, It is widely hoped that a theory of quantum gravity would allow us to understand problems of very high energy and very small dimensions of space, such as the behavior of black holes, and the origin of the universe. In particular, contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles  , Much of the difficulty in meshing these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. See Quantum field theory in curved spacetime for a more complete discussion. Yuri.N., Obukhov, "Spin, gravity, and inertia", Physical review letters 86.2 (2001): 192. the text by Wald cited above. Mann (1991). nl:Kwantumgravitatiepl:Grawitacja kwantowa The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. Classical and Quantum Gravity. Another possibility is that there are new, undiscovered symmetry principles that constrain the parameters and reduce them to a finite set. There are two other points of tension between quantum mechanics and general relativity. Topological quantum field theory provided an example of background-independent quantum theory, but with no local degrees of freedom, and only finitely many degrees of freedom globally. Mann (1996). Miscellaneous articles needing expert attention, Articles needing expert attention from November 2008, Articles with invalid date parameter in template, Articles with unsourced statements since April 2009, Articles with unsourced statements since February 2008, An Exceptionally Simple Theory of Everything, [gr-qc/9512024] Introduction to the Effective Field Theory Description of Gravity, [astro-ph/0506506] Singularity-Free Collapse through Local Inflation, Interface of Gravitational and Quantum Realms, http://www.ias.ac.in/currsci/dec252005/2064.pdf. Many of the accepted notio… An indirect way of observing quantum gravity e ects is via the gauge / gravity correspon-dence, which relates quantum eld theories and quantum gravity. This problem must be put in the proper context, however. To a certain extent, general relativity can be seen to be a relational theory, in which the only physically relevant information is the relationship between different events in space-time. Mann, and T.C. pt:Gravitação quânticask:Kvantová gravitácia Loop quantum gravity is the fruit of an effort to formulate a background-independent quantum theory. .  Template:Unreferenced section Much of the difficulty in meshing these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. In this way, string theory promises to be a unified description of all particles and interactions. A. Sikkemma and R.B. Can Gravity and Quantum Particles Be Reconciled After All? One might expect that, as with electromagnetism, the gravitational force should also have a corresponding quantum field theory.  These particles act as a force particle similar to the photon of the electromagnetic interaction. Much of the difficulty in meshing these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background. At low energies, the logic of the renormalization group tells us that, despite the unknown choices of these infinitely many parameters, quantum gravity will reduce to the usual Einstein theory of general relativity. This can be thought of as due to an extreme separation of scales at which they are important. In this way, string theory promises to be a unified description of all particles and interactions. beyond the classical gravity or classical string theory approximation of the correspondence, then have an analogue in One version starts with the canonical quantization of general relativity.  Currently, there is still no complete and consistent quantum theory of gravity, and the candidate models still need to overcome major formal and conceptual problems. This hypothetical particle is known as the graviton. While there is no concrete proof of the existence of gravitons, quantized theories of matter necessitate their existence. Topological quantum field theory provided an example of background-independent quantum theory, but with no local degrees of freedom, and only finitely many degrees of freedom globally. This work shows that, in order to avoid violation of relativistic causality, the measurable spacetime around a spin-half particle's (rest frame) must be spherically symmetric - i.e., either spacetime is spherically symmetric, or somehow measurements of the spacetime (e.g., time-dilation measurements) should create some sort of back action that affects and changes the quantum spin. To analyze this question, thought experiments in the context of quantum information, have been suggested. While gravitons are an important theoretical step in a quantum mechanical description of gravity, they are generally believed to be indetectable because they interact too weakly.. For this reason, thought experiments are becoming an important theoretical tool. Where, for ordinary field theories such as quantum electrodynamics, a technique known as renormalization is an integral part of deriving predictions which take into account higher-energy contributions, gravity turns out to be nonrenormalizable: at high energies, applying the recipes of ordinary quantum field theory yields models that are devoid of all predictive power. :xxxvi–xxxviii;211–212 For a quantum field theory to be well defined according to this understanding of the subject, it must be asymptotically free or asymptotically safe. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. An example is the well-known calculation of the tiny first-order quantum-mechanical correction to the classical Newtonian gravitational potential between two masses.