A clock so precise that it can measure gravity

To measure the time at the limits of physics, scientists perfected atomic clocks that already possessed remarkable precision. Their new variant of these devices has a stability so great that it can measure the distortions of space-time.

Our modern world is obsessed with the passage of time, although this obsession is relative to our occupation. For the vast majority of people, reading the time on a watch or cell phone is enough.

If you are an astrophysicist seeking to understand black holes, even more precision will be necessary.

The creation of more accurate clocks is not exceptional. Such improvements appear at regular intervals in the scientific literature. But the announcement just made by researchers at the National Institute of Standards and Technology in the United States changes the game.

The latter developed a clock that shattered records of stability, reproducibility and uncertainty, characteristics that serve to establish the accuracy of atomic clocks. Its precision is such that it could even measure the influence of Earth’s gravity over the course of time!

The rhythm of the atom

All atomic clocks operate on the same principle. Atoms held under vacuum are exposed to certain types of electromagnetic radiation, such as microwave radiation. These waves will transmit some of their energy to the electrons that circulate around the atoms, gaining them, and then losing that energy at an incredibly steady rate.

This stable oscillation of electrons transiting through high and low energy levels is what defines the time, much like the regular beats of a pendulum clock, but on an immensely more accurate scale.

Each atom will oscillate according to a rhythm of its own, and the type of atom used is what will determine the accuracy of a clock. In 1967, the International System of Units defined the duration of one second by 9 billion oscillations of one atom of cesium-133.

This precision serves as a standard for our GPS system and all global telecommunications.

Nevertheless, this clock is not perfect and will lose the equivalent of one second every 200 million years. The researchers, in search of better, have since abandoned the cesium atoms for an atom whose oscillation frequency is even shorter: ytterbium.

By keeping about 1,000 of these atoms in place with laser beams, the researchers were able to obtain a time measurement almost 100 times more accurate than that of cesium clocks. This precision is such that, according to researchers, this clock would lose a second every 14 billion years, a period that exceeds the current age of the Universe.

At the extremes of space-time

A clock of this precision is of course beyond human needs. However, it is not in this context that it will be useful, but to measure the influence of gravity.

One of Albert Einstein’s greatest discoveries is that time is relative and can be influenced, from one observer to another, by speed and gravity.

The closer someone approaches the speed of light, or approaches a solid object like a black hole, the more time will pass slowly compared to someone who would have remained on Earth; this concept has been widely illustrated by science fiction movies, such as Interstellar .

But even a planet like the Earth can exert this effect. The closer we get to our center, the more time will be slowed down, whereas conversely, the higher we climb, the more it will be accelerated. These changes are however tiny and totally imperceptible for a human.

However, for the most accurate clock in the world, this difference will be measurable. The accuracy of atomic clocks using ytterbium will detect the tiny influence of Earth’s gravity. If we positioned them all around the Earth, it would be technically possible to measure the differences in Earth’s gravity to the nearest centimeter.

At the same time, these clocks could be useful not only for detecting celestial phenomena such as gravitational waves or dark matter, but also for signaling terrestrial phenomena such as earth movements caused by earthquakes or volcanic eruptions.

According to the researchers, however, it will be necessary to refine our knowledge of Earth’s gravity before we can make use of such accurate devices.

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