49 Fun Facts About Atomic Clocks

An atomic clock keeps time by monitoring what property of atoms?

Their resonant frequency is the one light frequency that makes the atoms jump between energy levels, so the clock tunes its light to it and counts the cycles.

What kind of energy-level split in the caesium-133 atom defines the SI second?

A hyperfine transition is driven by microwaves, so atomic clocks had to wait for the radar and radio research of the 1930s and 1940s to generate them.

Which Nobel-winning invention lets scientists count an optical clock's ticks?

An optical frequency comb bridges the gap between light's frequencies and the radio frequencies electronic counters can measure, so an optical clock's ticks can be counted.

Before caesium took over, the second was briefly redefined as a fraction of which period instead of the day?

The tropical year for 1900 was divided into 31,556,925.9747 seconds, and caesium's frequency was measured against that ephemeris second so the second's length carried over.

Whose 1873 Treatise proposed measuring time by the vibrations of light waves?

James Clerk Maxwell called light's vibration a more universal unit of time than the Earth's rotation, which then set the mean solar second.

Who built the first practical caesium atomic clock in 1955 at Britain's National Physical Laboratory?

Louis Essen later became a critic of Einstein's special relativity, publishing a book in 1971 that questioned the theory.

The first atomic-clock prototype, built in the late 1940s, measured transitions in which molecule?

Ammonia gave a clock hardly better than existing standards, so researchers turned almost at once to more promising caesium beam devices.

Which 1944 Nobel physicist built atomic-beam magnetic resonance equipment that led toward atomic clocks?

Isidor Rabi first publicly proposed building an atomic clock in a 1945 lecture, and The New York Times reported his idea to the public.

Norman Ramsey won the 1989 Nobel Prize in Physics for inventing what?

The separated oscillatory field method gave sharp atomic resonances without needing a perfectly uniform magnetic field, and it became the basis of the caesium clock.

Which company launched the rack-sized 5060 caesium clock in 1964, bringing caesium standards into calibration labs?

Hewlett-Packard carried two of its caesium clocks to Switzerland in 1964, matching US and Swiss time standards to about a microsecond.

Which caesium clock became a US primary time standard when brought online in 2014?

NIST-F2 came online in April 2014 to run beside NIST-F1, and NIST machined duplicate parts so that Italy's metrology institute in Turin could build a sister clock.

NIST's atomic clocks that set America's official civilian time are housed in which Colorado city?

Boulder is home to NIST's time lab, where the record-setting clocks run for only days or weeks at a time: their job is to calibrate the workhorse clocks that tick nonstop.

Roughly what timing error corresponds to a positional error of about 30 cm in satellite navigation?

1 nanosecond is how long light takes to cross about 30 cm. Computing pioneer Grace Hopper handed out wires of that length so her audiences could hold a nanosecond in their hands.

Relativity predicts GPS satellite clocks run how much faster per day than ground clocks?

38 microseconds a day is the net effect: weaker gravity speeds the orbiting clocks up by 45, and their speed slows them by 7. Left uncorrected, GPS positions would drift by about 10 km every day.

Which institution keeps the US military's master clock, which GPS follows?

The Naval Observatory has served out the time since 1845, when it began dropping a ball at noon so that navigators on nearby ships could set their chronometers.

In most atomic clocks the atoms only correct an oscillator that does the actual ticking, made from which crystal?

Quartz on its own drifts as it ages and as the temperature changes, so the clock keeps checking the crystal against its atoms and pulling it back into line.

Atomic clocks at radio telescopes time-stamped the data behind which 2019 first?

A black hole image needed radio dishes from Hawaii to the South Pole to act as one Earth-sized telescope, so each stamped its data with an atomic clock before the hard drives were flown to supercomputers to be combined.

What is occasionally added to atomic time so that civil time (UTC) stays in step with Earth's uneven rotation?

Leap seconds were added 27 times between 1972 and 2016. They crashed enough computer systems, including Reddit's and Qantas's in 2012, that the world's metrologists voted in 2022 to stop adding them by 2035.

