NUCLEAR CLOCKS
NUCLEAR TIME
The world’s first nuclear clock could go beyond just keeping ultraprecise time. It might teach us about the forces that hold the Universe together
WORDS: PROF JON BUTTERWORTH
ILLUSTRATION: SIMON PRADES
Humans have long sought ways to measure the world we live in. Having systems of measurement, and of units, helps us understand ourselves and our surroundings. This is true whether we’re building, buying and selling, or adopting theories of fundamental physics.
When we measure something, we’re essentially comparing the thing being measured against a standard benchmark. It’s therefore crucial that the bench on which we’ve made our notional mark is stable and not liable to change.
Time is generally measured in relation to frequency and our current benchmark for it comes from atomic clocks.
If we have something oscillating, say a pendulum, we define a benchmark time as the interval in which a particular number of oscillations occur. If something oscillates with a frequency of one hertz (Hz), then the time taken for one oscillation is one second.
Atomic clocks exploit the fact that the electrons inside an atom exist in very definite energy levels. If an electron jumps between two energy levels, it emits or absorbs a very precise and predictable amount of energy in the form of a photon – a quantum of light.
Frequency and energy are related in quantum mechanics: a photon is a quantum light wave that has an exact frequency associated with it. In a caesium atom, the frequency of light emitted in one atomic transition between two energy levels is always exactly 9,192,631,770Hz. This consistent frequency is used to define what a second is: one second is the time it takes for 9,192,631,770 waves of light from a caesium transition to arrive at a detector.
By observing how many electrons jump up and down between these energy levels, a laser can be tuned to exactly this frequency, and the frequency of the laser can then be used to measure time.
WHY GO NUCLEAR?