Scientists want to change the way a second is defined

A new optical clock ticks so consistently that if it had started at the dawn of the universe, it would have lost less than two minutes

Ian Johnston
Science Correspondent
Wednesday 08 June 2016 22:51
A six-hour workday trial has been abandoned at one retirement home in the Swedish cityome of Gothenberg despite workers reporting better health, reduced absenteeism and improved quality of patient care
A six-hour workday trial has been abandoned at one retirement home in the Swedish cityome of Gothenberg despite workers reporting better health, reduced absenteeism and improved quality of patient care

For nearly 50 years, the length of a second has been defined in the same way.

But researchers in Germany now believe they have found how to make the most accurate clock ever created, which keeps time so well that if it had started 14 billion years ago at the Big Bang it would have lost just 100 seconds.

While the change in accuracy would hardly be noticed by humans, it could actually be significant for GPS navigation – increasing the accuracy on the ground from a few metres to a few centimetres – as well as electrical power grids and computerised financial networks.

And because the clock uses a different way of measuring time, it could alter the length of the basic units of seconds, minutes and hours by a minuscule amount.

The way time is measured is still based on the idea of a pendulum, except atomic clocks measure the natural, regular back-and-forth movement of an atom of caesium.

Since 1967, the International System of Units (SI) has defined the second as the time that elapses during 9,192,631,770 cycles of the microwave signal produced by these oscillations.

However, just like a traditional pendulum clock, they can run fast or slow with an error of about a nanosecond every 30 days.

A paper published in the journal Optica describes a new clock that uses strontium atoms, which “tick” much faster than caesium. And they do so in the optical, rather than the microwave, part of the spectrum.

If a second was defined in terms of strontium, the equivalent SI unit would be about 429,000 billion cycles. This method of calculating the length of a second reduces the error to less than 0.2 nanoseconds in 25 days, the researchers said.

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Dr Christian Grebing, of the The National Metrology Institute of Germany who worked on the new clock, said: “Our study is a milestone in terms of practical implementation of optical clocks.

“The message is that we could today implement these optical clocks into the time-keeping infrastructure that we have now, and we would gain.”

Optical clocks, which use lasers to monitor atoms or ions in a vacuum chamber shielded from outside influences, have been known about for some time but have been plagued with problems because they are so technically complicated and have a tendency to stop working.

Dr Grebing said their method showed they could be made to work in a practically useful way, but added that formally redefining the length of a second should be delayed for perhaps another 10 years.

This would allow time – even if measured by inferior caesium atoms – for scientists to work out which optical clocks were the most accurate. Other rival versions use different elements, such as aluminium atoms and ions, instead of strontium.

“We want to improve the timekeeping infrastructure all over the world by building better and better clocks and integrating them into the time-keeping infrastructure," Dr Grebing said.

“What we demonstrated is a first step towards a global improvement of timekeeping.”

If the length of a second was redefined, the idea would be to make it as close as possible to the current second, but it would be likely to produce a tiny change, he added.

“If you change the definition, you want a very smooth change to minimise the change in the length,” Dr Grebing said.

“Probably the change would be less than the current uncertainty of a second defined by caesium atoms.”

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