Quantum gravity: loops or strings?
Konstantinos Doran
The great accomplishments produced through the theoretical physics of the 20 th century revolve around the success of two dominant theories: general relativity and quantum theory. Quantum theory is renowned for creating the standard model of particle physics, which is used to describe three of the four fundamental fields in our universe: the electromagnetic, weak and strong fields. 1 General relativity is responsible for describing the final field, gravity. This is the result of the interaction between mass and space and time. Gravity is the consequence of the curvature of spacetime. 2 Conversely, Quantum theory is a theory of matter and energy at a sub-atomic level based on the concept of quanta (packets) of discrete energy. Hence, each field consists of particles, of specific properties, that transfer the energy within the field – bosons. 3 However, the predictions and ramifications of the two paradigms contradict each other, even though neither theory has been invalidated. Thus, many presume that both exist within a grander, encompassing theory. This leads to the greatest unsolved problem in physics for the past 80 years: how can both theories be combined? Two main theories are currently developing solutions to merge quantum theory and general relativity: loop quantum gravity and string theory. Both attempt to describe gravity through a quantum mechanical lens, a quantum theory of gravity – quantum gravity. All theories of quantum gravity describe the gravitational field (viewed on the macroscopic scale) via particles of a quantum field (on a subatomic scale), thus continuing the regular description for all fundamental fields explained by quantum theory. 4 This introduced the idea of the graviton , the ‘ exchange ’ particle responsible for ‘ transmitting gravity ’ . When attempts were made to quantize general relativity and incorporate gravitational interactions into quantum mechanical calculations through renormalization, 5 all answers diverged to infinity, making it impossible to calculate the probability of a particle interacting with a graviton . Further to this, there is a fundamental disagreement when comparing quantum theory and general relativity – time. In quantum theory time is modelled on a flat spacetime, and is thus considered absolute and universal; conversely,
1 See Quantum Gravity (2012): Preface. 2 See An Introduction to Loop Quantum Gravity with Applications to Cosmology (2014): 1.
3 See An Introduction to Quantum Gravity (2011): 5. 4 See Introduction to loop quantum gravity. (2012): 4. 5 A procedure used to absorb nonsensical infinite results caused by divergent parts of a calculation and redefining these parts to obtain a measurable and finite quantity. See An Introduction to String Theory (2011): 5-6.
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