Graviton

The graviton is a hypothetical bosonic-type elementary particle that would be the transmitter of the gravitational interaction in most quantum gravity models.

This particle was theorized in 1930 by a group of scientists, after several complications with the creation of a theory of everything. It was known that light was a photon; the negative charge, an electron; the mass, a boson; but no matter what they tried, gravity did not connect correctly with the theories. To solve this, they tried to assimilate it to a particle. Still, his math was falling apart. For this reason, string theory was devised, to be able to make precise calculations.

According to the properties of the gravitational field, the graviton must be an even spin boson (2 in this case), since it is associated with a second-order classical tensor field. Regarding the mass of the graviton, experimental measurements give an upper bound of the order of m_g = 1.6 × 10⁻⁶⁹ kg, although it could be exactly zero.

Graviton and quantum gravity models

Quantum field theory postulates that interactions in nature are produced through the intermediation of gauge bosons or quanta associated with the fields that represent these interactions. The interaction of matter particles with those bosons that represent the force fields is interpreted in terms of emission or absorption of these quanta. Thus, electrodynamics is explained by photons or quanta of the electromagnetic field: photons are continuously emitted and absorbed by all electrically charged particles, so that the interactions between these photons produce the macroscopic forces that are familiar to us, such as electromagnetism. The weak interaction and the strong interaction can equally be understood in terms of W and Z bosons and gluons respectively.

Considering the widespread success of quantum theory in describing most of the basic forces in the universe, it seems natural to assume that the same methods will serve to explain gravity. Many attempts have been made to introduce the hitherto invisible graviton, which would function in a way analogous to that of the photon and the other gauge bosons. However, there are specific mathematical problems associated with the way gravity operates that have not allowed us to develop a quantum gravitational theory until now.

A quantum theory of gravitation requires that the graviton operate in a similar way to the photon, but unlike in electrodynamics, where photons do not act directly with each other but only with charged particles, gravity simply does not work in this way. so simple, since gravitons could interact with each other. Experimental facts show that gravity is created by any form of energy (and mass is only a particularly condensed form of energy, a relationship established by Einstein's famous equation), which is difficult to describe in terms similar to charge. electrical. To date all attempts to create a simple quantum theory of gravity have failed.

Detecting the graviton experimentally is a rather problematic task. These subatomic particles would carry very little energy, therefore detection would be very difficult due to the weak effects they would cause. The only way to detect them would be to look for cases in which the motion or energy of a body changes in a way that is different from that predicted by the general theory of relativity, but one of the basic principles of quantum gravity would be that they should more or less coincide with these relativistic predictions.

Graviton and string theory

String theories, including M-theory, assume gravitons to be strings or closed branes. This would explain the apparent weakness of his strength; According to these theories, gravitons would exert their influence beyond the three-dimensional universe in which we live, interconnecting various possible "parallel universes."^{[citation required]}

Energy and wavelength

While gravitons are supposed to be massless, they would still carry energy, just like any other quantum particle. Photon energy and gluon energy are also carried by massless particles. It is unclear what variables could determine graviton energy, the amount of energy carried by a single graviton.

Alternatively, if gravitons had mass, gravitational wave analysis would provide an upper bound for the mass of gravitons. The Compton wavelength of the graviton is at least 1.6x¹⁶ m, or about 1.6 light years, corresponding to a graviton mass no greater than 7.7x10^-23 eV/c². This relationship between wavelength and mass-energy results from using the Planck-Einstein relationship, the same formula that relates the electromagnetic wavelength and the energy of the photon.. However, if gravitons are the quanta of gravitational waves, then the relationship between the wavelength and the energy of the corresponding particle is fundamentally different for gravitons than for photons, since the Compton wavelength of the graviton does not is equal to the wavelength of the gravitational wave. In contrast, the lower limit of the Compton wavelength of the graviton is approximately 9x10⁹ times greater than the gravitational wavelength for event GW170104, which was ~ 1,700 km. The report did not give more details about the origin of this relationship. It is possible that gravitons are not the quanta of gravitational waves, or that both phenomena are related in a different way.

Experimental observation

Unambiguous detection of individual gravitons, although not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, located in an orbit close to a neutron star, would only be able to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the neutrino background, since the dimensions of the neutrino shield required would ensure collapse into a black hole.

Observations from the LIGO and Virgo collaborations have directly detected gravitational waves. Others have postulated that graviton scattering produces gravitational waves just as particle interactions produce coherent states. Although these experiments cannot detect individual gravitons, could provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower than c in a region with non-zero mass density to be detectable). Recent observations of gravitational waves have put an upper limit of 1.2 x 10^-22 eV/c² in the mass of the graviton. Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and Newtonian dynamics modified, they could point to gravitons having a mass other than zero.