Scientists Watch Atoms Fall to See Earth’s Changing Structure

Of the four fundamental forces, gravity is the most familiar—seemingly unchanging, ubiquitous, and maybe even a little boring. We first meet gravity while dropping our first toys, and later get to know it from the thrill of a rollercoaster and the scabs on our knees. As the years go by, it keeps our feet on the floor and our butts at our desks.

But gravity is far more dynamic than modest everyday interactions suggest. Over the planet, its attraction varies between 32.09 and 32.25 feet per second squared. In these tiny fluctuations, scientists have found, is a wealth of information about our planet’s structure—as long as they can measure the signals. And now they are developing the most precise gravity sensors ever made, by wielding the rules of quantum mechanics.

Physicist Babak Saif, who works at NASA’s Goddard Space Flight Center, in Maryland, has built an instrument that uses atoms to sense gravity. Since an object’s gravitational attraction is directly related to how massive it is, this device essentially weighs nearby material. The instrument is so sensitive that while they were testing it, it yielded different gravity measurements before and after the scientists broke for lunch, says Saif. “It was detecting the food in our stomachs,” he says.

The quantum sensor, which NASA has developed with Bay Area-based company AOSense, relies on some 100 million cesium atoms. The device launches the atoms inside a cylindrical column and times how quickly they fall. As dictated by quantum mechanics, the atoms behave both like particles and waves. Imagine them sloshing in the column like water waves; as an atom wave ripples up the column and back down, it overlaps with itself to create an interference pattern of crests and troughs. That pattern varies based on how fast the atoms rise and fall and can reveal tiny fluctuations in gravity.

Researchers want to launch a version of this machine into space to map Earth’s gravitational field, says NASA Goddard geophysicist Scott Luthcke. In particular, you can use gravity to monitor the masses of glaciers, detect changes in aquifer levels, and even observe how water and air move in a tsunami.

This gravity sensor would replace a pair of satellites currently orbiting Earth known as GRACE-FO. Over a month of measurements, the quantum sensor can detect a 1.5-centimeter change in sea level over an area the size of Hawaii’s Big Island, says Luthcke. Compared to the current satellite pair, it can map Earth’s gravity 10 times more precisely at four times the spatial resolution. Its high precision derives from a design that isolates the atoms from all forces except gravity, in part by keeping the particles in vacuum near absolute zero.

Other researchers want to use similar sensors for projects closer to Earth’s surface. At the University of Birmingham, in the UK, researchers have built a prototype gravity sensor, also based on atom interferometry, for planning infrastructure projects. Before civil engineers can start construction, they need to check that their blueprints do not interfere with buried sewage pipes, building basements, or hidden mine shafts, for example. Gravity sensors can detect underground structures 10 to 50 feet underground without drilling expensive boreholes, says Nicole Metje, a civil engineer at the University of Birmingham. While commercial gravity sensors are already available, civil engineers don’t widely use them because measurements take a long time, and the instruments frequently need to be re-calibrated. The quantum sensor shouldn’t have these issues, says Metje.

Metje and her colleagues think that the quantum sensor could help the construction of a planned high-speed rail in the UK known as HS2. Part of the HS2 railway, which connects the cities of Birmingham and Manchester, will snake through the so-called Black Country, a region known for its Industrial Revolution-era coal mines. “There are probably hundreds or thousands of mine shafts,” says physicist Kai Bongs of the University of Birmingham, who collaborates with Metje. Records of the mine shafts, many of them over a century old, have not been well-documented, so the sensor could help them cheaply assess where they are.

So far, they’ve demonstrated that the device can detect the presence of some 400 pounds of lead placed about a foot and a half away in a laboratory. While the precision pales in comparison to NASA’s instrument, this ground-based tool doesn’t need to measure mass from several hundred miles overhead in a satellite. The current sensitivity is good enough to detect a tunnel underground, says Bongs. They are now testing the device outside in different seasonal conditions.

Some scientists want to use quantum sensors to study gravity itself. Physicist Sergei Kopeikin of the University of Missouri develops experiments that can test general relativity, Einstein’s incomplete theory of gravity.

One of Kopeikin’s approaches is to measure the orbits of different astronomical bodies in the solar system very precisely. If you can carefully track the positions of the planets and moons, you can then compare their trajectories to the predictions of general relativity to look for discrepancies. But you need a precise map of the gravity fluctuations on the moon and Earth to do so, says Kopeikin.

“Imagine you have a ruler made of wood or iron,” he says. “If you increase the temperature, the size of the ruler will increase.” But atoms, made of a fixed number of protons, neutrons, and electrons, are identical by virtue of nature. Quantum sensors exploit atomic properties that do not change at the whims of the environment.

And because gravity is everywhere, these sensors can be repurposed for a variety of applications. You can use gravity measurements to track how Earth’s surface moves after an earthquake or to discover pockets of oil underground, for example. “Gravity is the most pervasive force we humans know of,” says Luthcke. Since you can’t hide from it, you might as well work with it.

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