Editing Force (section)

=== Quantum field theory ===
{{main|Quantum field theory}}
[[File:Beta Negative Decay.svg|thumb|Feynman diagram for the decay of a neutron into a proton. The [[W boson]] is between two vertices indicating a repulsion.]]
In modern [[particle physics]], forces and the acceleration of particles are explained as a mathematical by-product of exchange of momentum-carrying [[gauge boson]]s. With the development of [[quantum field theory]] and [[general relativity]], it was realized that force is a redundant concept arising from [[conservation of momentum]] ([[4-momentum]] in relativity and momentum of [[virtual particle]]s in [[quantum electrodynamics]]). The conservation of momentum can be directly derived from the homogeneity or [[Symmetry in physics|symmetry]] of [[space]] and so is usually considered more fundamental than the concept of a force. Thus the currently known [[fundamental forces]] are considered more accurately to be "[[fundamental interactions]]".<ref name="final theory">{{cite book |last=Weinberg |first=S. |year=1994 |title=Dreams of a Final Theory |publisher=Vintage Books |isbn=978-0-679-74408-5}}</ref>{{rp|199–128}}

While sophisticated mathematical descriptions are needed to predict, in full detail, the result of such interactions, there is a conceptually simple way to describe them through the use of [[Feynman diagram]]s. In a Feynman diagram, each matter particle is represented as a straight line (see [[world line]]) traveling through time, which normally increases up or to the right in the diagram. Matter and anti-matter particles are identical except for their direction of propagation through the Feynman diagram. World lines of particles intersect at interaction [[Vertex (graph theory)|vertices]], and the Feynman diagram represents any force arising from an interaction as occurring at the vertex with an associated instantaneous change in the direction of the particle world lines. Gauge bosons are emitted away from the vertex as wavy lines and, in the case of virtual particle exchange, are absorbed at an adjacent vertex.<ref name=Shifman>{{cite book |first=Mikhail |last=Shifman |title=ITEP lectures on particle physics and field theory |publisher=World Scientific |year=1999 |isbn=978-981-02-2639-8}}</ref> The utility of Feynman diagrams is that other types of physical phenomena that are part of the general picture of [[fundamental interaction]]s but are conceptually separate from forces can also be described using the same rules. For example, a Feynman diagram can describe in succinct detail how a [[neutron]] [[beta decay|decays]] into an [[electron]], [[proton]], and [[neutrino|antineutrino]], an interaction mediated by the same gauge boson that is responsible for the [[weak nuclear force]].<ref name="Shifman"/>