Force

=== Gravitational ===
[[File:GRAVITY A powerful new probe of black holes.jpg|thumb|Instruments like GRAVITY provide a powerful probe for gravity force detection.<ref>{{cite web |title=Powerful New Black Hole Probe Arrives at Paranal |url=http://www.eso.org/public/announcements/ann15061/ |access-date=13 August 2015}}</ref>]]
Newton's law of gravitation is an example of ''action at a distance'': one body, like the Sun, exerts an influence upon any other body, like the Earth, no matter how far apart they are. Moreover, this action at a distance is ''instantaneous.'' According to Newton's theory, the one body shifting position changes the gravitational pulls felt by all other bodies, all at the same instant of time. [[Albert Einstein]] recognized that this was inconsistent with special relativity and its prediction that influences cannot travel faster than the [[speed of light]]. So, he sought a new theory of gravitation that would be relativistically consistent.<ref>
{{cite book
 | last1=Misner |first1=Charles W. |author-link1=Charles W. Misner
 | last2=Thorne |first2=Kip S. |author-link2=Kip Thorne
 | last3=Wheeler |first3=John Archibald |author-link3=John Archibald Wheeler
 | year=1973
 | title=Gravitation
 | title-link=Gravitation (book)
 | publisher=[[W. H. Freeman]]
 | location=San Francisco
 | isbn=978-0-7167-0344-0
 | pages=3–5
}}</ref><ref>
{{Cite book
 |last=Choquet-Bruhat |first=Yvonne |author-link=Yvonne Choquet-Bruhat
 |url=https://www.worldcat.org/oclc/317496332
 |title=General Relativity and the Einstein Equations
 |date=2009
 |publisher=Oxford University Press
 |isbn=978-0-19-155226-7 |location=Oxford |oclc=317496332
 |pages=37–39
}}</ref> [[Mercury (planet)|Mercury]]'s orbit did not match that predicted by Newton's law of gravitation. Some astrophysicists predicted the existence of an undiscovered planet ([[Vulcan (hypothetical planet)|Vulcan]]) that could explain the discrepancies. When Einstein formulated his theory of [[general relativity]] (GR) he focused on Mercury's problematic orbit and found that his theory added [[Perihelion precession of Mercury|a correction, which could account for the discrepancy]]. This was the first time that Newton's theory of gravity had been shown to be inexact.<ref>{{cite news |last1=Siegel |first1=Ethan |title=When Did Isaac Newton Finally Fail? |url=https://www.forbes.com/sites/startswithabang/2016/05/20/when-did-isaac-newton-finally-fail/#6fdc279675f5 |access-date=3 January 2017 |work=Forbes |date=20 May 2016}}</ref>

Since then, general relativity has been acknowledged as the theory that best explains gravity. In GR, gravitation is not viewed as a force, but rather, objects moving freely in gravitational fields travel under their own inertia in [[geodesic|straight lines]] through [[curved spacetime]] – defined as the shortest spacetime path between two spacetime events. From the perspective of the object, all motion occurs as if there were no gravitation whatsoever. It is only when observing the motion in a global sense that the curvature of spacetime can be observed and the force is inferred from the object's curved path. Thus, the straight line path in spacetime is seen as a curved line in space, and it is called the ''[[external ballistics|ballistic]] [[trajectory]]'' of the object. For example, a [[Basketball (ball)|basketball]] thrown from the ground moves in a [[parabola]], as it is in a uniform gravitational field. Its spacetime trajectory is almost a straight line, slightly curved (with the [[radius of curvature (applications)|radius of curvature]] of the order of few [[light-year]]s). The time derivative of the changing momentum of the object is what we label as "gravitational force".<ref name=Kleppner />