Quantum gravity is one of the biggest unsolved problems in modern physics. It seeks to unify two of the most successful theories in science: quantum mechanics, which governs the world of particles and forces, and general relativity, which describes the structure of spacetime and gravity. Despite their individual successes, these two theories are fundamentally incompatible at extreme scale, such as inside black holes or at the moment of the Big Bang.
The Conflict Between General Relativity and Quantum Mechanics
Albert Einstein’s theory of general relativity, formulated in 1915, describes gravity as a curvature of spacetime caused by mass and energy. It works exceptionally well at large scales, explaining planetary motion, black holes, and the expansion of the universe. However, it is a classical theory, meaning it does not incorporate the probabilistic nature of quantum mechanics.
On the other hand, quantum mechanics, developed in the early 20th century, governs the behavior of subatomic particles. It describes how particles exist in superposition, entangle with one another, and behave probabilistically rather than deterministically. The success of quantum mechanics has led to technologies like semiconductors, lasers, and quantum computing.
The problem arises when we try to apply quantum mechanics to gravity. At very small scales, such as near singularities inside black holes, general relativity breaks down, predicting infinities and paradoxes that make no physical sense. A quantum theory of gravity would resolve these contradictions, explaining how gravity behaves at the smallest scales.
String Theory
String theory suggests that fundamental particles are not point-like objects but rather tiny vibrating strings. Different vibrational modes of these strings correspond to different particles, including the hypothetical graviton, the quantum carrier of gravity. String theory naturally incorporates gravity and unifies all forces within a single framework. However, it requires extra dimensions beyond the familiar four (three spatial dimension and time), making experimental verification challenging.
Loop Quantum Gravity
Loop quantum gravity is a different approach that attempts to quantize spacetime itself. It suggests that space and time are made up of discrete, loop-like structures, like the pixels in a digital image. This approach removes the singularities found in general relativity, potentially explaining what happens inside black holes and at the beginning of the universe.
Quantum Gravity and Black Holes
Black holes are considered laboratories for quantum gravity because they involve both extreme gravity and quantum effects. The famous Hawking radiation, predicted by Stephen Hawking, suggests that black holes emit quantum radiation and can eventually evaporate. This prediction combined elements of quantum mechanics, relativity, and thermodynamics. It suggested that black holes can evaporate over time, shrinking as they lose energy. However, the full quantum gravitational explanation of this process is still unknown, making it an important problem in modern physics.
RELATED STORIES:
https://plato.stanford.edu/entries/quantum-gravity/
https://www.space.com/quantum-gravity.html
https://www.scientificamerican.com/article/is-gravity-quantum/
https://physics.aps.org/articles/v18/37
https://www.sciencedirect.com/topics/physics-and-astronomy/quantum-gravity
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