Gravity does not fit into the Standard Model of particle physics, which is the current framework that describes the fundamental forces and particles, including electromagnetism, the strong and weak nuclear forces, and the known elementary particles.
The Standard Model successfully combines quantum mechanics and special relativity to explain the behavior of particles and their interactions, except for gravity. Here are a few reasons why gravity is not included in the Standard Model:
Different mathematical description: Gravity, as described by general relativity, is formulated in terms of the curvature of spacetime caused by mass and energy. In contrast, the other fundamental forces in the Standard Model are described by gauge theories, which use different mathematical frameworks and equations.
Weak gravitational strength: Gravity is significantly weaker compared to the other forces, such as electromagnetism and the strong and weak nuclear forces. This discrepancy in strength poses challenges for unifying gravity with the other forces within the Standard Model.
No known quantum theory of gravity: While the other forces in the Standard Model can be described using quantum field theory, a consistent and widely accepted theory of quantum gravity that reconciles general relativity with quantum mechanics has not been developed yet. The quantization of gravity encounters conceptual and technical difficulties, such as the treatment of singularities and infinities.
Renormalization issues: The mathematical procedure of renormalization, which is successful in the context of the Standard Model, encounters problems when applied to gravity. The equations of general relativity become non-renormalizable, making it challenging to handle infinities and obtain meaningful predictions.
Efforts to incorporate gravity into the Standard Model, and to develop a theory of quantum gravity more generally, are ongoing areas of research. The most notable approach is string theory, which attempts to unify all fundamental forces, including gravity, within a single theoretical framework. Other approaches, such as loop quantum gravity and causal dynamical triangulation, also aim to provide a quantum theory of gravity.
However, at present, the full integration of gravity into the Standard Model remains an open question in theoretical physics, and further theoretical developments and experimental evidence are required to achieve a comprehensive understanding of gravity at the fundamental level.