BBC Knowledge Asia Edition2

One Mississippi, two

Mississippi, three... though

this way of counting seconds

works pretty well for playing

hide-and-seek, the atomic

clock is vastly superior when

it comes to accuracy. It

could run for 300 million

years and would not stray

from perfect time by one

second. Whereas counting

using words relies on how

fast you are speaking, the

atomic clock gets its

information from electrons

jumping around in atoms.

Such precision is needed for

GPS systems and global

telecommunication. Since

2014, The National Institute

of Standards and

Technology’s atomic clock

‘NIST-F2’ has set the

standard time in the US.

So how can atoms define a

second? Atoms contain

electrons, which change

energy levels when hit with

radiation at a certain frequency,

causing them to emit light. The

frequency at which electrons

of caesium atoms will change

state is 9,192,631,770Hz, and

is known as the ‘natural

resonance’ of the caesium

atom.

In 1967, the second was

defined as the duration of

9,192,631,770 periods of the

radiation corresponding to

the transition between two

electron energy levels of the

caesium-133 atom.

HOW IT WORKS

The ‘NIST-F2’ atomic fountain clock

Do insects sleep?

MICROWAVE CAVITY

3. DETECTOR
   Measures the light emitted by
   atoms

LASERS

LASER

LASER

CAESIUM ATOMS

2. PROBE LASER
   Excites the caesium atoms,
   causing them to absorb and
   re-emit light
   
   
   1. Lasers cool caesium
      atoms and push them into a
      ball, which is sent upwards.
      When the ball of atoms
      reaches the top of the clock,
      it then falls back down like a
      fountain
   
   
   
   

The microwave

frequencies are

measured; the one that

causes the caesium

atoms to emit light is used

to define a second

Microwaves interact with

the caesium atoms

PHOTOS: MATT COLE/FLPA ILLUSTRATION: SAM FALCONER

Yes. They don’t have eyelids, so

they don’t close their eyes like

we do. Cockroaches, however,

will fold down their antennae

when they sleep, which has the

similar purpose of protecting

delicate sensory organs.

When asleep, insects aren’t

just resting – sleeping praying

mantises will droop downwards

and sleeping bees are harder to

startle than those that are

having a rest.

Laboratory experiments

have shown that fruit flies that

are forced to stay awake are

slower at learning their way

round simple mazes than fruit

flies that are allowed sufficient

sleep. LV