Gravity is one of the four fundamental forces that glue the stuff of the Universe together. Because the three non-gravitational forces (electromagnetism, and the weak and strong nuclear forces) have been successfully described as arising from the exchange of force-carrying subatomic particles, theorists suspect a similar ‘quantum’ description also exists for gravity.
The force carrier of the electromagnetic force that binds together the atoms in your body is the photon. For the weak nuclear force, which operates within the cores, or nuclei, of atoms, there are three force carriers: the W-, W+ and Z0 bosons; and, for the strong nuclear force, eight kinds of gluons.
The hypothetical force carrier of gravity has been christened the ‘graviton.’
The properties of the graviton aren’t hard to deduce. In quantum theory, the greater the energy required to conjure a force-carrying particle out of the vacuum, the quicker that energy must be paid back. This means a heavy particle like a W+ boson, whose creation requires a lot of energy, exists for a short time, during which it can barely travel at all, explaining the weak force’s tiny range.
In contrast, the range of gravity is infinite, which implies no energy is required to make a graviton. It must, therefore, have zero mass.
Another property that can be deduced for the graviton is its ‘spin.’ In the quantum world, spin is connected to rotations of space.
The fundamental building blocks of matter – quarks and leptons – have spin-½, which means they don’t behave the same after being rotated through 360°, but only after 720°. The force carriers of the three non-gravitational forces all have spin-1, which means they behave the same after a rotation of 360°.
The graviton, however, must have spin-2, because only a spin-2 particle interacts with all matter, a fundamental feature of ‘universal’ gravity. It takes a rotation of 180° to bring it back to the same state.
Of course, it’s possible there’s no description of gravity in terms of the exchange of gravitons. Most physicists, however, believe that gravity can be quantised. The problem is the mathematical techniques used to quantise the other forces fail in the case of gravity.
The only framework that holds any hope is string theory. Here, the fundamental particles are envisaged not as point-like, but as different vibrations of strings of mass-energy.
One of the selling points of string theory is that one such vibrating string has the properties of a graviton. Unfortunately, the theory has serious problems. Not only are the strings postulated to be much smaller than an atom (and therefore undetectable), but in order to reproduce all the fundamental forces, the strings must vibrate in 10 dimensions of space-time – six more than we observe.
Even more seriously, the mathematics of string theory is so complex that it’s yet to make any testable predictions.
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The difficulty of detection
But, just because we don’t have a quantum theory of gravity, doesn’t mean gravitons don’t exist. We know that a light wave is composed of photons travelling together. And, by analogy, we suspect that a gravitational wave – a ripple in the fabric of space-time, first detected in 2015 coming from merging black holes – should be made of gravitons travelling together.
We can dim a light source until a sensitive detector registers the rat-a-tat-tat of individual photons. But detecting individual gravitons is formidably hard. The reason is that gravity is extremely weak – 10,000 billion, billion, billion, billion times feebler than the electromagnetic force – and, in quantum theory, weakness is synonymous with infrequent interaction with matter.
Gravitons, in other words, are hardly ever stopped by atoms, making them phenomenally difficult to detect.
To convey some idea of the elusiveness of gravitons, think of neutrinos. These are created by sunlight-generating nuclear reactions in the Sun and about a trillion pass through you every second without being stopped. Gravitons interact with matter 1,000 million, trillion times less frequently than neutrinos.
We can increase the chances of a neutrino being stopped by a particle by putting a lot of them in its way – using a detector with a large mass. This would also be the strategy for detecting a graviton. However, because of its phenomenally weak interaction with matter, we’re talking about a detector that utterly dwarfs a neutrino detector.
The biggest neutrino detector is IceCube at the South Pole, which consists of one cubic kilometre of Antarctic ice. To have any chance of detecting a graviton, however, a detector would need to have the mass of Jupiter (any bigger and a detector would shrink under its own gravity and become a brown dwarf – a hot failed star).
In 2006, Tony Rothman of Princeton University and Stephen Boughn of Haverford College considered the possibility of using a Jupiter-mass detector. The most powerful source of gravitons considered was Hawking radiation, from evaporating black holes, about one per cent of which should be in the form of the particles. Hawking radiation turns out to be negligible for stellar-mass black holes and significant only for small black holes.
The best sources of gravitons are therefore hypothetical mini black holes created in the extreme and violent conditions of the Big Bang and which have survived until the present day.
Rothman and Boughn made the highly optimistic assumption that, in the Milky Way, as much mass is tied up in mini black holes as in stars, and that the average distance of all these black holes is the distance from Earth to the centre of the galaxy. They concluded that a Jupiter-mass detector would take more than the lifetime of the Universe to register a single graviton.
Steps in the right direction
Such a detector would look for electrons ejected from atoms by gravitons, the gravitational analogue of the ‘photoelectric effect’ in which an electron is kicked out of an atom by a photon. A huge problem, however, would be distinguishing such electrons from those created by neutrinos.
This could only be achieved with a light-years-thick layer of shielding around the detector, which is unfeasible. “I’d bet my house that nobody in this Universe will ever detect a graviton,” says Rothman. “It may be the graviton should be considered a metaphysical, rather than a physical, entity.”
Recently, however, hope has grown that we may be able to explore the properties of gravitons experimentally.
Previously, in the laboratory, researchers have constructed liquid analogues of black holes, in which they have searched for hypothetical Hawking radiation. And now, an international team of scientists, led by Prof Jiehui Liang of Nanjing University in China, claims to have found an analogue of a spin-2 particle in a ‘fractional quantum Hall effect’ liquid.
In this ultra-cold system, confined to two dimensions by a strong magnetic field, electrons behave in a collective manner, like a large flock of starlings during a murmuration. Crucially, the murmuration has the property of a spin-2 particle.
Of course, this isn’t a real graviton. Nevertheless, it raises the possibility of furthering our understanding of these particles. And such an understanding may help physicists break through the roadblock that’s currently stymying their attempts to find the elusive quantum theory of gravity that will unify the fundamental forces into a single theory of everything.
About our expert
Tony Rothman is a semi-retired theoretical physicist who taught at Princeton and Harvard Universities. He has published non-fiction and fiction novels and written various stage plays alongside his academic work. He has been published in Foundations of Physics, European Journal of Physics, and Astrophysics and Space Science (to name a few).
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