What are gravitons and are they real? © Alamy

What are gravitons, and do they really exist?

Most theorists believe in the existence of gravitons but finding them and proving their existence is difficult.

Asked by: Graham Pollard, Saddleworth

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Gravitons are at the heart of arguably the biggest challenge in theoretical physics: the search for the ‘theory of everything’ – a set of equations describing all of the forces and particles in the Universe.

For decades, theorists have struggled to unify Einstein’s theory of gravity, known as General Relativity (GR), with quantum theory, which describes the subatomic world. That’s because each theory takes a radically different view of the force of gravity.

According to Einstein, matter distorts the very fabric of space and time around it, creating the effect of an attractive force field. But quantum theory describes all forces in terms of so-called ‘exchange particles’, flitting from place to place. In the case of gravity, those particles are known as ‘gravitons’.

Most theorists believe that gravitons must exist, because quantum theory has successfully explained every other force of nature. But not everyone agrees. No theory claiming to unify quantum theory with GR has been successfully verified, and this has raised suspicions that perhaps gravity isn’t like any other force – in which case gravitons may not exist.

Even if they do, finding them is another matter. Quantum theory predicts that as gravity has an effectively infinite range, the graviton must have an incredibly low mass. Studies of gravitational waves from colliding black holes suggest that the graviton must be at least a billion, billion, billion times lighter even than the electron.

Gravity is also by far the most feeble fundamental force in nature. This means that any graviton detector must be incredibly massive and placed near a powerful source of gravitons. Calculations suggests that even a detector with the mass of Jupiter orbiting a bizarre object like a neutron star (a potential strong source of gravitons) would struggle to find anything.

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