Time might feel like something we've got a pretty good handle on, but scientists are trying to find new ways to measure it ever more accurately. In recent tests run by the National Institute of Standards and Technology (NIST), experimental atomic clocks have achieved record performance in three metrics, meaning these clocks could help measure the Earth's gravity more precisely or detect elusive dark matter.
The NIST clocks are made up of 1,000 ytterbium atoms, suspended in a grid of laser beams. These lasers "tick" trillions of times per second, which in turn cause the atoms to consistently flicker between two energy levels like a metronome. Measuring these allows atomic clocks to keep time incredibly precisely, in some cases losing only a single second over 300 million years.
But there's always room for improvement, in this case by adding thermal and electric shielding to the devices. By comparing two experimental atomic clocks, NIST scientists have now reported three new records in the devices at once: systematic uncertainty, stability and reproducibility.
Systematic uncertainty refers to how accurately the clock's ticks match the natural vibrations of the atoms inside it. According to the team, the atomic clocks were correct to within 1.4 errors in a quintillion (a 10 followed by 18 zeroes).
Stability is a measure of how much the clock's ticks change over time. In this case, the NIST clocks were stable to within 3.2 parts in 1019 (a 10 with 19 zeroes) over the course of a day.
And, finally, reproducibility is measured by comparing how well two atomic clocks remain in sync. Checking the two clocks 10 times, the team found that the difference in frequency of their ticking was within a quintillionth.
"Systematic uncertainty, stability, and reproducibility can be considered the 'royal flush' of performance for these clocks," says Andrew Ludlow, leader of the project. "The agreement of the two clocks at this unprecedented level, which we call reproducibility, is perhaps the single most important result, because it essentially requires and substantiates the other two results."
In fact, this new level of precision means that the atomic clocks could help us measure gravity more precisely than ever before. Since gravity is known to distort the passage of time – famously demonstrated by the way atomic clocks tick slower in space than on the ground – the atomic clocks can work backwards from measuring time to measuring the effects of gravity.
Applications for that include measuring the geoid – the Earth's gravitational shape – to within 1 cm (0.4 in), making it several times more precise than the current best technology. Atomic clocks could also help detect gravitational waves, which are ripples in the very fabric of spacetime caused by cosmic cataclysms. They could even help out in the hunt for dark matter, which is so far only known through its gravitational interactions.
The research was published in the journal Nature.
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