Which stars spin so steadily that astronomers check the best of them against atomic clocks?

Pulsars are collapsed stars whose radio beams sweep past Earth like lighthouse lamps. The steadiest rival atomic clocks over decades, so astronomers time dozens of them to catch passing gravitational waves.

Since 1983, which unit has been defined by how far light travels in a set fraction of a second?

The metre is how far light travels in 1/299,792,458 of a second, which turned every precise ruler into a clock reading: the speed of light became a fixed number that can no longer be measured, only used.

In the 1971 Hafele-Keating experiment, four caesium clocks were flown around the world aboard what?

Commercial airliners on ordinary scheduled flights carried the clocks once eastward and once westward. The eastbound clocks came home behind the ones left on the ground and the westbound clocks ahead, as relativity predicted.

Which element runs the small, cheap atomic clocks found in phone networks and aboard GPS satellites?

Rubidium clocks fly on GPS satellites but drift by a few billionths of a second a day, so steadier clocks on the ground correct them. The same cheap design keeps phone networks and TV stations in step.

Which commercial atomic clock is the steadiest over short spans, which makes it the usual timekeeper at radio telescopes?

Hydrogen maser: its atoms amplify their own microwave signal. The maser came before the laser, which borrowed both the idea and most of the name.

The ticking heart of the chip-scale atomic clock that NIST demonstrated in 2004 was about the size of what?

A grain of rice was all the room its atoms needed. The clock was meant to fit a soldier's backpack, and it also found a market with oil and gas prospectors, who use it to time sensors on the ocean floor.

Optical clocks tick hundreds of trillions of times a second, but caesium's microwave clocks manage roughly how many?

Ten billion, near enough: 9,192,631,770 beats is the count that defines the second. Light's faster waves slice each second far more finely, just as a ruler with finer marks measures smaller things.

Which element, the one that colours fireworks red, ticks at 429 terahertz in JILA's record-setting lattice clocks?

Strontium atoms are read out thousands at a time, and averaging over them all at once is what makes lattice clocks the most stable ever built, ahead of clocks that watch a single ion.

In 2020 Japanese physicists tested Einstein by comparing atomic clocks at the foot and near the top of which landmark?

Tokyo Skytree's upper clock, 450 m up in slightly weaker gravity, ran a touch fast, as Einstein predicted. The team wants portable clocks that read height from the flow of time, to watch the ground swell at volcanoes.

In an optical lattice clock, atoms are held in place by what?

Intersecting laser beams make an egg carton of light with an atom resting in each hollow. It ought not to work, since the trapping light should skew the ticking, but at certain 'magic' wavelengths the shifts cancel.

A JILA optical clock unveiled in 2015 would stay within a second over roughly what span?

The age of the universe, about 15 billion years by the team's sums. Clocks this good have overtaken the caesium clocks that define the second, so the world's timekeepers are preparing to redefine it.

NIST's 'quantum logic' clock, named the world's most precise in 2010, keeps time with a single ion of which element?

Aluminium keeps superb time, but its state cannot be read directly, so a magnesium ion is trapped beside it. When the aluminium ticks, both ions move, and the magnesium flashes to report it.

In 2021 a JILA optical clock detected time dilation between the top and bottom of an atom cloud how tall?

One millimetre was enough: atoms at the top of the cloud ticked measurably faster than those at the bottom. The same effect means you age very slightly faster upstairs than down, which a 2010 test confirmed over 33 cm.

A 'nuclear clock' keeps time with a transition in the nucleus of which isotope?

Thorium-229 has the smallest nuclear energy jump known, low enough for an ultraviolet laser where other nuclei would need X-rays. A nucleus also shrugs off the stray fields that disturb electrons, so such a clock could be steadier still.

NASA's Deep Space Atomic Clock, launched in 2019, kept time with ions of which toxic heavy metal?

Mercury ions kept it steady enough to guide a spacecraft from on board. Today a probe sends a signal to Earth and waits for clocks there to work out where it is; with a clock like this one it could navigate alone.

