Mapping the Earth

Proving Einstein right is cool; proving him wrong would be spectacular. More precisely mapping the gravitational field of the Earth might not make the headlines in the same way, but it could deliver major benefits for the economy and quality of life.

The field of science concerned with determining Earth’s shape is called geodesy. Imagine you have a flat lakebed. If it filled with water, you would expect the water to have a flat surface. But if there was a deposit of a dense mineral such as gold under one end of the lakebed, the mineral’s gravity would pull water toward it, altering the shape of the water’s surface.

Geodetic surveys measure a combination of height and gravity known as “geopotential,” explained Derek van Westrum, a scientist with NOAA’s National Geodetic Survey, in a recent podcast. The geopotential predicts where water will go, which is of great interest to civil engineers, emergency planners and others tasked with designing infrastructure and keeping people safe from floods and storms. And because they are sensitive to gravity, clocks can measure this geopotential directly.

This can be a problem if your goal is to tell time accurately. Atomic clock labs must correct for the gravitational difference between their elevation and sea level, to compare their time measurements to those of labs that sit at different elevations.

Van Westrum and colleagues helped the NIST Boulder lab do this in 2015. The geodesists measured the lab’s geopotential to within around 1 part in a billion and installed a survey marker in the lab’s floor. One part per billion is quite a bit less precise than an atomic clock’s measurements of atomic frequencies, but it was good enough to give NIST confidence in its time measurements.

If your goal is to measure gravity, however, the fact that atomic clocks are sensitive to gravity could be a major boon.

Current state-of-the-art GPS-based survey equipment or gravity meters used to measure height above sea level in the field take hours and are accurate only to within centimeters. Today’s best atomic clocks can detect height differences of less than a centimeter, and in theory they can do so very quickly.

But these clocks are complex and fragile; they require expensive lasers and scientists on hand to run them. They are almost always operated in controlled laboratories. Commercial prototypes are just starting to appear, and they will need refinement before they are ready to be used at scale.

If state-of-the-art atomic clocks could be packaged up and hardened to operate in the field, however, they could open up a revolutionary new way to map the Earth’s topography. Van Westrum envisions a global network of atomic clock-based gravity sensors spaced every 10 to 100 kilometers, constantly monitoring for underground water or lava flows, or subtle shifts in Earth’s tectonic plates.

This atomic clock network could warn us when a volcano is about to erupt or an earthquake is about to strike. It could point the way to mineral deposits hidden underground. And it would provide nuanced information on where floodwater is likely to go, aiding engineers, planners and emergency personnel. It could also change how surveying is done.

“There’s a whole team ready to go,” Van Westrum says. “When these clocks are ready, we know what we’ll do with them.”

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