In the framework of quantum field theory (QFT) and its extensions, such as string theory, the gravitational force is treated differently compared to the other fundamental forces (electromagnetic, weak, and strong forces). This distinction arises due to several reasons:
Gravitational Weakness: The strength of the gravitational force is significantly weaker compared to the other fundamental forces. In the realm of particle physics, the gravitational force is negligibly weak in comparison to the other forces at energy scales accessible by current experiments. This weak strength makes it challenging to incorporate gravity into the framework of QFT in a straightforward manner.
Incompatibility with QFT: The theory of gravity, general relativity, and QFT are mathematically inconsistent when naively combined. In particular, gravity is described by a classical geometric theory (general relativity), whereas QFT describes particle interactions within a quantum framework. Merging these two frameworks in a consistent manner has proven to be a long-standing challenge and is an active area of research.
The Graviton and Quantum Gravity: In QFT, particles are associated with corresponding force-carrying particles, known as gauge bosons (e.g., photons for the electromagnetic force). However, the particle associated with gravity, called the graviton, has not been observed experimentally yet. The graviton is predicted by a hypothetical theory called quantum gravity, which aims to reconcile general relativity and quantum mechanics. However, the details of quantum gravity are not yet fully understood.
String Theory and Gravity: String theory is a candidate theory of quantum gravity that seeks to unify all fundamental forces, including gravity. In string theory, particles are replaced by tiny, vibrating strings. The theory naturally incorporates gravity, and the graviton emerges as one of the string's vibrational modes. However, string theory is a highly complex and mathematically challenging framework, and many aspects of it are still actively researched.
In summary, the gravitational force does not appear in the same way as the other fundamental forces in QFT and related theories due to its weak strength, mathematical incompatibility with QFT, the absence of experimental evidence for the graviton, and the ongoing theoretical challenges in formulating a consistent theory of quantum gravity, such as string theory.