The definition of the second evolves: optical atomic clocks will be used, 100 thousand times more precise than the cesium clock

Scientists are about to redefine the basic unit of time measurement. The unit will neither be longer nor shorter in duration, but will be more precise and much more powerful.

It is claimed that there can be no modern civilization as we understand it without measure. Without a standard unit, measurement would also be meaningless.

For nearly 150 years, measurement scientists around the world have reached a consensus on the use of standard units of measurement determined by extremely strict rules under the aegis of the International Bureau of Weights and Measures (BIPM) based in Paris.

Today, the office organizes seven basic units: time, length, mass, electric current, temperature, luminous intensity and quantity of matter. These units have become indispensable parts of science, technology and commerce.

Scientists constantly use these units in their studies. In 2018, the definition of the kilogram, which is the measure of mass, the ampere, which is the unit of electric current, the kelvin, which is the unit of temperature, and the mole, which is the unit of substance, has been modified. Currently, all units except moles are indexed to a single measure, time.

For example, the definition of meter is defined as the distance traveled by light in 299 million 792 thousand 458ths of a second. The kilogram is also defined as indexed to the second with a slightly more complex calculation.

Physicist Noel C. Dimarcq, Chairman of the BIPM Advisory Committee, says: “Currently, the units are not all autonomous, they are all indexed to the second. So when you go to the grocery store, when you say 1 kilo of potatoes please, you actually want a number of seconds worth of potatoes.”

But for the first time in more than half a century, scientists are about to change the definition of the second. Because new generation watches can now measure it much more clearly.

In June, BIPM measurement specialists will prepare the final list of criteria to be applied for the new definition. Speaking to The New York Times, Dimarcq said he expects all of these criteria to be met by 2026 and the new definition to be approved by 2030.

Since the global measurement system is built on the second, its definition must be done with great care. In other words, as long as the definition changes, the duration must remain the same.

Time was originally defined by the rotation of the earth on itself in a day. Using the 12-number sequence, ancient Egyptian astronomers defined a day as 24 hours by dividing day and night into 12-hour units.

But these hours could lengthen or shorten according to the position of the Earth around the Sun. Two thousand years ago, Greek astronomers needed to fix the clocks to calculate the movements of the Moon and decided to divide a day into 24 units of fixed length. The same astronomers decided to divide the clocks by the old Babylonian method of 60. Then the minutes were again divided by 60 to reach the second.

The first part of 24 hours, that is, one in 1440 of an average day, was accepted as a minute, and one in 86,400 was considered a second. This definition remained valid until 1967.

But this definition had its problems. The Earth’s rotational speed was gradually decreasing, so the days were gradually getting longer. Of course, the duration of a second too. These small changes have increased significantly over time. The world clock has lost about 3 hours in 2000 years.

For this reason, basing the definition of the second on unstable astronomical movements has led scientists to reflect. By the late 1960s, the broadcast of radio waves, where even instantaneous changes were significant, demanded topicality.

For this reason, the measurement specialists turned to the movements of the particles inside the atom, which never slowed down. The scientists turned to the cesium 133 atom, a liquid metal at room temperature.

Scientists placed cesium atoms in a vacuum environment and exposed them to microwave energy. With this method, the emission of photons from cesium atoms whose wavelength was determined and a data was obtained by counting the photons.

Accordingly, in 1967, the definition of the second was determined as 9,192,631,770 radiation cycles, corresponding to the transition between the two energy levels in the ground state of the unexcited cesium-133 atom at temperature ambient.

But even this definition was not precise enough, so scientists started working on a new definition. For this, optical atomic clocks have been developed. Although these clocks work on a principle similar to cesium clocks, they work with atoms with much faster magnetic resonance.

There are already a large number of optical atomic clocks. The strong points are ytterbium, strontium, mercury and aluminum. But so far no one has been selected.

Judah Levine of the US National Institute of Standards and Technology (NIST) pointed out that optical clocks are not yet ready to be used as a reference. Although these clocks measure very small atoms, many are larger in volume than a dining table and are very difficult to operate. However, the frequency obtained from these clocks is 100,000 times faster than the microwave energy of cesium clocks. They are therefore much more sensitive.

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