Physicists have discovered a way to measure gravity on a quantum scale: ScienceAlert

By acting on a small particle levitating in a magnetic trap, physicists have measured the smallest gravitational force ever recorded.

The particle weighed only 0.43 grams. The force of gravity acting on the scale was attonewtons (10^-18Newton). This is small enough to be on the edge of the quantum realm, raising the possibility of discovering how classical physics and quantum mechanics interact.

“For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together.” says physicist Tim Fox from Leiden University and the University of Southampton, who led the research.

“We have now succeeded in measuring gravitational signals at the smallest mass ever recorded, meaning we are one step closer to understanding how they work in tandem.”

The problem of quantum gravity is perhaps best described as intractable, at least so far. It's about our frameworks for explaining the universe.

Classical physics – gravity – explains how things work on most scales. When you get very small, down to atomic and subatomic scales, gravity no longer works to explain what we see.

That's why physicists use quantum mechanics, and that's great. But, just as classical physics cannot be applied to quantum scales, quantum mechanics does not work on classical scales. However, somehow, the universe works. This leads scientists to believe that a solution between the two frameworks has yet to be found.

One potential way to investigate the problem is to examine gravity on very small scales. However, this is more difficult than it may seem: gravity is everywhere in the universe, and extracting a quantum scale signal in Earth's gravitational environment is not easy.

To circumvent this dilemma, Fuchs and his team used what is called a superconducting magnetic trap. A small trap made of tantalum is cooled to a critical temperature of 4.48 K (-268.67 °C, or -451.6 °F).

In the chamber, particles are lifted. This consists of three 0.25mm neodymium magnet fields A glass ball with a diameter of 0.25 mm is stuck together to form a single particle with a mass of about 0.43 grams.

The device is suspended from springs in a collective spring system to protect the experiment from external vibrations, and the cryostat is placed on air dampers to reduce vibrations from the building.

Finally, an electrically operated wheel was placed with a set of three 2.45 kg copper blocks to create the gravity gradient. This produced a measurable effect on the particle – a gravitational force of just 30 ton.

It's the smallest scale at which physicists have measured gravity, beating a record set just three years ago by two 90-milligram balls of gold.

Researchers say this is just the first step. Now that they have proven the effectiveness of their experiment, they aim to push it much further.

“From here we will begin to reduce the size of the source using this technique until we reach the quantum world on both sides.” Fox says. “By understanding quantum gravity, we can solve some of the mysteries of our universe – such as how it began, what happens inside black holes, or unifying all the forces into one big theory.”

There will always be more to do, but now we're starting to feel like the answers are just a quantum leap away.

The team's research has been published in Advancement of science.