What is an atomic clock? Is there an absolutely exact clock?

"You have a second?". The question, to pot soon, may seem very simple. But what is a second? What do scientists use to measure it accurately? In this article, we will explain the answer to both questions and discover the importance of a certain chemical element in the art of measuring time.

You probably barely remember how the elements are arranged on the periodic table. Like the capitals of the world or the list of Gothic kings, it is something that little by little we are forgetting. But if you start reciting: hydrogen, lithium, sodium, potassium, rubidium… Surely you know what comes next. Indeed: the cesium. Cesium is a gray metal that, despite being solid at room temperature, melts in your hands. Cesium is an element that has been used since 1967 to measure time as precisely as possible.

How does a watch work?

Any clock works by measuring the number of "blows" that a vibrating object gives, that resonates. In a wall clock, each swing of the pendulum causes the gears to move. In a digital watch, electrical energy vibrates a crystal (usually quartz) and an electronic counter measures the oscillations. In a cesium watch, a plate of this metal is heated. Thus, cesium atoms are obtained in the vapor state. These atoms are of two different classes (or energies), depending on the property of their outermost electron: the spin. Thanks to a magnet, we can separate the less energetic atoms and take them to a chamber. Once there, microwave radiation will be used (see diagram of the figure) to convert them into energetic atoms. When they return to their natural state, they will emit light that we can capture with a sensor like that of a photo camera. Each "fall", each drop in energy, is like an oscillation.

A cesium 133 atom produces 9,192,631,770 oscillations in one second. As it never emits one more, nor one less, we can define the second thanks to this chemical element. The clocks that use these systems are known as atomic clocks. They are also made with other elements, such as hydrogen or rubidium. The most accurate clock in the world, the USN-Master Clock of the United States Navy, is a combination of several dozen atomic clocks of these elements and will not go out of adjustment for 30 million years.

Even more precise clocks

Even more precise clocks than the Master Clock are currently being researched. In the NIST (the official Institute of standards and Measurements of the USA) they have developed a more advanced clock (called "quantum logic") that will only lose one second every 100 million years. At the Paris Observatory, they are trying to go even further and design a clock that will not go out of adjustment in 32,000 million years. That's more than twice the age of the universe, so if they succeed, we'll have accurate clocks until… time runs out.