Since it was first proposed, the graviton (the particle theorised to be responsible for the gravitational force) has long been considered undetectable, but this may no longer be the case.
The graviton is based on the theory of quantum mechanics, which describes particles as having a specific energy value rather than continuous values as seen in waves. This framework explains forces in terms of an ‘exchange particle’, where forces are ‘carried’ by the particle between objects, like two basketball players throwing a ball to each other. In this scenario, the basketball players are the two objects that experience the force, the ball is the exchange particle, and the force felt by both players is the force transmitted by the particle. According to this theory, each fundamental force has its own exchange particle. Similar to how the photon, the smallest component of the electromagnetic force, transmits the force between
charged particles, gravitons would transmit the gravitational force between objects. Currently, the relevant exchange particle has been directly or indirectly observed for all other fundamental forces. Gravity is the only exception, and the graviton has remained unconfirmed.
Unlike electromagnetism, which is strong and relatively easy to detect and study, gravity is very weak and only becomes noticeable on massive scales, for example the Moon orbiting the Earth. This makes the graviton extremely hard to detect, as its individual effects are extremely subtle, and any attempt has previously been considered virtually impossible. However, new research may have found a way to detect a graviton after all.
A team of researchers, led by Dr. Igor Pikovski at the Stevens Institute of Technology in New Jersey, has proposed an experiment that, when combined with the detection of gravitational waves, could provide the first evidence of the graviton. Firstly, a bar of material, such as beryllium, would be cooled to absolute zero (0 Kelvin or -273.15 °C, the lowest theorised temperature), placing all its atoms in their lowest possible energy state. The team then waits
until a gravitational wave passes through the bar, which would cause the atoms to excite to a higher energy level. This excitation would suggest a possible interaction with a graviton. The observation would then be compared with data from the Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected a gravitational wave in 2015, to confirm that a gravitational wave did indeed pass through the bar. This ensures the excitation was caused by a gravitational wave and not a background source. Unfortunately, the quantum sensing technology needed to detect this subtle change in energy does not yet exist.
As our current models of gravity describing large and small scales are not compatible, the discovery of the graviton would have a profound effect on our understanding of physics. Quantum Theory and Einstein’s General Relativity (the theory that describes gravity as the warping of space-time), are not currently unified. As every other fundamental force can be successfully described by quantum theory, finding proof of the graviton’s existence would be evidence that gravity can also be quantised. This would mark a major step forward in the search for the ‘Theory of Everything’, a unified framework that explains all the fundamental forces and describes the workings of the universe at every scale.
Despite this promising development, some physicists remain sceptical that the experiment would provide definitive proof of the graviton’s existence. They argue that the same effect could be observed simply with a gravitational wave that is not quantised into gravitons, meaning this experiment would not prove the particle’s detection. In other words, they believe the result could occur even if gravity behaves as a continuous field or wave, rather than as
individual particles, as it does in non-quantum (classical) theory. Still, even the possibility of detecting gravity’s quantum signature has been enough to generate significant interest. This newly proposed experiment may not give one hundred percent certainty, but it represents an important and exciting step towards uncovering finding proof of quantised gravity.
Article written by Daisy Young, a 4th year Physics and Astrophysics masters student at the University of Edinburgh.
Article edited by Eleanor Stamp, a Neuroscience PhD student at the Institute of Genetics and Cancer, University of Edinburgh, and an Online News Editor for EUSci.
RESOURCES:
https://www.ligo.caltech.edu/page/timeline
https://www.nature.com/articles/s41467-024-51420-8
https://www.quantamagazine.org/it-might-be-possible-to-detect-gravitons-after-all-20241030/
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.044009
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