Does the graviton really exist?

Do gravitons really exist?” asks us on the Facebook page Science and the future Levi Tooker. This commentary, chosen during the Question of the Week ritual, brings us to the heart of one of the fundamental interactions of the universe.

What is a graviton?

This particle is a quantum (the smallest indivisible measure, ed.) gravitational energy in the same way that a photon is a quantum of light“, explains Science and the future Pierre Vanhove, researcher at the Institute for Theoretical Physics and author of the chapter “Does the graviton exist?“from the book”What is gravity? The great challenge of physics“(Editions Duno). The graviton will have zero mass and a spin (an intrinsic property of the particle associated with its rotation) equal to two.”The existence of a photon is predicted by quantifying the electromagnetic force between two charged bodies. A graviton is a theoretical prediction of a quantitative estimate of the force of gravitational attraction between two massive bodies.“ continues Mr. Vanhov.

Does this particle exist?

If the existence of the photon has been proven experimentally, then the existence of the graviton has not yet been the subject of direct experimental observation or confirmation. “But we hope it exists says the physicist. The consistency of the theory of fundamental interactions suggests the introduction of the graviton as a particle-intermediary force of gravity. Just as the photon mediates the electromagnetic force, the Z, W+, and W- bosons serve the weak force, while the gluons serve the strong force.“.

4 fundamental interactions of the Universe:

– I’electromagnetic interaction : As the name suggests, it governs all electrical and magnetic phenomena. It can be attractive or repulsive: elements with the same electrical charge repel each other, and elements with opposite charges attract. “This interaction is associated with the existence of electric charges and is largely responsible for the adhesion of atoms due to the binding of electrons (negative electric charge) attracted by the nucleus of the atom (positive electric charge).“, explains the Atomic Energy Commission (CEA) on its website. Its intermediary particle, as Pierre Vanhove points out, is a photon.

– I’weak interaction or weak interaction: it is responsible for Beta radioactivity, i.e. radiation, called Beta, is emitted by some radioactive nuclei that decay under the influence of this fundamental interaction. It is this process that makes it possible, for example, to date carbon-14, a radioactive isotope that converts to nitrogen-14 when beta radiation is emitted. Therefore, its intermediary particles are Z, W+, W- bosons.

– I’strong interaction or strong force: this interaction ensures the cohesion of the atom’s nucleus. “This interaction is especially responsible for the nuclear reactions that take place in the Sun.“, points out CEA. The intermediary particles are gluons.

– I’gravitational interaction : This is the force of attraction. This is the weakest of these four fundamental forces.

Let’s start again: the gravitational interaction would thus have a graviton as an intermediary particle, but the existence of the latter has never been directly proven. But, “the use of the graviton in theoretical calculations is well known, and we know how to use the “quantum formalism” involving the graviton for many theoretical calculations, ranging from primary cosmology to gravitational waves.“, assures Pierre Vanhove.

Thus, physicists are not completely ignorant: they know how to manipulate the graviton, and the calculations made turn out to be very effective. String theory shows, for example, that the graviton can be viewed as the product of two photons. “This gives hope that one day the experiment will be able to detect (directly or indirectly) a sign of the existence of the graviton as a quantum of gravitational force.hopes the physicist. But what experience? The mystery remains.

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