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Given the technological advances of the present, we need clocks that will keep time to a very high accuracy. The answer to this requirement is the atomic clock. The previous generation of clocks which consisted of a quartz crystal oscillator was performing efficiently. But the best of such oscillators would be late by a nanosecond after an hour of efficient performance. Translate this into distance measurements made with the help clocks and you have a more tangible error – in a matter of six weeks this would translates into an error in measurement of distance by 300 kilometres. So if this clock were used to gauge the position of a spacecraft it would be very prone to error. To improve on this atomic clocks were invented.
Roughly speaking, an atomic clock contains an element like cesium or calcium and a source of microwave radiation. Atoms consist of a nucleus which has protons and neutrons and these are surrounded by a cloud of electrons, which occupy discrete energy levels or states. When excited by a microwave, the electron in the cloud can absorb some of the incident radiation and get excited to a higher, excited state. But for this to happen, the incident microwave radiation has to match the characteristic frequency of the cesium or calcium atom. By tuning the microwave source and observing at what frequency the transition takes place, the exact value of the characteristic frequency is calculated and from this the time can be measured accurately. Frequency refers to the number of waves that cross a particular point in time in one unit of time. From this definition, by counting the number of waves, the time can be measured.
NASA’s Deep Space Atomic Clock misses a second once in 10 million years. The official definition of a second today is given by the frequency needed to make electrons transition between two levels in a cesium atom. But the state of the art is a strontium clock that loses 3.5 beats only once in 10 quintillion beats (a quintillion in one followed by 19 zeros).
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