WHEN YOU’RE ON A MOUNTAINTOP, TIME REALLY FLIES.

SLOWS AT LOW ALTITUDES, RESEARCH SHOWS

The Global Positioning System, or GPS, is a way of measuring signals from satellites to figure out exactly where you are. It is extraordinarily accurate — more than enough for driving directions, for example — but not perfect. There are discrepancies of several centimetres.

Part of the problem is the effect of relativity, Albert Einstein’s famous theory that explained gravity as a distortion of space and time. GPS measurements are always a little bit off because the immense gravity of Earth bends the space-time around it, just as Einstein predicted.

A new experiment with an ultra-precise optical atomic clock, however, offers a glimpse of possible future mapping technology, in which location can be calculated simply by testing tiny distortions in the flow of time, caused by Earth’s gravity.

The experiment, reported Monday in Nature Physics, involved the first-ever field test of an optical atomic clock outside of a laboratory, which itself is being hailed as a “significant milestone,” given how sensitive the clocks are to environmental disturbance. But the exciting aspect for future map-makers was the comparison of time flow at two separate locations: one at a lab in Torino in northwest Italy, the other 1,000 metres higher in the French Alps, at the Laboratoire Souterrain de Modane, a scientific facility in the middle of the 13-kilometre Fréjus Road Tunnel, which runs under a high mountain pass.

The difference in altitude was the key.

There are two main ways the flow of time can be distorted. One is that time goes slower at high velocities. So astronauts do not age as fast as people on Earth, a quirk that has been exploited by many sci-fi movies.

But there is another way. Time goes more slowly under higher gravity. One result of this, for example, is that over the 4.5-billionyear age of the Earth, the core is sometimes said to be about 30 months younger than the crust. Another result, which this new experiment sought to measure, is that time moves slower at lower altitudes, when the observer is closer to the centre of the Earth, and thus under more gravity.

“While the effect is typically small on Earth, atomic clocks can measure it,” writes Andrew D. Ludlow of the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colo., in a commentary in the same journal. “In this way, an optical (atomic) clock becomes a gravity sensor …”

Like computers in their early days, modern atomic clocks are unwieldy and finicky. This one was the size of a suitcase, bolted to a table inside a car trailer, which allowed it to be moved from its normal home in Braunschweig, Germany, to the French Alps.

It can measure time to incredible precision, with errors of only about one part in 10,000,000,000,000,000,000. It does this by cooling and trapping atoms so their electrons can interact with a laser at frequencies as high as that of light waves. Time is thereby broken up into what Ludlow calls “ultrafine intervals,” which can be counted to measure the passage of time.

“Doing this is much easier said than done,” Ludlow notes, and so it was.

The experimenters had to deal with increased “seismic noise” from intermittent explosions because workers are building a new escape tunnel. Their special laser, made of sapphire crystal and titanium, overheated. And the temperature kept fluctuating.

Still, the team of European researchers was able to show the time passed more slowly at the lower altitude in Torino, and faster high in the middle of the mountain. And they were able to use that discrepancy to calculate the difference in altitude.

They had, in effect, measured space by measuring time.

“By carrying out the first successful measurements of a transportable optical clock outside the lab, the authors have showcased these systems’ tremendous potential and affirmed their bright future,” Ludlow wrote.

The researchers acknowledged, however, that their measurements of the altitude of the tunnel were still considerably less precise than the most precise measurements available by more traditional methods.

The same kind of work is behind the ongoing official redefinition of the second by the International Committee for Weights and Measures. The second, as the fundamental unit of time, was once defined as a fraction of a year, but the more accepted definition today is in terms of a number of oscillations (about 9 billion) of a kind of cesium atom.

The research also has potential applications in other tests of the predictions of theoretical physics, such as the existence of dark matter. As the Italian researchers say, atomic clocks “are already being used to test physical theories and herald a revision of the International System of Units.”

WHILE THE EFFECT IS TYPICALLY SMALL ON EARTH, ATOMIC CLOCKS CAN MEASURE IT.