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50 free Atomic Clocks trivia questions with answers — science & nature quiz, new questions added Aug 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 50 questions with answers and explanations. Play the quiz
Q 01An atomic clock keeps time by monitoring what property of atoms?
Their resonant frequency
Transitions between quantised energy levels are driven by very specific frequencies of radiation, which is what the clock locks onto.
Q 02What kind of energy-level split in the caesium-133 atom defines the SI second?
A hyperfine transition
It is the tiny energy split in the atom's ground state, driven by microwaves at about 9.19 GHz.
Q 03Exactly how many periods of caesium-133's clock transition make one second?
9,192,631,770
The other numbers are rubidium's frequency, hydrogen's 21-cm line and the speed of light in metres per second.
Q 04Before the caesium definition, the second was defined as a fraction of what?
The tropical year 1900
There were deemed to be 31,556,925.9747 seconds in that year.
Q 05Whose 1873 Treatise first proposed measuring time by the vibrations of light waves?
James Clerk Maxwell
He argued it would beat the Earth's rotation, which then defined the mean solar second.
Q 06Who built the first practical caesium atomic clock in 1955 at Britain's National Physical Laboratory?
Louis Essen
Ironically, Essen was a lifelong critic of Einstein's relativity, publishing a book attacking it in 1971.
Q 07The first atomic-clock prototype, built in 1949, measured transitions in which molecule?
Ammonia
It was accurate but not more so than the best quartz clocks; caesium took over in 1955.
Q 08Which 1944 Nobel physicist built atomic-beam magnetic resonance equipment that led toward atomic clocks?
Isidor Rabi
His Nobel citation was 'for his resonance method for recording the magnetic properties of atomic nuclei'.
Q 09Norman Ramsey won the 1989 Nobel Prize in Physics for inventing what?
The separated oscillatory field method
Now called Ramsey interferometry, it gives narrower resonances; he developed it in 1949.
Q 10What was the first commercial atomic clock, sold by the National Radio Company in the 1950s, called?
Atomichron
Hewlett-Packard followed in 1964 with its rack-mounted 5060 caesium clock.
Q 11Which caesium clock became a US primary time standard when brought online in 2014?
NIST-F2
Cooling its microwave region with liquid nitrogen removed the black-body radiation error that limited its predecessor.
Q 12NIST-F2 was rated to neither gain nor lose a second in at least how long?
300 million years
Its predecessor NIST-F1 was rated at 100 million years.
Q 13Roughly what timing error corresponds to a positional error of about 30 cm in satellite navigation?
1 nanosecond
Light travels about 30 cm in a billionth of a second, which is why GPS satellites carry atomic clocks.
Q 21In the 1971 Hafele-Keating experiment, four caesium clocks were flown around the world aboard what?
Commercial airliners
They flew east then west and confirmed relativity's predicted time differences against clocks at the US Naval Observatory.
Q 22What is the clock transition frequency of rubidium-87 used in rubidium clocks, roughly?
6.8 GHz
Rubidium standards are cheap, small and used everywhere from telecoms to satellites, but count only as secondary representations of the second.
Q 23Which type of atomic clock has the best short-term stability, making it a favourite for radio astronomy?
Hydrogen maser
Q 14Relativity predicts GPS satellite clocks run how much faster per day than ground clocks?
38 microseconds
Uncorrected, that would send positions drifting by kilometres within a day.
Q 15GPS Time runs at a constant offset from International Atomic Time of what size?
19 s
Like TAI, GPS Time never inserts extra seconds; receivers subtract a broadcast offset to get UTC.
Q 16Europe's Galileo satellites carry two rubidium clocks and two of which other kind?
Passive hydrogen masers
The second-generation Galileo masers are about two feet long and weigh 40 pounds.
Q 17International Atomic Time (TAI) is a weighted average of roughly how many clocks worldwide?
450
They sit in about 80 institutions, and the scale is published a few weeks in arrears.
Q 18What is added to atomic time to keep civil time (UTC) within 0.9 s of Earth's rotation?
Leap seconds
The first was introduced in 1972 and the 27th, at the end of 2016, was the last one added through 2025.
Q 19After 27 leap seconds since 1972, the last in 2016, by how many seconds did UTC trail TAI?
37
That is the initial 10-second offset of 1972 plus 27 one-second insertions, the last at the end of 2016.
Q 20In 2022 the General Conference on Weights and Measures voted to abandon the leap second by which year?
2035
After that the difference between TAI and UTC will stay fixed.
It uses hydrogen's 1.4 GHz hyperfine transition but drifts over the long term as its cavity ages.
Q 24A chip-scale atomic clock demonstrated by NIST in 2004 was about the size of what?
A grain of rice
It drew just 125 milliwatts and became commercially available in 2011.
Q 25Optical clocks tick around 10^15 Hz. Microwave clocks like caesium run near what frequency?
10^10 Hz
Higher frequency means more 'ticks' to average, which is why optical clocks are so much more stable.
Q 26Which element's optical lattice clock first beat caesium's precision, ticking at 429 terahertz?
Strontium
JILA demonstrated a strontium clock precise to one part in 10^18 in 2015.
Q 27Who proposed the optical lattice clock in 2001 and built the first one in 2003?
Hidetoshi Katori
His insight was trapping atoms in a laser lattice at a 'magic wavelength' that does not disturb the clock transition.
Q 28In an optical lattice clock, atoms are held in place by what?
Intersecting laser beams
The beams form a stable 'egg-crate' pattern of light with an atom in each pocket.
Q 29An optical lattice clock drifts one second in about how long, a thousand times longer than a caesium clock?
30 billion years
That is more than twice the current age of the universe.
Q 30NIST's 2010 'quantum logic' clock, first to beat caesium's precision, used which element's ions?
Aluminium
In 2019 a single-aluminium-ion version reached a frequency uncertainty of 9.4 parts in 10^19.