A Fresh Look At Keeping Time

May 26, 2019

So, it happened. And you probably didn’t notice. But the long-awaited, new definition of the kilogram went into effect this past week.

The metal cylinder that has defined the kilo for over a century has now been replaced with a mathematical equation based on a number known as the Planck constant.

It was the last physical artefact defining any unit of measurement in the international system, so its replacement marks an important point in efforts to define those units in ways that are permanent and immutable – based on constants of nature, rather than somebody’s foot or a hunk of metal. But it’s certainly not the end of the effort.

Now they’re taking on the second with renewed vigor. You know, the base unit of time in the International System of Units. That second. 

Physicists have already redefined the second once, but they’ve agreed to revise it again as new technologies are making it possible to be even more precise.

Since 1967, the definition of a second has been associated with atomic clocks.

What’s that?

“Atomic clocks aren't so different from any other type of clock,” said Andrew Ludlow, a physicist with the National Institute of Standards and Technology.

Picture a pendulum clock. The pendulum swings back and forth, once a second. Then the gears and the face of the clock count up the number of swings of the pendulum.

“An atomic clock is pretty similar,” he said. “We'll have some type of internal oscillator – in this case it's the electron of an atom that essentially oscillates back and forth. And then we have some mechanism for counting those oscillations. When we when we get to a certain number of counts, we call that a second.”

Using cesium, that’s about nine billion oscillations per second.

Ludlow is leading the development of something called “optical” atomic clocks, which could soon define the second again.

If you thought nine billion oscillations per second was a lot of tick-tocks, optical clocks have even more.

“If we could peer down into the electron, we'd see the oscillations of the electron…could be many hundreds of thousands of times faster,” he said.

That puts the operation into a different part of the electromagnetic spectrum. It’s no longer in the microwave domain where traditional atomic clocks operate, but rather in the optical domain.

“So, the oscillation rate of these clocks might be hundreds of terahertz,” he said.

It turns out that kind of precision can be quite useful. It could improve navigation and communication systems. It could be leveraged to measure other quantities in physics. It’s a better yardstick for science.