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Along with Einstein’s theory of general relativity, the 20th century introduced quantum theory as a leading topic in physics. Quantum theory began to navigate the concepts surrounding quantum gravity. Quantum gravity explored the ways in which quantum mechanics characterizes and explains gravity. This particular subject in theoretical physics studies the quantum effects of compact astrophysical objects where gravity is very influential.
Furthermore, quantum gravity is the unifying term for the theories that seek to collectively describe gravity with the fundamental forces of physics. Quantum gravity postulates the theoretical entity of gravitation, therefore making it a distinguishable field of physics.
Unified Theories of Quantum Gravity
General relativity approaches gravity as a classical, deterministic theory that cannot be treated quantum mechanically. However, quantum mechanics and gravity must be unified in order to develop a successful theory. There are several approaches to quantum gravity; leading among these are string theory, supergravity and canonical quantization.
The theoretical structure of point-like particles being substituted for one-dimensional strings is known as string theory. The theory explains how one-dimensional strings spread and diffuse through space, as well as how they behave with each other. The string is visually conceived as any other particle, but its mass and charge are determined by its vibrational state. The vibrational state of the string relates to the gravitation, which is the particle that holds gravitational force. Due to the inclusion of gravitation, string theory is deemed a unified theory of quantum gravity.
Supergravity, also known as SUGRA, is a theory in the modern field that integrates supersymmetry, relating to fermions and bosons, and general relativity. In consideration of these theories, canonical quantization is the procedure taken for quantizing classical theory while also maintaining the original structure. Canonical quantization appeals to quantum gravity for two main reasons: it sheds light on geometrical characteristics and adequately approaches conceptual issues of quantum gravity.
The mystifying, conceptual issues of quantum gravity include the interpretation of the wave function in the universe and the essence, or potential problem, of time. Discordantly, string theory requires mathematical stability to addresses quantum gravity.
Why We Need Quantum Gravity?
As quantum gravity attempts to unite general relativity and quantum mechanics, its questions remain important and puzzling. The two leading theories that explain particles and their interaction with each other are the standard model of particle physics and general relativity.
The standard model of particle physics covers electromagnetism and the nuclear forces and general relativity assesses gravity. Despite the success and stability of these two theories, if we attempt to combine them they crumble. The only known way to create a bridge between the standard model and general relativity, and the reason we need quantum gravity, is by applying the theory of gravity to these two reputable theories.
Problems with Quantum Gravity
As quantum gravity advances, there remain limits to its study. The most pressing issue is experimental testing; any energy levels necessary to examine the conjectures of quantum gravity are unfeasible in present day laboratory tests. In addition, the question of whether space and time are continuous remains unanswered, therefore physicists cannot confirm whether gravity is quantized.
Despite the physical limitations of quantum gravity, theoretical concerns in its evaluation as a theory of general relativity also exist. An association with general relativity forms varying assumptions about the universe at a macroscopic scale, versus the assumptions made by quantum mechanics at a microscopic scale.
Integrating these theories brings about the renormalization problem. This fundamentally means that all of these forces combined results in an infinite value and unsolvable equations. Nonetheless, renormalization is not suited to a quantum interpretation of gravity.
Other theories unmentioned that are also considered to be in the field of quantum gravity include loop quantum gravity, twister theory, noncommutative geometry, Euclidean quantum gravity, and the Wheeler-DeWitt equation. These theories foreshadow a different conception of space and time, rather than a merging of separate concepts.
As quantum gravity struggles to be strengthened and proven due to its complex translation and experimentation, it remains a difficult field to evaluate. The question of where the conclusive theory of quantum gravity lies remains a mystery for 21st century physicists.
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