Gravitons, the particles thought to carry gravity, have never been seen in space – but something very similar has been detected in a semiconductor
Physicists have been searching for gravitons, the
hypothetical particles thought to carry gravity, for decades. These have never
been detected in space, but graviton-like particles have now been seen in a
semiconductor. Using these to understand gravitons’ behaviour could help unite
the general theory of relativity and quantum mechanics, which have long been at
odds.
“This is a needle in a haystack [finding]. And the paper
that started this whole thing is from way back in 1993,” says Loren Pfeiffer at
Princeton University. He wrote that paper with several colleagues including
Aron Pinczuk, who passed away in 2022 before they could find hints of the
elusive particles.
Pinczuk’s students and collaborators, including Pfeiffer,
have now completed the experiment the two began discussing 30 years ago. They
focused on electrons within a flat piece of the semiconductor gallium arsenide,
which they placed in a powerful refrigerator and exposed to a strong magnetic
field. Under these conditions, quantum effects make the electrons behave
strangely – they strongly interact with each other and form an unusual
incompressible liquid.
This liquid is not calm but features collective motions
where all the electrons move in concert, which can give rise to particle-like
excitations. To examine those excitations, the team shined a carefully tuned
laser on the semiconductor and analysed the light that scattered off it.
This revealed that the excitation had a kind of quantum spin
that has only ever been theorised to exist in gravitons. Though this isn’t a
graviton per se, it is the closest thing we have seen.
Ziyu Liu at Columbia University in New York who worked on
the experiment says he and his colleagues knew that graviton-like excitations
could exist in their semiconductor, but it took years to make the experiment
precise enough to detect them. “From the theoretical side, the story was kind
of complete, but in experiments, we were really not sure,” he says.
The experiment isn’t a true analogue to space-time –
electrons are confined to a flat, two-dimensional space and move more slowly
than objects governed by the theory of relativity.
But it is “extremely important” and bridges different
branches of physics, like the physics of materials and theories of gravity, in
a previously underappreciated way, says Kun Yang at Florida State University,
who was not involved in the work.
However, Zlatko Papic at the University of Leeds in the UK
cautions against equating the new finding with detection of gravitons in space.
He says the two are sufficiently equivalent for electron systems like those in
the new experiment to become testing grounds for some theories of quantum
gravity, but not for every single quantum phenomenon that happens to space-time
at cosmic scales.
Connections between this particle-like excitation and
theoretical gravitons also raise new ideas about exotic electron states, says
team member Lingjie Du at Nanjing University in China.