Programme 1

Background

The Atomic Clock

In the 1950s scientists developed a very accurate way of measuring time, called the atomic clock or the caesium clock. Caesium (Cs) atoms can be made to flip between two different states if a beam of microwaves at a particular frequency is fired at them. Caesium atoms are fed into a chamber containing microwaves. The atoms vibrate between two energy levels at an almost constant frequency. The second is now defined as 9,192,631,770 vibrations of a caesium atom.

The magnets split the atoms into the two states. The states are separated by a second set of magnets. A beam of atoms of one state only is then fed into a detector where their frequency is measured. This beam is then used to oscillate the original beam in a feedback loop to make sure the original beam stays at the correct frequency.

The three scientists who invented this technique — Norman Ramsay, Hans Dehmelt and Wolfgang Paul — won the Nobel Prize for Physics in 1989.

Leap Seconds

The Earth spins on its axis once every 24 hours — almost. In fact it is slowing down. Every so often, at the end of either July or December, a ‘leap second’ is added, to keep our time in harmony with the rotation of the Earth. Between 1972 and 2000, 21 leap seconds were added.

Super Atomic Clocks

‘Super atomic clocks’ are now being developed which use ion traps to measure time a thousand times more accurately than today’s atomic clocks. The idea is that atoms of Ytterbium (Yb) would be stripped of electrons to form ions and then cooled by lasers to just above absolute zero. Ytterbium has been chosen because it vibrates at an even higher frequency than caesium. These clocks would be accurate to 1 second in the age of the Universe (15,000,000,000 years)!

For more information on atomic clocks, see:

http://www.nobel.se/physics/laureates/1989/index.html
http://www.npl.co.uk/npl/publications/atomic/