The fundamental problem between gravity and quantum physics arises from the incompatibility between the two prevailing theories that describe the behavior of these two fundamental forces: general relativity and quantum mechanics.
General relativity, formulated by Albert Einstein, provides a framework for understanding gravity as the curvature of spacetime caused by massive objects. It successfully explains the behavior of gravity on large scales, such as the motion of planets and the expansion of the universe.
On the other hand, quantum mechanics is the theory that describes the behavior of matter and energy at the microscopic level. It has been tremendously successful in explaining the behavior of fundamental particles and their interactions, except for gravity.
The main issue arises when attempting to apply quantum mechanics to gravity. The mathematics and concepts used in quantum mechanics are incompatible with the framework of general relativity. This incompatibility manifests in several ways:
Quantum Nature of Gravity: General relativity treats gravity as a classical field, while quantum mechanics describes other fundamental forces, such as electromagnetism and the strong and weak nuclear forces, in terms of quantum fields and particles (such as photons and gluons). Incorporating gravity into the quantum framework requires a consistent quantum theory of gravity.
Infinities and Renormalization: Quantum field theories involve calculations that can produce infinite values. These infinities arise when trying to calculate the behavior of particles and fields in gravitational contexts. To make sense of these infinities, physicists employ a technique called renormalization. However, this approach has not been successfully applied to the gravitational field, indicating that a more comprehensive understanding is needed.
The Measurement Problem: Quantum mechanics involves the principle of superposition, where particles can exist in multiple states simultaneously until they are measured or observed. However, applying this principle to the fabric of spacetime, as described by general relativity, raises conceptual challenges. It remains unclear how to reconcile the probabilistic nature of quantum mechanics with the deterministic nature of general relativity.
Efforts to reconcile gravity and quantum mechanics have led to various approaches, such as string theory, loop quantum gravity, and other quantum gravity theories. However, a complete and widely accepted theory that successfully unifies gravity and quantum mechanics, known as a theory of quantum gravity, is still elusive. This is considered one of the most significant open questions in theoretical physics and an active area of research.
Finding a theory of quantum gravity would provide a more complete and consistent understanding of the fundamental forces of nature and potentially shed light on phenomena such as the behavior of black holes, the early universe, and the nature of spacetime itself.