Which pioneer of the laser cooling used in atomic clocks first chilled atoms in optical molasses at Bell Labs?

Steven Chu chilled atoms by bombarding them with laser light, much as enough ping-pong balls could halt a rolling bowling ball. To the atoms the light is like a thick liquid, hence the name optical molasses.

NIST chilled one caesium clock with liquid nitrogen mainly to cut which source of error?

Black-body radiation from warm chamber walls nudges the atoms' ticking, and the deep chill all but silenced it. But the cooling made the clock such a chore to run that NIST retired it in 2015, a year after launch.

Why does caesium suit an atomic clock better than hydrogen does?

Its heavier atoms move much more slowly, so each spends longer in the microwaves and its frequency is less blurred by motion. The soft, gold-tinted metal's energy levels also shrug off stray magnetic fields.

Caesium, the metal that defines the second, was in 1860 the first element discovered using which instrument?

A spectroscope split the light of a flame fed with German spa water and showed two blue lines that no known element makes. Bunsen and Kirchhoff, who had invented the instrument a year earlier, named their find from a Latin word for blue.

NIST, keeper of America's civilian atomic clocks, went by what name from 1901 until 1988?

The National Bureau of Standards was founded by Congress so that American industry had national standards to measure against.

Consumer 'atomic' wall clocks and watches actually stay accurate how?

By receiving radio time signals sent out from a real atomic clock, they correct an ordinary clock inside. Most tune in after midnight, because the long-wave signal carries best when it is dark along the whole path.

What is a clock called in which laser-cooled atoms are tossed up through a microwave cavity and fall back?

Atomic fountain clocks probe each atom twice, on the way up and on the way down, about a second apart. In older clocks hot atoms streak straight through in a split second, and a longer look gives a sharper reading.

By US rules, computer clocks must stay within 50 milliseconds of NIST's atomic time when they time-stamp what?

Stock market trades are made by machines in fractions of a second, so only closely matched time stamps can show who got there first. Before a 2016 rule change, the clocks were allowed to be a full second out.

Physicists compare atomic clocks made from different elements in the hope of detecting what?

Dark matter, if it is a sea of ultralight particles, would nudge the constants of nature that set each atom's tick. Different elements would shift by different amounts, so two unlike clocks would drift apart.

Which internet protocol keeps your computer's clock in step with atomic time servers?

NTP stamps each request as it leaves and as the reply returns, so your computer can work out the network delay and allow for it. David Mills designed it in 1985, which makes it older than the web.

The metric system's bureau, which also averages the world's atomic clocks, sits just outside which city?

Paris lies just east of the bureau's park at Saint-Cloud. No clock shows world time exactly: the average is worked out afterwards, and each month the bureau tells every lab how far its clocks had strayed.

In January 2017, weeks after it went live, which satellite navigation system was found to have failing atomic clocks?

Galileo, Europe's rival to GPS, kept every satellite working through the failures. Each one carries four atomic clocks so that a spare can take over when one stops.

Ytterbium, used in top atomic clocks, is named after a village in which country?

Sweden's village of Ytterby, near Stockholm, gave its name to four elements: yttrium, ytterbium, terbium and erbium. All were teased out of an unusual black rock found in its quarry in 1787.

Which company's rocket carried Europe's ACES atomic clocks into orbit in April 2025?

SpaceX flew ACES to the space station on a Falcon 9, and its two clocks were sent there to test Einstein's relativity from orbit.

In 2024 the White House asked NASA to set up an official time standard, kept by atomic clocks, for which place?

The Moon's weaker gravity makes clocks there run fast by millionths of a second a day. That is enough to throw off spacecraft that navigate by timed radio signals, so lunar missions need a shared clock of their own.

Physicists have made atomic clocks more precise by getting their atoms to act as one through which quantum effect?

Entanglement ties the atoms together so that what happens to one is felt by all, which lets part of the random quantum noise in each reading cancel. MIT physicists showed it working in a clock in 2020.