Quantum theory, as currently understood and formulated, faces challenges when it comes to incorporating gravity into its framework. The main reason for this difficulty lies in the fundamental differences between quantum mechanics and general relativity, which is the theory of gravity proposed by Albert Einstein.
Incompatibility of frameworks: Quantum mechanics and general relativity are based on different mathematical formalisms and conceptual frameworks. Quantum mechanics describes the behavior of particles and fields at the microscopic scale, while general relativity describes the geometry of spacetime and the behavior of gravity at large scales. The mathematical structures and equations of these theories are fundamentally different and do not easily fit together.
Quantizing gravity: One of the key obstacles is the quantization of gravity itself. While successful methods exist for quantizing other fundamental forces, such as electromagnetism or the strong and weak nuclear forces, attempts to quantize gravity have encountered significant challenges. The non-renormalizability of gravity poses a major hurdle, resulting in infinities and inconsistencies in the calculations. This issue implies that our current understanding of quantum field theory is incomplete when it comes to gravity.
Nature of spacetime: Quantum mechanics assumes a fixed background spacetime in which particles and fields evolve. In contrast, general relativity describes gravity as the curvature of spacetime, where the distribution of matter and energy influences the geometry. Combining these two perspectives in a consistent way is a major challenge. Understanding the quantum nature of spacetime itself is an active area of research and an essential aspect of a quantum theory of gravity.
Quantum entanglement and locality: Quantum mechanics involves entanglement, where the states of particles can be instantaneously correlated regardless of distance. General relativity, on the other hand, relies on the principle of locality, which states that interactions cannot occur faster than the speed of light. Reconciling these two principles within a quantum theory of gravity is an ongoing problem.
Energy scales and observations: Gravity becomes most relevant on cosmic scales and in extreme conditions, such as black holes or the early universe. Currently, there is a lack of experimental evidence and observational data that directly probe the quantum nature of gravity at these energy scales. The energies required to directly observe quantum gravitational effects are currently beyond the reach of our experimental capabilities.
To address these challenges, several proposed theories aim to unify quantum mechanics and gravity, such as string theory, loop quantum gravity, and causal set theory. However, these theories are still under development and are subject to ongoing research and scrutiny.
In summary, while quantum theory and general relativity have been highly successful in their respective domains, combining them into a consistent theory of quantum gravity remains a major theoretical challenge. It is an active area of research where scientists strive to develop a more comprehensive framework that can fully explain the behavior of gravity at the quantum level.