As accurate as accurate gets

These optical lattice clocks are about 100 times more accurate than the atomic clocks currently used. But it’ll take us a few million years to notice!

These clocks are so precise they can measure red shift,which is the phenomenon of time slowing down around objects with a great gravitational pull and check changes in a single centimetre of the Earth’s surface.

In the new method,Ytterbium atoms are collected in an optical lattice formed by a laser beam bounced off a mirror. Once the atoms are in place,in what scientists have called an “eggbox”,another laser beam is focused at them to measure the electronic shifts on each atom.

HOW TIME IS MEASURED

historically,time was measured by the Earth’s rotation. But this was inaccurate as scientists established that the Earth wobbled on its axis,making some days longer than others.

THE ATOMIC clock is currently the most accurate way to measure time,based on the amount of energy required to “spin a cesium atom”.

A SECOND,as established in 1967 when the International System (SI) unit of time was first defined,is counted when a Cesium atom emits 9,192,631,770 oscillations of radiation,using an apparatus called “cesium fountain”.

A CLOCKWORK CESIUM

It’s a device which uses the exact frequency of microwave spectral lines emitted by Cesium atoms. The frequency of 9,192,631,770 hertz (cycles/second) provides the fundamental unit of time.

The accuracy of the clock is 2 nanoseconds per day or one second in 1,400,000 years. It was the most accurate realisation of a unit until the lattice clock.

The clock operates by exposing cesium atoms to microwaves until they vibrate at one their resonant frequencies and then the corresponding cycles are counted as a measure of time. The frequency is from when the outermost electron of Cesium jumps from a lower to a higher orbit.

The first atomic clock was built by the US National Bureau of Standards in 1949. But instead of cesium atoms,the first clock used ammonia atoms and was off by a second every four months. Scientists switched to Cesium,and by 1964,clocks became so precise that it took 6,000 years for them to gain or lose a second. Today’s atomic clocks are off by one second after 6 million years.

The part of the atomic clock that actually keeps time is a standard quartz crystal oscillator. The oscillations of the Cesium atoms check the frequency of the quartz device,giving the clock its accuracy.

In 2008,scientists wound all the atomic clocks around the world ahead by exactly one second,in order to synchronise Coordinated Universal Time (UTC),the international standard for atomic clocks,with the Earth’s rotation. It wasn’t the clocks that were off but the planet.

The Earth loses about two milliseconds each day due to a variety of reasons: space dust,magnetic storms,solar winds,Earth’s own atmosphere,and moon’s gravitational tug.

The seemingly preposterous idea,first proposed by Albert Einstein in the theory of special relativity,that someone living above the sea-level ages faster than people at sea-level is true. In 2010,scientists positioned two atomic clocks 30 cm apart above sea-level,and found that the higher of the two clocks ran slightly faster. In statistical terms,a mountain-dweller would age about 90 billionths of a second faster over a 79-year lifetime!

WHY CESIUM?

Cesium was discovered in 1860 by Robert Bunsen,the inventor of Bunsen burner. An isotope of Cesium,with mass number 133 which is rare,is used for atomic clocks. Cesium was chosen for these measurements because all of its 55 electrons are in stable orbits. The outermost electron is not disturbed by the other electrons held close to the nucleus by electromagnetic force. The cesium atom thus can be kept in tight vacuum of 10 trillionths of an atmosphere,so that they radiate in a narrow spectral line whose frequency can be accurately determined.