Ultra-precise atomic optical clocks could help us redefine the length of the second

(ORDO NEWS) — The definition of the second, the most fundamental unit of time in our modern system of measurement, has not been updated for over 70 years (give or take a few billionths of a second).

But that may change in the next decade or so: Ultra-precise optical clocks powered by visible light are on their way to redefining the second.

These new versions of atomic clocks are, at least in theory, much more accurate than gold cesium clocks, which measure the second based on the vibrations of cesium atoms when exposed to microwaves.

“You can think of it as equivalent to a ruler with marks every millimeter, as opposed to a stick that only measures 1 meter,” Jeffrey Sherman, a researcher at the National Institute of Standards and Technology’s Division of Time and Frequency in Boulder, Colorado, told Live Science.

In June, the Bureau International des Poids et Mesures may publish the criteria needed for any future definition of the second, reports The New York Times. So far, no optical clock is ready for prime time.

But a new definition could be formally approved as early as 2030, Sherman said.

A new type of optical clock could help expose dark matter – the invisible stuff that exerts gravitational pull; or find the remnants of the Big Bang called gravitational waves – ripples in space-time, predicted by Einstein’s theory of relativity.

Fundamental unit of measure

The modern standard second is based on a 1957 experiment with an isotope, or variety, of cesium. When pulsed with microwave energy of a certain wavelength, cesium atoms are in the most “excited” state and release the largest possible number of photons, or units of light.

This wavelength, called the natural resonance frequency of cesium, causes cesium atoms to “tick” 9,192,631,770 times every second.

This original definition of the second was tied to the length of the day in 1957 – and that, in turn, was tied to variables such as the rotation of the Earth and the position of other celestial objects at that time, according to The New York Times.

In contrast, optical atomic clocks measure the vibrations of atoms, which “tick” much faster than cesium atoms, when pulsed with light in the visible range of the electromagnetic spectrum. Because they can tick much faster, they could theoretically determine the second with a much finer resolution.

There are several contenders to replace cesium as the reigning chronometer, including strontium, ytterbium and aluminium. Each has its pros and cons, Sherman said.

To make such a clock, researchers must suspend and then cool the atoms to absolute zero, and then pulse them with the finely tuned color of visible light needed to maximize the excitation of the atoms.

One part of the system shines light on the atoms, while the other counts the vibrations.

But some of the biggest challenges come with making sure the laser emits the exact color of light — say, a certain shade of blue or red — that is needed to make the atoms resonate, Sherman said.

The second step – counting the vibrations – requires a so-called femtosecond laser frequency comb, which sends out pulses of light at tiny intervals, Sherman said.

According to Sherman, both elements are incredibly complex engineering solutions and could take up an entire lab room by themselves.

Application of optical clock

Why do scientists need increasingly accurate atomic clocks to measure the second? This is not just an academic pursuit.

Time doesn’t just march to its own drum; according to Einstein’s theory of relativity, it is distorted by mass and gravity.

As a result, at sea level, where the Earth’s gravitational field is stronger, time can tick infinitely slowly, and at the top of Everest, where it is even slightly weaker.

Finding these minute changes over time could also reveal evidence of new physics.

For example, the influence of dark matter has so far only been detected in the distant dance of galaxies circling each other, in the bending of light around planets and stars, and in the remnants of light from the Big Bang.

But if clumps of dark matter lurk closer to home, ultra-precise clocks that record tiny time dilations might be able to detect them.

Similarly, gravitational waves rock the fabric of space-time, they compress and stretch time. Some of the largest gravitational waves are being detected by the Laser Interferometric Gravitational Wave Observatory, a multi-thousand-mile-long light relay that measures spikes in space-time resulting from cataclysms such as black hole collisions.

But a battalion of atomic clocks in space can detect these time dilation effects for much slower gravitational waves, such as cosmic microwave background waves.

“These are so-called primordial gravitational waves, which could be leftovers from the Big Bang,” Sherman says.


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