Such clocks are useful, and necessary, first of all to check certain physical theories, for instance, special and general relativity. According to these theories, time flows differently depending on whether the subject is moving or not, and depending on how close or far it is from the centre of the earth. To check these theoretical predictions, clocks had to be taken on board planes then rockets. But, of course, it was necessary to have clocks with guaranteed total stability for the duration of the experiments. Another question has been niggling physicists for decades: are physical constants... constant? For instance, has the speed of light always been what measurements show it to be today? The experimental protocols set up to answer these types of questions require time measurements of physical processes with a level of precision only atomic clocks can provide. Another example: to increase the power of telescopes, they have to be linked. Therefore, it is necessary to co-ordinate the arrival of the signals from the different telescopes with the utmost precision.
But atomic clocks also have a use in everyday life. In global positioning systems (GPS), the positioning of an object on the ground is based on signal transmission time measurements; any inaccuracy in the measurement of signal propagation has an effect on the accuracy on the ground. Only atomic clocks, taken on board satellites, can provide the accuracy and stability required for useable geolocation. The final example and perhaps the most important is the telecommunications sector. The signals bombarding us must be sampled in time at regular intervals upon emission but also upon reception using the same measurement. If the emitter and the receiver aren’t synchronised, this will cause problems. The greater the synchronisation, the higher the transmission flow; the quicker the sampling, the more information per unit of time can be transmitted. High-speed internet is only possible thanks to the existence of atomic clocks.
But what’s a second?
Atomic clocks have another use: as timekeepers. Or, to be more precise, to provide the definition of the second as a unit of time.
Up until 1960, the second was defined as being the 3,600th of a 24th of an average day, i.e., the average time it takes the earth to rotate on its axis with the sun as its reference. But over time, the earth turns more and more slowly! Obviously, this is minimal: in 100 years, the earth will take approximately 2 milliseconds more than today to accomplish a complete rotation. Our planet rotated on its axis in approximately 23 hours 200 million years ago and it will complete a rotation in 25 hours in 200 million years. Moreover, regardless of this general slowing down in the long term, there are also irregularities and seasonal variations. In other words, every day is different. To understand this, think of an ice-skater: when he spins around with his arms outstretched and then lowers them by his sides, he increases the speed of rotation. The same is true of the earth whose speed of rotation changes according to the distribution of its constitutive masses; this happens, for instance, when there is an earthquake. Owing to these two effects, it is difficult to base a precise definition of a second on the rotation of the earth. That is, if we want the time interval known as a second to remain constant over the centuries and even the millenniums. Therefore, in 1960, the decision was taken to base the definition of the second on the revolution of the earth around the sun, a more stable phenomenon than its rotation. From this moment on, the second was defined as the duration of this revolution divided by the exact number of days it requires (slightly more than 365), then by 24 and finally by 3600, i.e., 1/31556925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. Its duration is equivalent to that of the second based on the rotation of the earth such as it was on average at the end of the 19th century. This new definition of the second only lasted until 1967, when time became atomic and no long astronomical. The accuracy of atomic clocks is such that they exceed all other time measurement systems. Hence, they were awarded the “honour” of serving as the standard for the definition of the second. That is why the second is currently defined as being the time it takes for a certain electromagnetic wave emitted by a caesium atom to oscillate 9192631770 times.
