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Scientists look to redefine the length of a second

Everyone needs to know the time, right? Getting the time right is important for countless parts of our society, and it’s so integral that there is a time standard for the world, governed by atomic clocks. You may not think there was much to say about the second, but a recent study by two teams of scientists in Boulder, Colorado might mean that atomic clock signal could get much more accurate – and, if that’s the case, it would mean we could redefine the second more precisely. Time as we know it may be about to imperceptibly change.

It might be useful to explore why a second is the length that it is in the first place. Time as we measure it, is to an extent arbitrary – seconds, minutes, hours are all just units used to mark the passage of time. But any unit could have appeared – why doesn’t a day have ten hours? The answer is that ancient civilisations were looking to mark the amount of time it takes for the Earth to turn once about its axis, or for it to rotate once about the sun, and both of these times are fairly stable. All the units we use, then, are just derivatives of planetary motion.

Time as we know it may be about to imperceptibly change

The first pendulum clock was invented by Christiaan Huygens in the 17th century, and his basic idea of an oscillator with a resonance informed later clocks. The idea was improved in the 18th century by John Harrison, who realised that smaller, higher frequency oscillators have more stable and pure resonances, which makes clocks more reliable. Most everyday clocks now use a tiny piece of quartz in the shape of a miniature tuning fork. Up until 1955, you had to correct these clocks by checking them against a regular astronomical phenomenon (like the Sun) but, appropriately, the atomic age saw us refine our clocks with a recourse to the atom.

Since the 1960s, the second has been defined by atomic clocks made of caesium atoms – to be more technical, we’ve understood the second since 1967 as exactly “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom” at a temperature of absolute zero. What this means is that these atoms absorb and emit light at a particularly frequency, and the stability of this radiation means it is consistently accurate. Over the course of a few hundred million years, the current generation of atomic clocks is accurate to within one second.

It has been calculated that these newer atomic clocks are 100 times more accurate than the caesium clock

Caesium atoms tick about nine billion times per second, but the research team have operated a network of “optical atomic clocks”. These are based on different elements, including aluminium, strontium and ytterbium, and they tick much faster – consequently, it’s possible to divide a second into even smaller slices. The study pitted the clocks against each other, sending information across an optical fibre and through an open-air link, and they found uncertainties of less than a quadrillionth of a percent. It has been calculated that these newer atomic clocks are 100 times more accurate than the caesium clock, but scientists will still need to conduct more tests on these and other atomic clocks to better understand their properties before the second is officially redefined.

This won’t radically alter time from our everyday perspective – according to physicist Jun Ye, who was involved in the collaboration, if the clock was set back to the beginning of the universe, there would only be a second’s difference to a caesium atomic clock. And it’s not even as shocking as it initially sounds – scientists redefine these essential units as their measurements become more accurate (the definition of the kilogram was altered in 2019).

But it is very important for scientists, especially as super-sensitive sensors could allow much more accurate measurements in experimental set-ups. Optical clocks have already been used to detect the difference in the Earth’s gravitational field, and it could detect a passing wave of dark matter or allow us to explore Einstein’s theory of relativity. It’s possible that we could sense the stress of the Earth’s crust and perhaps even predict volcanic eruptions. A minutely-altered second may not sound like much, but the ramifications could be incredible.

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