Q 01/05

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

A) Their resonant frequency

B) Their temperature

C) Their radioactive decay rate

D) Their mass

Answer · why

A) Their resonant frequency

Transitions between quantised energy levels are driven by very specific frequencies of radiation, which is what the clock locks onto.

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

It is the tiny energy split in the atom's ground state, driven by microwaves at about 9.19 GHz.

Q 03/05

Exactly how many periods of caesium-133's clock transition make one second?

A) 299,792,458

B) 1,420,405,752

C) 6,834,682,611

D) 9,192,631,770

Answer · why

D) 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 04/05

Before the caesium definition, the second was defined as a fraction of what?

A) The mean solar day of 1950

B) The lunar month

C) The tropical year 1900

D) A sidereal day

Answer · why

C) The tropical year 1900

There were deemed to be 31,556,925.9747 seconds in that year.

Q 05/05

Whose 1873 Treatise first 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

He argued it would beat the Earth's rotation, which then defined the mean solar second.