Graviton

The graviton is the hypothetical elementary particle responsible for mediating the gravitational interaction in theories of quantum gravity.

Conceptually, the graviton plays for gravity the same role that the photon plays for electromagnetism.

In the Standard Model of particle physics:

• the electromagnetic interaction is mediated by the photon
• the weak interaction by the W and Z bosons
• the strong interaction by gluons

Gravity, by contrast, is still described at the macroscopic scale by Albert Einstein’s general relativity, a classical theory that has not been quantized.

The graviton emerges from the effort to quantize gravity, that is, to describe it using the same conceptual and mathematical framework employed for the other fundamental interactions. It represents a potential point of convergence between general relativity and quantum mechanics.

If experimentally confirmed, the graviton would mark a decisive advance toward a unified description of the fundamental interactions.

However, unlike the mediators of the other fundamental forces, the graviton has never been observed experimentally. It therefore remains a theoretical construct.

Theorized physical properties of the graviton

The graviton is expected to exhibit the following properties:

• Spin 2
  The graviton must have spin 2. This requirement is structural in nature: the source of gravity is not a vector current, as in electromagnetism, but the energy-momentum tensor, which is a second-rank tensor. In physical terms, an interaction that couples to energy, mass, pressure, and energy fluxes demands a mediator more complex than a spin-1 boson.
• Massless
  The graviton is predicted to be massless. This expectation rests on two independent arguments. First, gravity has an infinite range. Second, gravitational waves propagate at the speed of light. Since only massless particles, such as photons, can propagate at the speed of light in vacuum, it follows that the graviton should likewise be massless.

  Note. In principle, the graviton could possess an extremely small but nonzero mass. Experimental observations nonetheless impose an upper bound on its mass: \[ m_g \lesssim 1{,}76 \times  \ 10^{-23} \text{eV}/c^2 \] This bound is consistent, within experimental uncertainties, with a vanishing mass. A finite graviton mass has also been proposed as a possible explanation for the observed dynamics of galaxies without invoking dark matter.

• Charge and stability
  The graviton carries no electric charge, no color charge, and is expected to be stable. In this respect, it is entirely neutral with respect to all interactions described by the Standard Model.

Historical origin of the concept

As early as the nineteenth century, Pierre-Simon Laplace speculated about a finite propagation speed of gravity mediated by intermediate entities, although his considerations remained firmly within a classical framework.

With the advent of quantum theory, physicists began to interpret the graviton as the elementary quantum of gravity. At the microscopic level, the force that binds us to the Earth would then arise from a continual exchange of such quanta. By analogy, just as light can be understood as a flux of photons, a gravitational wave may be viewed as a coherent state composed of a large number of gravitons.

An early notion of quantized gravitational radiation already appears in Einstein’s 1916 work.

The term graviton was introduced in 1934 by Dmitry Blokhintsev and Fyodor Galperin, and later adopted by Paul Dirac in 1959.

Despite this long intellectual history, the graviton continues to raise several unresolved theoretical issues.

• Non-renormalizability
  When general relativity is quantized in a straightforward perturbative approach, ultraviolet divergences arise that cannot be eliminated through renormalization. Consequently, the theory loses predictive power at the Planck scale, and calculations become uncontrolled. This remains the primary obstacle to constructing a consistent quantum field theory of the graviton.
• The role of string theory
  In string theory, fundamental particles are not point-like but instead correspond to one-dimensional strings. Within this framework, the graviton emerges naturally as a massless vibrational mode of the string. Nevertheless, string theory currently faces significant challenges because, despite its mathematical sophistication, it has yet to receive experimental validation.
• The practical impossibility of observing a graviton
  Detecting a graviton is extraordinarily difficult because gravitational interactions are exceedingly weak at microscopic scales. The interaction cross section between gravitons and matter is so small that a detector with a mass comparable to that of Jupiter, placed near a neutron star, one of the strongest known gravitational sources, would register on the order of one graviton every ten years. Experiments such as LIGO and Virgo detect classical gravitational waves, not the individual quanta that constitute them.

Despite nearly a century of theoretical investigation, the graviton remains experimentally unverified. Its existence has yet to be established.

And so on.