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Quantum Gravity in the Making: The Fusion of Quantum Mechanics and General Relativity
We always appreciate it when explanations add up. There is a sort of soothing and satisfying effect that comes along and suffuses us when things make sense.
Precisely for those reasons, what has irked many theoretical physicists for already several decades is the absence of the theory of quantum gravity, which intends to put gravitational and quantum behaviour under one coherent roof regardless of the energy scale.
This article sketches a brief overview of the quantum gravity landscape and a couple of its leading candidate theories, namely M-theory, loop quantum gravity, causal dynamical triangulations, and asymptotically safe gravity.
The Outline of the Marriage
Quantum mechanics is the ensemble of the laws of physics that accurately account for the kinetics of atomic and subatomic particles as well as radiation (light) at minute distance scales. The theory explains behaviour that we do not normally observe in our macroscopic world, for instance, establishing an instantaneous connection over large distances (quantum entanglement), crossing barriers otherwise impenetrable to larger objects (quantum tunnelling), and taking different paths at the same time (quantum superposition).
It is this theory — more correctly, it is quantum field theory, which is quantum mechanics intertwined with classical field theory and special relativity — that explicates the dynamics of the most fundamental building blocks of our Universe, collectively captured in the Standard Model of particle physics. In addition to fermions, such as the electron, the quarks, and the neutrinos, that model also contains bosons, including the photon and the Higgs boson.
In contrast, gravitation is not described within the contours of the Standard model, but by Albert Einstein’s theory of general relativity, which posits that the curvature of spacetime is what we experience as gravity. In the words of physicist John Wheeler: “Spacetime tells matter how to move; matter tells spacetime how to curve.”
Theories of quantum gravity look at regimes where both gravitational fields and quantum effects grow very strong. This usually occurs beyond the Planck scale…
