Atomic Clocks Trivia
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49 free Atomic Clocks trivia questions with answers — science & nature quiz, new questions added Sep 2026.
Every phone, satellite and stock exchange runs on time kept by atomic clocks, and this quiz explains how. It starts with the basics: why an atom's resonant frequency makes a better pendulum than a quartz crystal, which element defines the SI second and by exactly how many cycles, and who built the first practical caesium clock in 1955. Then it follows the technology from ammonia masers and the Atomichron to NIST-F2, hydrogen masers, rice-grain chip-scale clocks and the strontium optical lattice clocks that would lose a second in 30 billion years. There are questions on what the clocks are used for (GPS, TAI and UTC, leap seconds and their planned abolition, high-frequency trading, radio time signals) and what they have proved (the Hafele-Keating airliner flights, gravitational time dilation across a single millimetre), plus the people: Maxwell, Rabi, Ramsey, Essen, Wineland, Katori. Easy questions suit anyone who has wondered why their phone knows the time; the expert tier asks about the Dick effect and Allan deviation. For clocks of every other kind, try our Clock quiz. Every answer was checked against Wikipedia's articles on atomic clocks, time standards and the scientists involved, and the supporting sentence is attached to each question.
30 of 49 questions with answers and explanations. Play the quiz
Q 01An atomic clock keeps time by monitoring what property of atoms?
Their resonant frequency
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.
Q 02What kind of energy-level split in the caesium-133 atom defines the SI second?
A hyperfine transition
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.
Q 03Which Nobel-winning invention lets scientists count an optical clock's ticks?
Optical frequency comb
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.
Q 04Before caesium took over, the second was briefly redefined as a fraction of which period instead of the day?
The tropical year
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.
Q 05Whose 1873 Treatise proposed measuring time by the vibrations of light waves?
James Clerk Maxwell
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.
Q 06Who built the first practical caesium atomic clock in 1955 at Britain's National Physical Laboratory?
Louis Essen
Louis Essen later became a critic of Einstein's special relativity, publishing a book in 1971 that questioned the theory.
Q 07The first atomic-clock prototype, built in the late 1940s, measured transitions in which molecule?
Ammonia
Ammonia gave a clock hardly better than existing standards, so researchers turned almost at once to more promising caesium beam devices.
Q 08Which 1944 Nobel physicist built atomic-beam magnetic resonance equipment that led toward atomic clocks?
Isidor Rabi
Isidor Rabi first publicly proposed building an atomic clock in a 1945 lecture, and The New York Times reported his idea to the public.
Q 09Norman Ramsey won the 1989 Nobel Prize in Physics for inventing what?
The separated oscillatory field method
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.
Q 10Which company launched the rack-sized 5060 caesium clock in 1964, bringing caesium standards into calibration labs?
Hewlett-Packard
Hewlett-Packard carried two of its caesium clocks to Switzerland in 1964, matching US and Swiss time standards to about a microsecond.
Q 11Which caesium clock became a US primary time standard when brought online in 2014?
NIST-F2
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.
Q 12NIST's atomic clocks that set America's official civilian time are housed in which Colorado city?
Boulder
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.
Q 13Roughly what timing error corresponds to a positional error of about 30 cm in satellite navigation?
1 nanosecond
Q 21In the 1971 Hafele-Keating experiment, four caesium clocks were flown around the world aboard what?
Commercial airliners
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.
Q 22Which element runs the small, cheap atomic clocks found in phone networks and aboard GPS satellites?
Rubidium
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.
Q 23Which commercial atomic clock is the steadiest over short spans, which makes it the usual timekeeper at radio telescopes?
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.
Q 14Relativity predicts GPS satellite clocks run how much faster per day than ground clocks?
38 microseconds
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.
Q 15Which institution keeps the US military's master clock, which GPS follows?
Naval Observatory
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.
Q 16In most atomic clocks the atoms only correct an oscillator that does the actual ticking, made from which crystal?
Quartz
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.
Q 17Atomic clocks at radio telescopes time-stamped the data behind which 2019 first?
Black hole image
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.
Q 18What is occasionally added to atomic time so that civil time (UTC) stays in step with Earth's uneven rotation?
Leap seconds
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.
Q 19Which stars spin so steadily that astronomers check the best of them against atomic clocks?
Pulsars
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.
Q 20Since 1983, which unit has been defined by how far light travels in a set fraction of a second?
Metre
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.
Hydrogen maser
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.
Q 24The ticking heart of the chip-scale atomic clock that NIST demonstrated in 2004 was about the size of what?
A grain of rice
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.
Q 25Optical clocks tick hundreds of trillions of times a second, but caesium's microwave clocks manage roughly how many?
Ten billion
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.
Q 26Which element, the one that colours fireworks red, ticks at 429 terahertz in JILA's record-setting lattice clocks?
Strontium
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.
Q 27In 2020 Japanese physicists tested Einstein by comparing atomic clocks at the foot and near the top of which landmark?
Tokyo Skytree
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.
Q 28In an optical lattice clock, atoms are held in place by what?
Intersecting laser beams
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.
Q 29A JILA optical clock unveiled in 2015 would stay within a second over roughly what span?
The age of the universe
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.
Q 30NIST's 'quantum logic' clock, named the world's most precise in 2010, keeps time with a single ion of which element?
Aluminium
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.