"Quantum gravity is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics."
Theoretical attempts to reconcile gravity with the principles of quantum mechanics.
General Relativity: An understanding of the classical theory of gravity, including the concept of spacetime curvature and gravitational waves.
Quantum Mechanics: An understanding of the principles of quantum mechanics, including wave-particle duality, uncertainty principle, and quantum states.
Quantum Field Theory: An understanding of the principles and methods of quantum field theory, which describes the behavior of matter and fields at the quantum level.
Particle Physics: An understanding of the properties and interactions of elementary particles, including the concept of symmetry breaking and the Higgs boson.
Black Holes: An understanding of the properties and behavior of black holes, including the event horizon and singularities.
Cosmology: An understanding of the large-scale structure and evolution of the universe, including the expansion of the universe and the early universe.
String Theory: An understanding of the principles and methods of string theory, which proposes that all fundamental particles are actually tiny strings vibrating in higher dimensional space.
Loop Quantum Gravity: An understanding of the principles and methods of loop quantum gravity, which is an alternative approach to merging quantum mechanics and general relativity.
Quantum Information: An understanding of the principles of quantum information and quantum computing, which are important for understanding the behavior of quantum systems.
Spin Foam Models: An understanding of the principles and methods of spin foam models, which are a type of approach to quantum gravity that involves the use of discrete spacetime geometries.
Non-commutative Geometry: An understanding of the principles and methods of non-commutative geometry, which is the study of geometries that do not obey the classical rules of commutativity.
Canonical Quantum Gravity: An understanding of the principles and methods of canonical quantum gravity, which is a particular approach to merging quantum mechanics and general relativity that is based on the Hamiltonian formulation.
Emergence: An understanding of the principles of emergence, which proposes that complex phenomena can emerge from simple underlying rules or structures.
Causal Sets: An understanding of the principles and methods of causal sets, which is a particular approach to quantum gravity that involves the use of discrete causal structures.
Asymptotic Freedom: An understanding of the principle of asymptotic freedom, which is a property of certain quantum field theories that allows them to be treated perturbatively at high energies.
Renormalization Group: An understanding of the methods of the renormalization group, which is a technique for handling the divergences that arise in certain quantum field theories.
Quantum Gravity Phenomenology: An understanding of the various proposals for experimental tests of quantum gravity, including the detection of gravitational waves and the search for violations of the equivalence principle.
Quantum Foundations: An understanding of the various interpretations of quantum mechanics and their implications for the foundations of physics, including the nature of reality and the role of measurement.
Loop Quantum Gravity: This approach aims to unify general relativity and quantum mechanics through a discretization of spacetime using loop-like structures.
String Theory: This type of quantum gravity proposes that the fundamental constituents of matter are not particles but extended objects known as strings.
Asymptotic Safety: This approach attempts to remove the divergences inherent in general relativity by assuming that the theory possesses a nontrivial ultraviolet fixed point.
Causal Dynamical Triangulation: This model relies on a discretization of spacetime in terms of triangular pieces which are then attached and analyzed.
Group Field Theory: This type of quantum gravity attempts to give a field-theoretic description of quantum gravity using intertwined fields over a group manifold.
Horava Gravity: This type of gravity involves a break from the relativistic symmetry of general relativity, leading to a conformal anomaly, and proposes novel causality structures.
Symmetric Manifold Theory: This theory postulates that the observable universe is a hypersurface in a higher-dimensional space, and the interactions between matter and gravity can be formulated as an effective field theory.
Noncommutative Geometry: This approach involves replacing spacetime coordinates' commutative algebra with a noncommutative version in a bid to reconcile gravity and quantum mechanics.
Scale Relativity: This theory assumes that the laws of physics must be invariant under changes in scale and that spacetime might not have a smooth structure.
Double Special Relativity: This theory posits that both the Planck length and the speed of light are invariant and proposes a new form of spacetime geometry that may explain dark matter and dark energy observations.
"It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars as well as in the early stages of the universe moments after the Big Bang."
"Three of the four fundamental forces of nature are described within the framework of quantum mechanics and quantum field theory: the electromagnetic interaction, the strong force, and the weak force."
"This leaves gravity as the only interaction that has not been fully accommodated."
"The current understanding of gravity is based on Albert Einstein's general theory of relativity, which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space."
"The gravitational singularities inside of black holes, the ad hoc postulation of Dark Matter, as well as Dark Energy and its relation to the Cosmological Constant are among the current unsolved mysteries regarding gravity; all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm."
"At distances close to the Planck length, like those near the center of the black hole, quantum fluctuations of spacetime are expected to play an important role."
"The most popular being M-theory and loop quantum gravity."
"All of these approaches aim to describe the quantum behavior of the gravitational field, which does not necessarily include unifying all fundamental interactions into a single mathematical framework."
"Such a theory is often referred to as a theory of everything."
"Other lesser-known but no less important theories include Causal dynamical triangulation, Noncommutative geometry, and Twistor theory."
"One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the Planck scale, around 10−35 meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators."
"Thought experiment approaches have been suggested as a testing tool for quantum gravity theories."
"In the field of quantum gravity, there are several open questions - e.g., it is not known how spin of elementary particles sources gravity."
"This field of study is called phenomenological quantum gravity." Note: Remaining questions not answered directly in the excerpt.