Quantum Gravity: Detection of Weak Gravitational Pull on Microscopic Particles Reveals How Force Operates at Subatomic Level

There are four fundamental forces which are responsible for shaping the universe that we inhabit. Among these, gravity differs because it is best described as a curvature of space-time, making it resistant to unifications with quantum theory.

Reconciling Gravity and Quantum Theory

Understanding the gravitational force at the quantum scale could help unveil the grand mysteries of the universe, but scientists have never fully understood how the force discovered by Isaac Newton works in the quantum realm. Even Albert Einstein, in his theory of general relativity, claimed that there is no realistic experiment that can show a quantum version of gravity.

For more than a century, experts have tried and failed to unite gravity with the rules of the subatomic world. Physicists have tried experiments to increase the sensitivity of gravity, like in general relativistic effects in atom clocks, in testing the equivalence principle, and in precision measurements of Newton's constant.

However, scientists have never tested gravity for small masses and on the level of the Planck mass. Measuring this force from classical sources in laboratory settings is contrasted by an increasing interest to understand gravitational phenomena from quantum states of source masses.

Measuring Microscopic Gravity

In a recent breakthrough, physicists at the University of Southampton have successfully detected a weak gravitational pull on a tiny particle. They believe that their discovery can pave the way to finding the quantum gravity theory.

In this experiment, the researchers used levitating magnets to detect gravity on particles which are small enough to border on the quantum realm. This allowed them to measure gravitational signals at the smallest mass ever recorded. The details of their findings are described in the paper "Measuring gravity with milligram levitated masses".

Led by Tim Fuchs, experts from Southampton collaborated with scientists from Leiden University in the Netherlands and from the Institute for Photonics and Nanotechnologies in Italy. Their experiment centered on a magnetic particle levitated above a superconductor cooled to 1/100 of a degree above absolute zero. This was done while being heavily protected against interference from vibrations.

The sophisticated setup involving superconducting devices measures a weak pull of just 30-attonewton. This was observed on a tiny particle that measures only 0.43 mg. The team then measured the almost negligible pull of the hovering particle using an electrical bicycle wheel. As the wheel is fitted with brass weights that revolve about one meter away, the weights were brought near the particle and then back again.

When the wheel started to spin, it caused the particle to move a bit like a swing. Gravity pulled it, let it go, and then pulled it again. The gravitational force between two objects is affected by their masses and the distance between them. This means that the larger and closer they are, the stronger the attraction.

The gravitational pull measured in the experiment may be too small, since an attonewton is one billionth of a billionth of a newton. It is definitely not yet quantum gravity, but it can be a stepping stone towards it, according to Fuchs.

By demonstrating that their equipment works, the research team hopes to measure the behavior of gravity between smaller and smaller particles which are increasingly influenced by the rules of quantum mechanics. This could take some time and the first of such experiments could take another five to ten years.