It’s time to dump her and move on

BIPM metrologists, along with experts from various countries, are preparing to change the way the second is measured. It is a very delicate operation, the result of which can be crucial in changing the way we understand the universe.

The second is the basic unit of time in the international measurement system.

But in fact, other basic units such as meter (length), kilogram (mass), ampere (current) and kelvin (temperature) are defined based on the latter.

The BIPM, for example, defines a meter as “the path of light in a vacuum in a period of 1 / 299,792,458 seconds.”

For millennia, humanity has used astronomy to define its units of time. But since 1967, the observation of atoms has been used to define the latter. This is because atoms act more accurately than the rotation of the Earth because they are not completely uniform.

Scientists have observed that the Earth has been rotating more slowly for millions of years. As a result, the days are an average of 1.8 milliseconds longer each century.

So, for example, 600 million years ago, Earth Day lasted only 21 hours. And worse, in 2020 several studies have shown that the planet has started to spin faster in the last 50 years.

So although it is imperceptible, the “astronomical second” is not always the same, as the atomic particles move more accurately and predictably.

Since 1967, the second has been determined by the oscillation of the particles of 133 cesium atoms under the influence of a special type of microwave. The device used for this measurement is called an atomic clock.

Under the influence of these microwaves, cesium-133 atoms act like a pendulum with a “balance” of 9,192,631,770 times per second.

Until then, the second one used as a reference for counting oscillations was based on the length of a day in 1957, determined by the behavior of the Earth, the Moon, and the stars. Now, the BIPM determined that the official measure of the latter would be calculated from the number of oscillations of the 133 cesium atom particles.

So, in short, it is defined as the time it takes for the current cesium to oscillate 9,192,631,770 times. But this definition seems outdated.

For about a decade, there have been optical atomic clocks that have the ability to observe the “tick” of atoms that oscillate much faster than cesium.

Some of them count the ticks of ytterbium, strontium, mercury, or aluminum, for example. It is as if the atomic clock is a magnifying glass that allows us to detect more oscillations and define the second one with greater precision.

And now there are dozens of optical clocks in many countries. It is hoped that, as some experiments have already shown, different measurements can be compared to prove the results obtained.

The BIPM intends to use optical atomic clocks to measure the latter, but is yet to determine the criteria for such a measurement. The most important thing is to prove the accuracy of the optical clocks, according to Gérard Petit, a researcher in the BIPM time group.

To date, the best comparisons have been made between optical clocks in the same laboratory. But Petite told BBC News World, the BBC’s Spanish-language news service, that the aim is to compare different clocks in different laboratories. And the element of the periodic table that will replace cesium as a reference must also be specified.

Moreover, optical atomic clocks are extremely complex devices, many of which require a complete laboratory to function.

Some of the challenges with these devices are, for example, emitting the type of laser radiation to accurately oscillate atoms with precise accuracy, or having ultra-fast laser pulses with minimal intervals so as not to lose the oscillations that need to be counted. , explained researcher Jeffrey Sherman, from the Time and Assistance Department of the National Institute of Standards and Technology (NIST) to the Live Science portal.

Gérard Petit stated that, if all goes according to plan, the criteria should be set to begin in June 2022, and that a new one should come into force in 2030.

“The operations and comparisons are complex,” he says.

What happens when I change the definition of the second one?

“Nothing,” says Petite with a laugh.

The main reason for updating the second is to keep things in order, as the world’s measurement structure depends on the second.

“It’s possible to live for a while with a definition that’s not the most accurate, but after a while it becomes incomprehensible,” Petit says.

“In practice, in everyday life, it may not change anything, but in science, a definition based on the best possible measurement is needed.”

Furthermore, measuring ultra-accurate time can help us understand phenomena that are not currently understood.

NIST explains, for example, that optical clocks were once used to measure the space-time distortion described by Einstein’s theory of relativity.

Optical clocks are so accurate that they can prove the difference between two clocks with a height difference of only one centimeter. That is, due to gravity, time is slower at sea level than at high altitudes, such as Mount Everest.

These ultra-precise clocks could also be used to detect the enigmatic dark matter that makes up 25% of the universe, but little is known about it. With new technology, scientists will be able to detect this unknown matter that affects ordinary matter and space-time.

And they can also provide clues about the first gravitational waves, which are echoes of the Big Bang that distort space-time, like a rock thrown into a lake. Atomic clocks may be able to detect these deformations and be able to provide further clues about the creation of the universe.

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