What Is Gravity Made Of?

Nature is governed by only four fundamental forces. We understand what three of them are made of and how they function, but the story of gravity—the force most familiar to us—remains a mystery. What is gravity made of? Does it have particles of its own? If so, how can they be identified?

The enigma of nature and the mysteries of the universe have fascinated humankind since ancient times. Yet, it is only in the last few centuries that we have begun to unravel its workings with considerable accuracy. Modern science now knows a great deal—though not everything—about the universe, its origins, and the secrets of nature. By piecing together knowledge accumulated over centuries, we now understand that nature, or the universe, operates under the governance of just four fundamental forces. These forces, along with specific particles, bind the vast cosmic family together. These four natural forces hold everything in the universe in place.

The four fundamental forces consist of the electromagnetic force, the strong nuclear force, the weak nuclear force, and gravitational force. The initial three forces are elucidated through the framework of the Standard Model of particle physics. Collectively, these forces are designated as "non-gravitational forces." Conversely, the force of gravity is characterized by the theory of relativity, specifically by general relativity. While the electromagnetic, strong, and weak nuclear forces explain the microcosmic world—like atoms—relativity accounts for the large-scale structure of the universe: planets, stars, and black holes.

The Standard Model describes the three fundamental forces by the exchange of force-carrying particles at the subatomic level. These three basic forces work to hold matter together. Thus far, the Standard Model has been notably successful as a theoretical model. For example, the electromagnetic force acts to hold atoms together by keeping the nucleus and electrons bound together. The particle that carries this force is the photon, better known as the particle of light.

On the other hand, the strong nuclear force interacts with particles in the nucleus through the exchange of gluons. This fundamental force is what binds quarks together, thus holding an atomic nucleus together. In addition, it explains the great amount of energy that is released in the process of nuclear fission and the detonation of atomic explosives.

The weak nuclear force acts through interactions between particles that are mediated by three carrier particles: W+, W−, and Z0. Like the strong nuclear force, the weak nuclear force acts only in the ultramicroscopic, or more accurately, the subatomic realm. However, unlike the strong force, the weak nuclear force has a unique property: it affects all particles. Even neutral particles, like neutrons, are acted upon by this force. Instead of simply binding particles together, the weak force performs other vital roles, such as changing the type of quarks. For example, in radioactive beta decay within a nucleus, this force converts a neutron to a proton.

Now the question arises: can this fourth force, gravity, be described in a similar fashion? How does gravity act between two bodies? Can this also be due to force-carrying particles?

Before answering these questions, it must be pointed out that gravity differs from the other three forces in some fundamental ways. First of all, gravity is much weaker compared to the other forces. Scientists estimate that it is 10³⁸ times weaker than the strong nuclear force, 10³⁶ times weaker than the electromagnetic force, and 10²⁹ times weaker than the weak nuclear force. And although it is so relatively weak, gravity still happens to be one of the most crucial forces in nature. Without gravity, the whole interconnected universe could never have come into being. The universe would have no planets, stars, or galaxies, just a cold and empty space with only scattered dust and gas.

Physicists think that gravity, like the other three fundamental forces, could also be understood in terms of force-carrying particles. They have explored many theoretical models for unification including gravity with the Standard Model. Such unification requires an understanding of the nature of these theoretical particles, known as gravitons, and how they would mediate the force of gravity.

In physics, gravitons are purely theoretical. The term "graviton" was coined from the English word gravity. Scientists have made guesses about what properties this hypothetical particle would have. For example, quantum theory states that the heavier a particle is, the more energy it takes to produce it, and the shorter its lifetime. Such particles can't travel very far because their effect is confined to close proximity. Consider the W+ boson, the mediator of the weak nuclear force. This particle is massive, requiring much energy to create, so it has very limited range, at most on the atomic size scale. Beyond this range, its effects become negligible. On the other hand, the photon, which mediates the electromagnetic force, has zero mass and can travel an infinite distance.

Gravity acts over infinite distances. For instance, across the enormous distance of our solar system, the gravitational force coming from the Sun keeps Earth and the other planets in their orbits. On a larger scale, gravity is responsible for the construction of giant objects such as the Milky Way galaxy and black holes, which are composed of millions of stars. It thus makes sense that no energy would be needed to create gravitons either, which means they must be massless too.

Researchers have also theorized on the spin properties of gravitons. In quantum mechanics, spin denotes a particle's intrinsic angular momentum, a fundamental property. Quarks and leptons, the constituents of matter, have a spin of 1/2. Because of this, their state does not return to its original state after a 360-degree rotation but only after a 720-degree rotation. By contrast, force-carrying particles of the Standard Model, such as photons, carry a spin of 1, which means they appear the same after a 360-degree rotation. By the same logic, scientists hypothesize that gravitons would have a spin of 2, because only those particles with spin 2 are able to interact with all matter. Particles with spin 2 are exactly the same even after a 180-degree rotation.

Whereas there have been attempts to model gravity as the exchange of gravitons with respect to the principles of particle physics, these theoretical particles may have no basis in reality. Nevertheless, gravity has been suspected by many physicists as also quantizable. While the problem lies with those mathematical tools that successfully provided proof for other forces yet failed to do so when applied to gravity, the arrival of string theory came somewhat as a savior for physicists. String theory postulates that fundamental particles are not singular, point-like objects, but rather oscillating strings of energy in which different vibrational modes correspond to different particles.

String theorists think that what appears as point-like particles, such as electrons, are actually vibrations of tiny strings. If we were to have a super-microscope, looking into an electron would not see a point particle but rather an incredibly small, vibrating string. According to the string theorists, other particles besides electrons are also manifestations of strings vibrating at different frequencies.

String theory also provides at least a glimmer of hope for understanding gravity. It predicts a particular class of string having properties similar to that of the graviton. However, string theory is still mysterious and is plagued with serious challenges. One prominent problem is that the strings it describes are incredibly tiny-much smaller than an atom. There is an even more baffling problem: string theory postulates that spacetime has ten dimensions, not the four we're used to—in fact, three dimensions of space and one of time. Those extra dimensions turn the math of string theory into something incredibly complicated and hard to deal with.

Because of these requirements, string theory has not yet provided any observable predictions. Experimental testing of the theory would require a huge and currently unattainable investment of effort and resources. Experimental physicists, in particular, are very skeptical of the theory. Their attitude is straightforward: if a hypothesis does not lead to testable or observable predictions, it is not a valid hypothesis, let alone a scientific theory.

This has left the scientific community divided. Some find the string theory to be a promising framework that could, one day, give a unification of physics; others regard it as a fancy speculation. The debate as to whether string theory is a scientific theory, or an over-ambitious hypothesis goes on.