Q 01/05

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

A) Their thermal vibration rate

B) Their resonant frequency

C) Their orbital electron speed

D) Their magnetic field strength

Answer · why

B) 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...

Q 02/05

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

A) A hyperfine transition

B) A nuclear isomer transition

C) A rotational transition

D) A vibrational transition

Answer · why

A) 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 03/05

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

A) Michelson interferometer

B) Magneto-optical trap

C) Optical frequency comb

D) Fabry–Pérot cavity

Answer · why

C) 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...

Q 04/05

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

A) The lunar month

B) The tropical year

C) The Saros cycle

D) The Metonic cycle

Answer · why

B) 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'...

Q 05/05

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

A) Heinrich Hertz

B) Michael Faraday

C) Lord Kelvin

D) James Clerk Maxwell

Answer · why

D) 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.