Atomic Clocks: A Tool for Building Investigative Timelines

Introduction

Time impacts every aspect of our lives, from our daily routines to the synchronization of global financial markets and the navigation of spacecraft across the solar system. For most of human history, time was measured by the sky: the arc of the sun, the position of the stars, and eventually by mechanical clocks that imitate these rhythms for practical use. While these clocks are still common today, the atomic age, beginning in the 1940s with the dawn of nuclear physics, changed how we define and measure time. By the mid-20th century, new technologies like telecommunications, navigation and computers needed precision far beyond gears and pendulums, and measuring the regular “ticks” of atoms provided the solution.

The world’s first atomic clock was developed by the U.S. National Bureau of Standards (later renamed the National Institute of Standards and Technology, NIST) in 1949 and measured the vibrations of ammonia molecules. By the 1950s, cesium-133 atoms had replaced ammonia, and in 1959 the NBS-1 atomic clock entered service as NIST’s primary frequency standard, becoming the “official clock” of the United States.

NBS-1, NIST’s primary frequency standard from 1959. “Nbs-1.jpg” by National Institute of Standards and Technology, Time and Frequency Division, via Wikimedia Commons. Public domain (U.S. government work).

As investigators, it’s critical that our timeline analyses are based on an accurate and reliable time standard. While many atomic clocks exist around the world, in the U.S. the NIST clock is the time standard most commonly used. This post explores what makes atomic clocks so accurate and how investigators can use them for timeline analysis.

The Role of NIST in Timekeeping

NIST, an agency of the U.S. Department of Commerce, is responsible for building and operating atomic clocks. Alongside the U.S. Naval Observatory (USNO), NIST maintains the national time standard, ensuring that the United States’ official time is synchronized with Coordinated Universal Time (UTC), the global standard. The time provided by NIST is so precise that UTC(NIST) and UTC(USNO) are equivalent at the 20-nanosecond level, an almost unimaginably tiny fraction of a second.

Through its website, time.gov, NIST offers the public direct access to this official time. When you visit the site, your device’s clock is compared to the NIST atomic clock, and the displayed time is corrected for network delays so you see the most accurate time possible.

How Clocks Work

Clocks work by counting a periodic event with a known frequency. For example, the pendulum in a grandfather clock repeatedly swings back and forth at the same rate, and the gears “count” the swings. The arm of the pendulum is adjusted so that each half-swing takes one second. One cycle per second equals one hertz (Hz).

In a quartz clock (a digital wristwatch, for example), a piece of quartz crystal is placed in an electronic circuit where it vibrates at a set frequency, typically 32,768 Hz. The circuit “counts” that frequency by dividing it by 32,768 to produce one-second ticks, which advance the seconds on the clock. Dividing a high frequency down to a low one increases accuracy.

A 32,768 Hz tuning-fork quartz resonator, the timekeeper inside most quartz watches. “QuartzDiapason.JPG” by Alain Le Rille, via Wikimedia Commons, licensed under CC BY-SA 2.5. Changes: centered on a wider white background. This adapted image is licensed under CC BY-SA 4.0.

How Atomic Clocks Work

An atomic clock keeps time by counting the tiny energy jumps, or “ticks,” of atoms, which happen in a perfectly steady rhythm as the electrons in the atom change levels. Unlike the Earth’s rotation, which wobbles and slows, or mechanical clocks, which can drift, atoms oscillate at a perfectly steady rate for as long as they exist.

Cesium atoms oscillate at a frequency of exactly 9,192,631,770 cycles per second. This frequency defines the length of one second in the International System of Units (SI).

Atoms change energy levels when they are exposed to very specific frequencies. If an atomic clock can generate the frequency that makes the atoms change, that frequency can be counted or divided down and compared. In this case, the generated frequency is the “tick.” The benefits are that these frequencies are natural properties and that they are very high, in the billions of hertz. If an atomic clock were off by 1 Hz at a frequency of 1 GHz (1 billion Hz), it would be off by one second in 31.7 years, or roughly 86 microseconds (0.000086 s) per day.

NIST’s best cesium oscillators, such as the NIST-F1 and NIST-F2 fountain clocks, can produce a frequency with an uncertainty of about 3 × 10⁻¹⁶. That translates to a time error of about 0.03 nanoseconds per day, or about one second in 100 million years.

NIST-F1 cesium fountain clock (1999). “Nist-f1.jpg” by National Institute of Standards and Technology, Time and Frequency Division, via Wikimedia Commons. Public domain (U.S. government work).

Why Is the NIST Atomic Clock So Accurate?

1. Unchanging atomic properties

• The frequency of atomic transitions (such as those in cesium or strontium) is a fundamental property of nature, unaffected by the environmental factors that plague other types of clocks, like weather or humidity.

• This means atomic clocks can maintain their accuracy over millennia.

2. Advanced cooling and trapping techniques

• By cooling atoms to near absolute zero, NIST clocks minimize the random motion that could blur the measurements.

• NIST scientists have developed a technique for measuring thousands of atoms simultaneously, increasing the precision of each tick.

3. Error reduction

• Innovations in laser technology and vacuum systems have reduced sources of error, such as the effects of stray electromagnetic fields on atoms.

• The latest clocks are so precise they can measure the tiny differences in time caused by changes in gravity over just a few centimeters, a direct demonstration of general relativity.

4. Continuous calibration and synchronization

• NIST’s clocks are constantly compared and synchronized with other national and international time standards, ensuring global consistency.

• The time signals sent to time.gov are corrected for network delays, so users receive the most accurate time possible.

The Impact of Atomic Clock Accuracy on Everyday Life

• GPS and navigation: GPS satellites rely on atomic clocks to calculate positions on Earth. Even a microsecond error could result in a navigation error of hundreds of meters.

• Telecommunications: Mobile phone networks and the internet depend on precise timing to synchronize data transfer and avoid signal collisions.

• Financial markets: Stock exchanges use atomic time to timestamp trades, ensuring fairness and regulatory compliance.

How time.gov Works

When you open time.gov, your browser checks your device’s local clock and then requests the official time from a NIST server. The server, synchronized with UTC(NIST), sends the time back, and the system measures the round-trip delay. The displayed time is corrected for this delay, so you see the most accurate time possible on your screen.

After that initial synchronization, the running clock on your device is based on its local oscillator, which may drift over time. For continuous accuracy, many operating systems and devices periodically resynchronize with NIST or other time servers using protocols like NTP (Network Time Protocol).

So Why Do I Need to Know All of This if I’m Just Trying to Build a Timeline?

That’s a lot of science, and some details may seem tangential to building a fire investigation timeline. But investigators must understand why their timelines are accurate if they intend to rely on them. A cornerstone of timeline analysis is synchronizing all “hard times” to a common reference. In the U.S., that reference is the atomic clock behind time.gov. Investigators need to align their clocks and also understand why this atomic standard is far more accurate than, say, a wristwatch.

Why Use TimeScene?

TimeScene aligns all of your event data to a single, accurate reference clock. It does this by synchronizing timestamps with an NTP server, a network service that distributes time derived from atomic clocks (more on NTP here). Investigators can focus on evaluating evidence and resolving cases instead of wrestling with complex calculations or inconsistent timelines, and be confident that their timelines are grounded in an atomic clock standard they can defend.

Image credits

Header image: “NIST-F2 cesium fountain atomic clock.jpg” by National Institute of Standards and Technology, Time and Frequency Division, via Wikimedia Commons. Public domain (U.S. government work).

Inclusion of these images does not imply endorsement of TimeScene by their creators.