signal broadcast from WWVB in Fort Collins hang in homes around the nation.

Another route for getting atomic time out of the lab and into the world is the internet. NIST operates around 20 servers synchronized to UTC(NIST) whose sole purpose is to provide the time upon request to devices on the internet, from routers to computers to other servers. NIST’s Internet Time Service receives more than a million hits per second, making it more popular than Google.

While it’s impossible to know exactly how many devices directly receive NIST time, a 2016 study estimated that 8.5% of all devices on the internet at the time had contacted just two of NIST’s time servers within the previous month. Many of these devices then relay the time to other machines. The synchronization of time across the internet is crucial for email time stamps, stock trades, the power grid and other critical functions. 

These days, however, we receive atomic time most directly through what’s in our pockets or on our wrists. Gone are the days when you might excuse yourself from being late for a meeting because your wristwatch was a few minutes slow. For better or worse, your phone and smartwatch always know the time to within a fraction of a second.

How is that possible? Phones and other smart devices contain internal electronic clocks that drift over time, like an ordinary wristwatch. But they get regular time updates from cell towers. Those towers each have a GPS receiver that picks up time signals from the 31 GPS satellites operated by the U.S. military. Each GPS satellite, in turn, has on board several atomic clocks based on the element rubidium.  

These atomic clocks are good, but they’re nowhere near as good as the best ground-based ones. If they weren’t corrected, GPS would start sending us astray, and our cellphone clocks and smartwatches would start running fast or slow. 

So the GPS clocks are regularly updated by a set of ground-based hydrogen masers, cesium beam clocks and rubidium fountain clocks operated by the U.S. Naval Observatory at the Schriever Space Force Base in Colorado.

By law, NIST and USNO are jointly responsible for U.S. time. They have agreed to sync their time scales to within 20 nanoseconds (billionths of a second) and compare them regularly via satellite. Because both NIST and USNO run such accurate and stable clocks and correct them regularly based on UTC, they rarely drift more than a few nanoseconds apart — despite having entirely independent sets of clocks halfway across the country from one another!

Many other countries produce time scales using variants of the process described above. Some run ensembles of clocks similar to those at NIST and USNO; others run a single cesium clock or hydrogen maser. The differences between each contributing country’s time scale and UTC are published monthly in a publication called Circular T, which is available on the BIPM website.

Most countries that don’t currently contribute to UTC use a commercial internet server with an embedded GPS receiver to distribute atomic time. Some countries also use the NIST internet time service.

The global atomic timekeeping symphony never stops. It’s playing when you wake up, while you’re working and playing, and as you sleep. In a world where harmony and cooperation can often seem in short supply, the way we keep time reminds us that when people and countries work together, they can produce something both useful and beautiful.

What is time?