Meet the gravitons: the key to the theory of quantum gravity

Imagine an invisible messenger, traveling through the cosmos at unimaginable speeds to communicate the force that keeps planets in orbit, galaxies in their majestic forms, and stars coupled in their cosmic dances.

Such a messenger, if it exists, would be the graviton: the hypothetical particle responsible for mediating the force of gravity. Despite appearing as a fundamental aspect to understanding the Universe, the graviton remains an enigma to modern science.

How can something so small be the key to unraveling the vast mysteries of the Cosmos?

The history of the graviton begins with the description of gravity itself. Isaac Newton, in 1687, formulated the law of universal gravitation, describing the force of attraction between two bodies as a function of their masses and the distance between them. For more than two centuries, Newton’s theory was the cornerstone of physics, accurately explaining the motions of planets and other celestial bodies.

However, in the early 20th century, Albert Einstein revolutionized our understanding of gravity with his theory of general relativity, first published in 1915. Einstein described gravity not as a force between masses, but as a curvature of spacetime caused by the presence of mass and energy.

This new view brought a deeper understanding, able to explain phenomena that Newton’s theory could not, such as the precession of Mercury’s orbit and the bending of light rays as they pass near massive objects.

However, while general relativity is incredibly successful in describing gravity on large scales, such as that of stars and galaxies, it does not integrate well with the other great theory of modern physics: quantum mechanics.

This, in turn, describes the other fundamental forces of nature — electromagnetism, the strong force, and the weak force — in terms of mediating particles, such as photons (for electromagnetism), gluons (for the strong force), and the W and Z bosons (for the weak force). For gravity to fit into this quantum framework, it is necessary to postulate the existence of its mediating particle, the graviton.

This hypothetical particle would be characterized by having no mass and no electric charge and being endowed with a spin of 2. If the graviton exists, it would be responsible for “informing” space-time how it should curve in response to the presence of mass and energy. In other words, the graviton would be the particle that communicates gravity from one object to another at a quantum level.

Despite the theoretical elegance of the graviton, its direct detection is one of the greatest challenges of modern physics. The main reason for this is the extreme weakness of the gravitational force compared to the other fundamental forces.

To put this in perspective, the electromagnetic force between two charged particles is about 1036 times stronger than the gravitational force between the same particles. This means that even if gravitons exist and are “acting” throughout the Universe, their effects are extremely subtle and difficult to measure directly.

Furthermore, any experiment designed to detect gravitons would be extraordinarily complex and would require sensitivity beyond our current technology. For example, gravitational waves — ripples in spacetime predicted by Einstein and confirmed experimentally in 2015 — are caused by catastrophic events such as the merger of black holes, but even these waves are incredibly difficult to detect.

Detecting individual gravitons would be much more difficult, since they are quantum entities that operate on an even smaller scale and in even weaker interactions.

The search for the graviton is, in many ways, a search for a new understanding of the Universe. If we can ever detect gravitons or indirectly confirm their existence, it would represent a huge advance in physics. For example, we could begin to develop a complete theory of quantum gravity, capable of describing the behavior of subatomic particles as well as the dynamics of black holes and the Big Bang itself. Physicists are currently exploring alternatives, such as string theory, which proposes that fundamental particles are actually vibrating strings in extra dimensions of space. The graviton, in this context, would be a specific vibration of these strings. However, string theory is also still a theoretical proposal without experimental confirmation. Until further advances are made, humanity will continue to push the boundaries of science and challenge the imagination, perhaps one day discovering how the Universe really works.