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'''General relativity''' is a [[theory]] of [[Space (physics)|space]] and [[time]]. The theory was published by [[Albert Einstein]] in 1915.<ref>O'Connor J.J. and E.F. Robertson (1996), "[http://www-history.mcs.st-and.ac.uk/HistTopics/General_relativity.html General relativity]". ''[http://www-history.mcs.st-and.ac.uk/Indexes/Math_Physics.html Mathematical Physics index]'', [http://www.st-andrews.ac.uk/maths/ School of Mathematics and Statistics], [http://www.st-andrews.ac.uk/ University of St. Andrews], Scotland, May, 1996. Retrieved 2015-02-04.</ref> 

The main idea of general relativity is that [[space]] and [[time]] are two different parts of [[spacetime]]. In general relativity, [[gravity]] does not work the same way that [[Sir Isaac Newton]] said gravity worked. According to Einstein, gravity changes not only space, but also time. ([[Time dilation]])

==Idea==
An important idea in general relativity is the "principle of equivalence". An example is that two people, one in an elevator on the surface of the Earth, and the other in an elevator in outer space but accelerating upwards at 9.8 m/s<sup>2</sup> will each observe the same behavior of an object they drop from their hands. The object will [[Acceleration|accelerate]] to the floor at 9.8 m/s<sup>2</sup> in either case.

This makes it impossible for either to distinguish whether or not they are at rest in a gravitational field.  Other versions of this type of "thought experiment" were used to show that light would curve in an accelerating frame of reference.<ref>{{citation|last1=Di Casola|first1=Eolo|last2=Liberati|first2=Stefano|last3=Sonego|first3=Sebastiano|title=Nonequivalence of equivalence principles|journal=American Journal of Physics|date=2015|volume=83|issue=39|doi=10.1119/1.4895342|pages=39–46|arxiv=1310.7426|bibcode=2015AmJPh..83...39D|s2cid=119110646}}</ref>

The force pulling the Earth towards the Sun is about the same as a second force. This second force is called the [[centrifugal force]]. The centrifugal force exists because the Earth moves sideways. This sideways motion makes the distance between the Earth and Sun increase. Since the Earth is being pulled towards the sun and moving away at the same time, it stays at about the same distance. This is also how the [[Moon]] orbits the earth. In this second case, Earth is the ball and the Moon is the object. 

The Sun and other objects with mass curve four dimensional spacetime fabric.<ref>{{Cite web|title=General Relativity|url=https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity/#From|access-date=2021-05-30|website=www.pitt.edu}}</ref>

==Predictions==
General relativity has predicted many things which were later seen. These include:
* As light gets closer to the Sun, it bends towards the sun twice as much as [[classical physics]] predicts (the system used before general relativity). This was seen in an experiment by [[Arthur Eddington]] in 1919.<ref>{{citation|first1=F. W.|last1=Dyson|first2=A. S.|last2=Eddington|first3=C.|last3=Davidson|title=A Determination of the deflection of light by the Sun's gravitational field, from observations made at the total eclipse of May 29, 1919|journal=Philosophical Transactions of the Royal Society A|doi=10.1098/rsta.1920.0009|volume=220|pages=291–333|date=1920|issue=571–581|bibcode=1920RSPTA.220..291D}}</ref> When scientists saw his experiment, they started to take general relativity seriously.
* The [[perihelion]] of the planet [[Mercury (planet)|Mercury]] rotates along its orbit more than is expected under [[Newtonian physics]]. General relativity accounts for the difference between what is seen and what is expected without it.
*[[Redshift]] from gravity. When light moves away from an object with gravity (moving away from the center of the valley), it is stretched into longer wavelengths. This was confirmed by the [[Pound-Rebka experiment]].
* The [[Shapiro delay]]. Light appears to slow down when it passes close to a massive object. This was first seen in the 1960s by space probes headed towards the planet [[Venus (planet)|Venus]].
* [[Gravitational wave]]s. They were first observed on 14 September 2015.<ref>{{cite journal |collaboration=LIGO Scientific Collaboration and Virgo Collaboration |last1=Abbott |first1=Benjamin P.|first2=R.|last2=Abbott|first3=T. D.|last3=Abbott|display-authors=1|title=Observation of Gravitational Waves from a Binary Black Hole Merger  |journal=Physical Review Letters |volume=116 |issue=6 |pages=061102-1–061102-16|year=2016 |doi=10.1103/PhysRevLett.116.061102 |pmid=26918975 |arxiv=1602.03837 |bibcode = 2016PhRvL.116f1102A |s2cid=124959784 }}</ref>

==Related pages==
* [[Special relativity|Special Theory of Relativity]]

== References ==
{{reflist}}

== Other websites ==
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[[Category:Basic physics ideas]]
[[Category:Relativity]]
[[Category:Albert Einstein]]