Quantum gravity
relativity expresses time as malleable and relative due to a curved spacetime. 6 These large disagreements prevent the two theories merging. New (theoretical) frameworks are required. We need either a new theory that resolves the mathematical complications of graviton interactions or one that adapts quantum mechanics to a curved spacetime. But why do we need a quantum theory of gravity? Although general relativity can make accurate predictions over most scales and distances, it struggles to accurately predict how gravity interacts at distances at subatomic scales and, as such, is inaccurate if used to investigate how the singularity in a black hole behaves. Furthermore, a quantum theory of gravity is vital to explain the early universe, microseconds after the Big Bang or even lead to an understanding of the origin of dark matter and dark energy. 7 Loop quantum gravity attempts to solve the problem of quantum mechanics on a curved spacetime. The key problem is that quantum theory doesn’t have background independence . Quantum theory relies on assumptions that everything occurs in a flat spacetime and that particles do not affect the spacetime that they inhabit. General relativity is background independent, as it defines how objects affect spacetime while interacting with other objects. 8 Loop Quantum Gravity preserves the background independence of general relativity and attempts to quantize it through describing the quantum evolution of the geometry of space. Loop quantum gravity does this by using an abstract space of connections rather than regular coordinates. 9 These connections, instead of using vectors, use spinors or Ashtekar Variables , which describe the quantum property called spin (angular momentum). -35 m), so that each point is connected to itself. This is where loops originate, with each loop considered an elementary closed circuit of gravitational field. The network of these loops is called a spin network . These networks describe any geometry of 3D space and form a space of fields , predicted by quantum field theory, while also describing a general relativistic description of space on a large scale. The reduction of space into finite quanta thus solves the apparent singularities of black holes within general relativity, as the miniscule volume of the black hole is not point- like but instead restricted to a loop. This stops infinite densities from occurring and leads to an interesting prediction. 10 If we are approaching a black hole, time moves more slowly as compared to a distant observer, as predicted by general relativity. Once we fall past the event horizon (the edge of our current understanding of black holes), we would expect the matter falling into the black hole to slow down due to quantum pressure ; 11 eventually, the pressure exerted would stop matter falling inwards and instead eject it These connections form closed loops of Planck length ( ℓ P ) (1.6×10 6 See Introduction to loop quantum gravity. (2012): 4. 7 See An Introduction to String Theory (2011): 5-6; Covariant Loop Quantum Gravity: An elementary introduction to Quantum Gravity and Spinfoam Theory (2014): 4-5. 8 See Introduction to loop quantum gravity. (2012): 5. 9 A connection is a mathematical function that describes how a vector changes as it moves between two points in space. See Loop Quantum Gravity Explained (2019) at time 7:08 for greater detail. 10 See Introduction to loop quantum gravity. (2012): 14-22; Loop Quantum Gravity Explained (2019) at time 8:10 for more information on spinors and spin networks and how loops work. 11 the same pressure responsible for keeping electrons in their orbit around a nucleus when considering the electron as ‘orbiting’ the atom classically.
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