January 16, 2019
Francesco Ricci stands at a table covered in a tangle of wires and tiny mirrors. “Here is where everything happens,” he says, pointing to a metal cylinder roughly the size of a cookie tin. A graduate student at the Institute of Photonic Sciences in Barcelona, Ricci is showing me a device he’s built to survey a foreign land: the nanoscape.
Zoom in on the nanoscape and you’ll glimpse atoms partnering up to form molecules, and proteins fastening to the surfaces of bacteria. But it’s difficult to study these Lilliputian characters clearly. “You cannot use everyday devices like a scale for measurement,” says Ricci. He gestures with macroscopic sausage fingers, each one capable of knocking the tiny mirrors on the table completely out of alignment. “You need more precise tools.” To that end, scientists have developed machines gentle enough to tug on strands of DNA and to grasp single atoms.
Ricci’s team has added another tool to their kit. Inside the cookie tin is a sensor that can register weights 100 million times lighter than a single E. coli bacterium. With such high sensitivity, physicists think these devices could pick up tiny signals pointing toward the unknown: a new type of gravitational wave, perhaps, or even dark-matter particles.
The heart of the instrument is a virus-sized, levitating glass bead, kept in suspension by an infrared laser that pelts it with a controlled formation of photons. Because the bead hovers in a vacuum, it experiences pretty much no friction, which means the gentlest of touches can knock it out of place. Chemists could weigh a single molecule, for example, by attaching it to the bead, nudging the bead with a carefully controlled force, and watching the rhythm of its swing. They could then calculate its mass from the pace: Lighter molecules swing faster.
The hallmark of Ricci’s instrument is its accuracy. Other scientists have developed similarly sensitive instruments—ones that can detect weight fluctuations as small as a single proton. But their readouts were far less reliable, says physicist Andrew Geraci of Northwestern University. Some sensors quoted weights that were off by 30 percent or more, equivalent to a bathroom scale error of about 50 pounds.
By contrast, Ricci’s sensor can achieve about 1 percent accuracy, equivalent to a bathroom scale offset by about 1.5 pounds. One goal for such precise sensors is to create high-resolution images of individual proteins and other molecules, says physicist Adrian Bachtold, a colleague of Ricci’s who is not involved in this work. Bachtold is developing similar sensors made of carbon nanotubes. For example, you could place a single molecule in a magnetic field, which rotates the molecule’s constituent atoms. Because distinct elements rotate at different rates, a nearby force sensor could detect the rate of rotation of the atoms to identify them.
Ricci’s sensor could also be adapted to study some of physics’ most baffling conundrums, says Geraci. For example, physicists have struggled for decades to explain why the laws of gravity, which accurately explain how stars move on a galactic scale, are incompatible with the microscopic rules of quantum mechanics. To get at that question, Geraci’s team is currently performing an experiment that levitates a nanoscale glass bead extremely close to a small gold mirror. They are trying to measure the tiny gravitational attraction between the two objects. If done precisely enough, they could rule out proposed ideas about the quantum nature of gravity. Ricci’s calibration technique could help them achieve this precision.
In addition, Geraci is building an instrument using levitated nano-beads to look for high-frequency gravitational waves—narrow ripples in spacetime that existing labs like LIGO were not designed to detect. When such a wave travels over the glass bead, it should alter the shape of the laser beam keeping the bead in suspension. The bead would move, and the sensor could detect that motion. Theorists predict that these gravitational waves should be rare, but nobody has really looked for them, says astrophysicist Nergis Mavalvala of the Massachusetts Institute of Technology.
The cost of such a detector is dirt cheap, relatively speaking. Geraci’s team has determined that the machine would only need to be about 3 feet long and could sit on a tabletop. Compare that to LIGO, whose two L-shaped instruments consists of arms stretching two and a half miles and have cumulatively cost over a billion dollars to build. “To detect high-frequency gravitational waves, you can get away with building something that is technically easier and cheaper,” says Mavalvala.
If detected, these narrower gravitational waves could aid the search for dark matter, a hypothesized substance that physicists think should make up 85 percent of the universe’s mass. Geraci and Asimina Arvanitaki, a physicist at the Perimeter Institute, have determined that a hypothesized dark-matter particle called an axion interacting with a black hole should produce these waves.
Scientists are only as acute as their tools. In the nanoscape, “it’s extremely difficult to understand what you’re measuring,” says Bachtold. A carefully calibrated sensor like Ricci’s helps physicists with the meta question: how well they can trust their own measurements.
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