Gravity is often referred to as the weakest of the four fundamental forces because its strength is significantly smaller compared to the other three forces: electromagnetism, the strong nuclear force, and the weak nuclear force. This characterization is based on experimental observations and measurements.
The strength of a force is determined by the coupling constant associated with that force. In the case of gravity, the coupling constant, known as the gravitational constant (G), is extremely small compared to the coupling constants of the other forces. This means that the gravitational force between two objects is much weaker than the electromagnetic force, the strong nuclear force, or the weak nuclear force between the same objects.
The gravitational force's weakness is evident in everyday life, where electromagnetic forces easily overcome the gravitational force in most situations. For example, a small magnet can lift a paperclip against the entire gravitational force of the Earth.
The reason for the relative weakness of gravity is still an open question in physics and an active area of research. One potential explanation comes from the framework of quantum field theory. In quantum field theory, all forces arise from the exchange of particles called force carriers. The force carriers associated with gravity, called gravitons, are hypothesized to have spin-2, while the force carriers of the other forces have spin-1. This difference in spin may contribute to the weaker strength of gravity compared to the other forces.
Additionally, the discovery of the Higgs boson has shed some light on the origin of particle masses. The masses of particles, including those that mediate forces, arise from their interactions with the Higgs field. The Higgs field is thought to couple more strongly with particles that have larger masses. Since the particles that mediate the strong, weak, and electromagnetic forces are much more massive than the graviton, their coupling with the Higgs field is stronger, resulting in a stronger force.
It's important to note that while gravity may be considered the weakest force, it has an unlimited range and affects all objects with mass or energy. Its influence extends across vast cosmic distances and plays a crucial role in shaping the structure and evolution of the universe.