Scientists take the smallest gravity measurement ever in Quantum Quest

Scientists have broken the record for the smallest gravity measurements using a technique they say has the potential to become much smaller. So small that it can help us determine whether gravity is quantized, and if so, how general relativity and quantum mechanics can be reconciled.

One of the great revelations of the early 20th century is that energy is not continuous, but exists in tiny packets known as quanta. This discovery, which was stunning in itself, led to numerous follow-up studies showing that other things were also being quantified. However, uncertainty remains about how far this extends: for example, do time and gravity also exist in packages so small that we have not been able to find them?

The existence of quantized gravity is widely considered to be the key to resolving the apparent incompatibility between quantum mechanics and general relativity, our best theory for representing gravity. However, decades of searching have failed to find evidence for this quantization, or to theoretically explain how it would work to a standard generally considered acceptable. New experiments bring us closer to that goal.

Formerly at Leiden University and now at the University of Southampton, Dr Tim Fuchs led a team that used a floating magnet to measure the effects of gravity on a particle weighing 0.43 milligrams (0.000015 ounces) when it was cooled to -273.14°C, one-tenth of a degree above absolute zero.

The super-cold conditions minimize the particle’s vibrations, allowing the team to measure a gravity of just 30 attonewtons (3*10).-17 N, or to write it out for impact 0.00000000000000003 N) on it. That’s still larger than the likely size of gravitational quanta, if they exist. However, Fuchs argues that the same technique can become even smaller until he discovers whether gravity can have any force, or is limited to discrete jumps. The approach is analogous to Robert Millikan’s first measuring the electron’s charge, showing that the total charge of an oil droplet is always a multiple of a specific number.

The ‘particle’ used, although small, is within our experience: it is the correct order for a grain of sand or sugar. The force is a lot smaller and is not caused by the gravitational pull of the entire Earth, but by blocks that weigh only 1 kilogram. A wheel tuned the weights so that their influence on the particle could be measured at different distances.

“For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together,” Fuchs said in a statement. “Now that we have successfully measured gravitational signals at the smallest mass ever measured, it means we are one step closer to understanding how this works together.”

“From here we will start scaling back the source using this technique until we reach the quantum world on both sides,” Fuchs continued. “By understanding quantum gravity, we could solve some of the mysteries of our universe – like how it started, what happens inside black holes, and uniting all the forces into one big theory.”

For this to happen, quantum gravity must be real, which some physicists doubt. If Fuchs’ work finds no sign of quantization at increasingly smaller forces, those voices will become louder.

Although the idea seems simple, gravity is very difficult to measure on a microscopic scale because it is so weak. It may not feel that way to us, crushed under the weight of a planet that doesn’t want to set us free. However, in the world of the little ones, gravity is completely overwhelmed by the power of the other three forces, and experiments must find ways to take that into account.

To do the work, the team needed advanced superconducting traps, precise magnetic fields and sensitive detectors protected from vibrations. “We are pushing the boundaries of science,” says co-author Professor Hendrik Ulbricht. “Unraveling these mysteries will help us unlock more secrets about the structure of the universe, from the smallest particles to the largest cosmic structures.”

The research is open access in Science Advances.

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