Editing Fundamental interaction (section)

=== Gravity ===
{{Main|Gravity}}
''Gravitation'' is the weakest of the four interactions at the atomic scale, where electromagnetic interactions dominate.

Gravitation is the most important of the four fundamental forces for astronomical objects over astronomical distances for two reasons. First, gravitation has an infinite effective range, like electromagnetism but unlike the strong and weak interactions. Second, gravity always attracts and never repels; in contrast, astronomical bodies tend toward a near-neutral net electric charge, such that the attraction to one type of charge and the repulsion from the opposite charge mostly cancel each other out.<ref>{{cite news |last1=Siegel |first1=Ethan |title=What Is The Strongest Force In The Universe? |url=https://www.forbes.com/sites/startswithabang/2016/04/26/what-is-the-strongest-force-in-the-universe/ |access-date=22 March 2021 |work=[[Starts With a Bang]] |date=2016 |language=en}}</ref>

Even though electromagnetism is far stronger than gravitation, electrostatic attraction is not relevant for large celestial bodies, such as planets, stars, and galaxies, simply because such bodies contain equal numbers of protons and electrons and so have a net electric charge of zero. Nothing "cancels" gravity, since it is only attractive, unlike electric forces which can be attractive or repulsive. On the other hand, all objects having mass are subject to the gravitational force, which only attracts. Therefore, only gravitation matters on the large-scale structure of the universe.

The long range of gravitation makes it responsible for such large-scale phenomena as the structure of galaxies and [[black hole]]s and, being only attractive, it slows down the [[expansion of the universe]]. Gravitation also explains astronomical phenomena on more modest scales, such as [[planet]]ary [[orbit]]s, as well as everyday experience: objects fall; heavy objects act as if they were glued to the ground, and animals can only jump so high.

Gravitation was the first interaction to be described mathematically. In ancient times, [[Aristotle]] hypothesized that objects of different masses fall at different rates. During the [[Scientific Revolution]], [[Galileo Galilei]] experimentally determined that this hypothesis was wrong under certain circumstances—neglecting the friction due to air resistance and buoyancy forces if an atmosphere is present (e.g. the case of a dropped air-filled balloon vs a water-filled balloon), all objects accelerate toward the Earth at the same rate. Isaac Newton's [[law of Universal Gravitation]] (1687) was a good approximation of the behaviour of gravitation. Present-day understanding of gravitation stems from Einstein's [[General Theory of Relativity]] of 1915, a more accurate (especially for [[cosmology|cosmological]] masses and distances) description of gravitation in terms of the [[geometry]] of [[spacetime]].

Merging general relativity and [[quantum mechanics]] (or [[quantum field theory]]) into a more general theory of [[quantum gravity]] is an area of active research. It is hypothesized that gravitation is mediated by a massless [[Spin (physics)#Fermions and bosons|spin-2 particle]] called the [[graviton]].

Although general relativity has been experimentally confirmed (at least for weak fields, i.e. not black holes) on all but the smallest scales, there are [[alternatives to general relativity]]. These theories must reduce to general relativity in some limit, and the focus of observational work is to establish limits on what deviations from general relativity are possible.

Proposed [[extra dimensions]] could explain why the gravity force is so weak.<ref>{{cite web|url=http://home.web.cern.ch/about/physics/extra-dimensions-gravitons-and-tiny-black-holes|title=Extra dimensions, gravitons, and tiny black holes|date=20 January 2012|author=CERN}}</ref>