When to Use This Tag

gravity covers the discussion of the attractive force of gravity independently of a specific theory, which could describe or explain this force. Hence, you should use the tag when comparing gravity to weak-force or when trying to compare various theories. If you are after particular calculations, prefer general-relativity or newtonian-gravity correspondingly.

#Introduction

Gravity is a force that has been observed to affect all bodies with non-zero mass or energy. There are currently two working explanations of gravity (in their respective area of usefulness), but no successful theory has been proposed to explain gravity on a quantum-mechanical level.

Newtonian Gravity newtonian-gravity

The original description of gravity is based on the assumption of an overall attractive force $\vec F$ between bodies with mass $m_1$ and $m_2$ at a given distance $\vec r$, given by

$$ \vec F = G \frac{m_1 m_2}{r^3} \vec r \quad.$$

This theory does not take into account the gravitational effect of energy, pressure, and stress and fails at large masses.

General Relativity (GR) general-relativity

GR models gravity as a variation of space and time itself: Large bodies and energy densities bend the four-dimensional spacetime in such a way that an attractive effect between bodies is created. In the limit of small energy/mass densities, GR reproduces newtonian-gravity.

Semi-Classical Gravity

Semi-classical gravity refers to the standard model or quantum field theory on a curved spacetime. In other words, gravity is treated as classical whereas everything else is treated as quantum.

Examples of major results from Semi-Classical Gravity include unruh-radiation, hawking-radiation, chandrashekar-limit, etc.

Quantum Gravity quantum-gravity

So far, no successful quantisation of gravity has been proposed or experimentally proven. Similarly to the gauge bosons $\gamma$, $W^\pm$, $Z^0$, and the various gluons, which mediate the electromagnetic, weak, and strong interactions, another boson, dubbed graviton, is assumed to mediate the gravitational attraction. From the various features of gravity (long-range, always attractive), it is assumed that the graviton is a massless spin-2 boson.

Note that the graviton is not to be confused with the Higgs mechanism, which creates the mass of the gauge bosons in the first place (and has nothing to do with gravity).

String Theory string-theory

One popular approach to quantum gravity is string-theory. string-theory has been successful in reproducing general-relativity in the low-energy, classical limit. String theory aims not only to be a theory of quantum gravity, but also a theory-of-everything, which means it also unifies the other forces, and matter, together. String theory reproduces General Relativity in the non-stringy limit by requiring conformal invariance to constrain the beta functions to vanish.

String theory requires extra dimensions for conformal anomaly to vanish, and it also requires supersymmetry to have fermions in its spectrum. Neither of these has been observed to a conclusive position, though the 125 GeV Higgs is a piece of strong evidence for supersymmetry (as in, the mssm, which has been shown to take place in certain realistic string vacua by Kumar, Acharya and Kane) and there has been a recent result hinting at third-generation superpartners being observed at the LHC.

Loop Quantum Gravity

loop-quantum-gravity is another well-known theory of quantum gravity that quantises general-relativity by using different variables, the Ashtekhar variables instead of the standard spacetime metric (with its corresponding le-cevita, or Christoffel connection.). Loop Quantum Gravity is formulated as a first-order theory, which means it uses the vielbin (specifically, the vierbin, a vielbin in 4-dimensional spacetime), i.e. the unit vector in curved spacetime. In fact, loop quantum gravity doesn't directly use the vwierbin, but the viewrbin is divided by the "Imirizzi parameter".

It is well-known that Loop Quantum Gravity produces a discrete, or granular, picture of spacetime; This makes it not lorentz-invariant, which is considered a big problem for loop quantum gravity since Lorentz invariance has been very well-tested to the scale of the Planck length. Sen (2013) also showed that Loop Quantum Gravity does not produce a continuous, or smooth picture, of spacetime at large scales. Furthermore, loop quantum gravity does not incorporate the standard model interactions. This means that loop quantum gravity would need serious refinement.

Related theories

Supergravity supergravity and Kaluza - Klein theory kaluza-klein

One related theory is kaluza-klein theory, Kaluza - Klein Theory attempts to show that General Relativity in a 4 + 1 -dimensional spacetime reduces to general-relativity in a 3 + 1 - dimensional spacetime PLUS Maxwell's electromagnetism (maxwell-equations) in a 3 + 1 - dimensional spacetime.

supergravity is an extension to kaluza-klein which also covers the weak-force] and the [strong-force. To be consistent, it requires supersymmetry, in order to allow fermions too. supergravity also arises in the low-energy, classical limit of super - string-theoryies.