Scientists are trying to redefine how we measure time – here’s why


Everyone needs to know the time. Ever since 17th-century Dutch inventor Christiaan Huygens made the first pendulum clock, people have thought about good reasons for measuring time more accurately.

Getting the timing right is important in many ways, from running a railroad to making millisecond trades in the stock market. Now, for most of us, our clocks check themselves against a signal from atomic clocks, like those on board Global Positioning System (GPS) satellites.

But a recent study by two teams of scientists in Boulder, Colo., Could mean that those signals will become much more precise, paving the way for us to effectively redefine the latter more precisely. Atomic clocks could become so precise, in fact, that we could begin to measure previously imperceptible gravity waves.

Brief history of time

Modern clocks still use Huygens’ basic idea of ​​an oscillator with a resonance – such as a pendulum of a fixed length that will always move back and forth with the same frequency, or a bell that rings with a tone. specific. This idea was greatly improved in the 18th century by John Harrison who realized that smaller, higher frequency oscillators have more stable and purer resonances, making clocks more reliable.

Credit: Andrew Seaman

Nowadays, most everyday clocks use a tiny piece of quartz crystal in the shape of a miniature musical tuning fork, with very high frequency and stability. Not much has changed with this clock design over the past hundred years, although we’ve done better to make them cheaper and more repeatable.

The huge difference these days is the way we check – or “discipline” – quartz clocks. Until 1955, you had to keep correcting your clock by comparing it to a very regular astronomical phenomenon, like the Sun or the moons of Jupiter. Now we are disciplining the clocks against the natural oscillations inside atoms.

The atomic clock was first built by Louis Essen. It was used to redefine the second in 1967, a definition that has remained the same since.

It works by counting the flip frequency of a quantum property called spin in electrons in cesium atoms. This natural atomic resonance is so sharp that you can tell if your quartz crystal clock signal is moving away in frequency by less than one part in 10¹⁵, or one millionth of a billionth. A second is officially defined as 9,192,631,770 spin reversals of cesium electrons.

The fact that we can make such disciplined oscillators with precision makes frequency and time the most accurate measurements of any physical quantity. We send atomic clock signals all over the world and in space via GPS. Anyone with a GPS receiver in their mobile phone has access to a time-measuring device with amazing precision.

Read more: Why we’ll probably never have a perfect clock

If you can measure time and frequency accurately, there are all kinds of other things that you can measure accurately as well. For example, measuring the frequency of spin reversal of certain atoms and molecules can tell you the strength of the magnetic field they are experiencing, so if you can find the frequency accurately, you are also finding the field strength accurately. The smallest possible magnetic field sensors work this way.

But can we make better clocks that allow us to measure frequency or time even more precisely? The answer could still be that of John Harrison, turn up the frequency.

The cesium spin flip resonance has a frequency corresponding to microwaves, but some atoms have nice sharp resonances for optical light, a frequency a million times higher. Optical atomic clocks have shown extremely stable comparisons with each other, at least when a pair of them is placed only a few feet apart.

Scientists wonder if the international definition of the second could be redefined to make it more precise. But to achieve that, the different optical clocks that we would use to keep time accurately must be reliable to read the same time, even if they are in different laboratories thousands of miles apart. So far, these long-range tests haven’t been much better than microwave clocks.

Better clocks

Now, using a new way of linking clocks with ultra-fast lasers, researchers have shown that different types of optical atomic clocks can be placed within a few miles of each other and still match in 1 part on 10¹⁸. This is just as good as previous measurements with pairs of identical clocks a few hundred meters apart, but about a hundred times more accurate than before with different clocks or long distances.

The authors of the new study compared several clocks based on different types of atoms – ytterbium, aluminum and strontium in their case. The strontium clock was located at the University of Colorado and the other two were at the US National Institute of Standards and Technology down the road.

A diagram showing three atomic clocks compared at a distance from each other.