This Tiny Particle Could Upend Everything We Know About Gravity—And the Universe—Scientists Say (2024)

EVERYWHERE YOU LOOK, you can see gravity’s fingerprint. It’s in the path the moon takes around Earth each night and the humbling thump when you wipe out on an icy patch of sidewalk.

The force of gravity governs our daily life and can even help us detect the collision of black holes billions of light years away. Yet, even though gravity is indispensable on the human scale and in the cosmos, it’s barely noticeable when you evaluate the world at the quantum level.

For decades, scientists have dreamed of finding a way to reconcile both gravity’s effects on the classical and quantum scale through complex ideas like string theory or loop quantum gravity. A unified theory of gravity could be the key to solving other big questions in the universe as well—like how the Big Bang began or what makes up dark matter. Yet, while both ideas have their own merit in theory, actually being able to detect the small effects of gravity on the quantum level is another matter entirely.

That’s where new research published earlier this year in Science Advances comes into play. In this work, a research group from the U.K., Netherlands, and Italy designed an experiment so sensitive that it can measure a gravitational force equal to one-quintillionth of a Newton (on the scale of 1 attoNewton) on a particle weighing only 0.43 milligrams. For reference, the gravitational force of one Newton is roughly equivalent to the force of gravity pushing down on an apple sitting on a table.

Tjerk Oosterkamp, Ph.D, is a senior author on the paper and a professor of theoretical physics at Leiden University in the Netherlands. He says that even though the gravitational force his team measured was on a very tiny particle—in fact, the tiniest particle to date to have such a force measured—he stresses that this measurement is still “a million miles away” from demonstrating quantum gravity.

“What we’re saying is that this is a step on the way towards measuring quantum gravity effects,” Oosterkamp explains.

Being able to measure these effects could be an important first step toward a clearer understanding of quantum gravity—which could unlock secrets about the very origin of the universe itself.

YOU CAN THINK ABOUT gravitational effects like a sound wave. To detect a quieter noise, an audio recorder needs to be more sensitive and it needs to filter out background noise. Similarly, the smaller an object, the “quieter” its gravitational force.

To “hear” the gravitational force on their 0.43-milligram particle, Oosterkamp and his colleagues needed to design an experiment to listen very closely while filtering out non-gravitational vibrations, like the random motion of particles buzzing and colliding that creates thermal energy. The cooler the experiment, the fewer stray vibrations to remove.

To do this, the team relied on a combination of tools to increase sensitivity, including: a dilution refrigerator (similar to the kind used to cool down quantum computers) to minimize thermal energy, a mass-spring system to absorb environmental vibrations, and a superconducting “trap” to levitate the small particle to isolate it from any lingering vibrations. A second 2.4-kilogram source mass was placed nearby to create a gravitational force for the levitating particle; two objects with mass are required in such an experiment so that one source’s gravitational force can act upon the other, much like Earth and the moon.

According to Oosterkamp, building this contraption to operate under such extremely cold conditions—very close to absolute zero, or -273.15 degrees Celsius—is what sets this result apart. It’s also why he thought the experiment might never take place to begin with.

“It was unexpected that this actually works,” Oosterkamp says. “I showed my efforts to a retired colleague when he revisited the lab, and he saw all these masses and springs suspended from this very cold plate in our dilution refrigerator, and he asked ‘Why do you expect you can even cool this Christmas tree?’”

Because of these precautions to eliminate excess vibrations, the team was able to measure a 30-attoNewton gravitational force on the levitating test particle.

Yasunori Nomura, Ph.D., is a professor of theoretical physics at UC Berkeley whose work focuses on quantum theory and quantum gravity. Nomura says that while this experimental design could play a role in isolating gravitational forces on even smaller particles, it may still have limitations when attempting to measure quantum gravity itself.

“This measurement is a step toward directly observing gravitational forces in a truly quantum regime,” Nomura says. However, one sticking point, he says, is that the effects of quantum gravity are thought to only become significant at extremely small scales. “Reaching these scales with current measurement techniques, including levitating a small mass in superconducting traps, is impossible,” Nomura says.

Nomura says there may also be other approaches to measuring quantum gravity that avoid directly measuring small particles at all.

WHILE OOSTERKAMP’S GRAVITY DETECTOR may not be measuring quantum gravity effects anytime soon, he hopes that it could soon play a role in detecting large gravity effects instead. In particular, he hopes to use it as a tool to increase the sensitivity of experiments looking for gravitational waves—the ripple effects in spacetime left behind by large gravitational events like colliding black holes. Experiments like the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) and Italy-based Virgo gravitational wave observatory (VIRGO) are already detecting these ripples by measuring very small changes in the path of a laser across multiple kilometers.

“We’re hoping to build the successor to LIGO/VIRGO, which is called the Einstein Telescope,” says Oosterkamp. This telescope is planned to be built in Europe in the mid-2030s and would be a next-generation gravitational wave detector. “They [the LIGO/VIRGO team] can teach us about even lower vibrations, and we tell them what we know about cooling things.”

Rana Adhikari, Ph.D., is a professor of physics at CalTech who has contributed to LIGO. He agrees that learning how to limit vibrations through cooling will play an important role in future gravitational wave detectors.

“The most interesting part [of this work] is how they are able to get the temperature so low and maintain such exquisitely low acceleration noises,” Adhikari says. “Future gravitational wave detectors operated [under cold conditions] will need to build on the foundation of this work. Being able to operate at such a low temperature would eliminate nearly all of the thermodynamic noise sources that we struggle with.”

And while Oosterkamp’s work may not yet pave a clear path toward measuring quantum gravity, Adhikari says that it’s likely one of many puzzle pieces that will unlock this world-changing scientific discovery.

“This [work] is a great example of how experimental ingenuity can lead to making measurements of the universe in a new way,” Adhikari says. “The road towards quantum gravity will be decorated with experiments of ever increasing sensitivity.”