A teeny device can measure subtle shifts in Earth’s gravitational field

There’s a new entrant in the competition to develop ever-tinier instruments that can detect changes in our planet’s gravitational field — a tabletop device roughly the size of two smartphone stacked together.

Created by a team of researchers in China, the instrument has been used to measure the Earth’s tides over the course of several days, physicist Pu Huang of Nanjing University and his colleagues report in the Mar. 22 Physical Review Letters.

Researchers have long sought to build lightweight and cost-effective gravity-measuring instruments. Called gravimeters, such devices can detect tectonic plates shifting, sense the movement of underground water, reveal hidden oil and gas reserves, and track magma within volcanoes to provide data for predicting eruptions, along with many other applications.

Yet most instruments with enough sensitivity for such experiments remain stuck inside of laboratories, requiring bulky equipment to create protective vacuums or cool their parts to extremely low temperatures (SN: 11/7/19). Portable gravimeters tend to be extremely expensive objects the size of household appliances (SN: 2/28/22). But far smaller ones exist, such as a prototype for a postage stamp–size gravimeter, reported in 2016, with enough sensitivity for some basic geophysics applications.

The strength of gravity between two objects depends on their masses and the distance between them. Gravitational changes can therefore be measured by something like a weight on a spring. If such an apparatus is moved closer to or farther from the ground, or over abrupt changes in the terrain’s density, the Earth’s gravitational pull on the weight will change, and the length of the spring will correspondingly change very slightly.

Instead of a weight and spring, Huang and his team used two magnets, one levitated by the other. The levitated magnet acted like the weight, moving up and down in response to a shifting gravitational field. The intangible force of magnetism between the magnets worked akin to the solid spring. A laser measured changes in the position of the levitated magnet, allowing the researchers to watch the Earth’s gravitational field fluctuate over several days in response to tidal forces from the moon. They also detected two earthquakes, one off the coast of Honshu, Japan, and one in the Bali Sea of Indonesia.

The new device is not quite as sensitive as the world’s best portable gravimeters, notes physicist Xuejian Wu of Rutgers University in Newark, N.J., who wasn’t involved in the work. These larger top-of-the-line devices can detect changes to gravitational fields of around one part in a billion, whereas this one can do so to about one part in 60 million.

But, Huang says, the team has plans to increase the sensitivity while shrinking their device even further. Currently, most of its bulk comes from magnetic shielding and other supporting equipment. But the heart of the instrument — the tiny, levitated magnet — is smaller than a grain of rice and weighs less than two coffee beans. Within the next couple years, Huang says, the researchers hope to integrate the entire system into a chip-sized device that can be placed on drones for fieldwork.