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I [[klassisk mekanik]] er '''gravitation''' en [[kraft (fysik)|tiltrækningskraft]] som er mellem alle [[Partikel (fysik)|partikler]] ([[stof (fysik)|stof]]) med [[masse (fysik)|masse]] i [[universet]]. Resultatet af gravitationen er tyngdekraften.
 
I [[Albert Einstein|Einsteins]] [[Generel relativitetsteori|generelle relativitetsteori]] er '''gravitation''' ikke en [[kraft]], men en egenskab ved [[rummet]] – eller mere eksakt [[rumtid]]en. Faktisk bliver enhver form for [[energi]] i bevægelse (f.eks. [[foton]]er; [[lys]]) "bøjet" om enhver form for energi (f.eks. masser)! Det skyldes netop ikke "tyngdekraften", fordi fotoner ikke har masse.<ref>[https://web.archive.org/web/20170909030828/http://www.desy.de:80/user/projects/Physics/ParticleAndNuclear/photon_mass.html Deutsches Elektronen-Synchrotron: What is the mass of a photon?] Citat: "...Alternative theories of the photon include a term that behaves like a mass, and this gives rise to the very advanced idea of a "massive photon". If the rest mass of the photon were non-zero, the theory of [[Kvanteelektrodynamik|quantum electrodynamics]] would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it is if photons have zero rest mass...experiment...An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields...The new limit is 7 × 10-17 [[eV]]..."</ref> Men fordi rummet krummer om enhver form for energi, vil lyset følge rummets krumning.
 
== Gravitation i klassisk mekanik ==
[[Albert Einstein|Einsteins]] [[relativitetsteori]] forudsiger at gravitationens udbredelseshastighed skal være konsistent med [[lysets hastighed]].
<ref>[http://www.sciencedaily.com/releases/2003/06/030619075759.htm June 19, 2003, sciencedaily.com: Berkeley Lab Physicist Challenges Speed Of Gravity Claim] Citat: "...According to Einstein's General Theory of Relativity, light and gravity travel at the same speed, about 186,000 miles (300,000 kilometers) per second. Most scientists believe this is true but the assumption was that it could only be proven through the detection of gravity waves..."</ref>
<ref>[https://web.archive.org/web/20160730212313/http://www.astronomycafe.net:80/qadir/q2226.html Ask the Astronomer: If nothing can travel faster than light, why does gravity take affect instantly?] Citat: "...There is no observation that justifies this 'Newtonian' statement, and lots of evidence from special and general relativity that gravity obeys the speed limit set by the speed of light..."</ref>
<ref>[https://web.archive.org/web/20160730212747/http://www.astronomycafe.net:80/qadir/q1399.html Ask the Astronomer: Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us? ] Citat: "...Because all indications are that gravity travels at the speed of light, we can only be affected by where a body is at the SAME time we see the light from it..."</ref>
Gravitationens udbredelseshastighed kan derfor ikke være større end lysets hastighed (f.eks. øjeblikkelig). Et objekts '''tyngdefelt''' eller rettere gravitations virke, er med tiden hele universet.
 
* [[Merkur (planet)|Merkurs]] bane om [[solen]].
* [[Sort hul (astronomi)|Sorte huller]].
* Et par [[neutronstjerne]]rs spirallering mod hinanden passer meget nøjagtig med Einsteins ligningers forudsigelser.<ref>[https://web.archive.org/web/20160919170229/http://www.jodrellbank.manchester.ac.uk/news/2006/einstein/ 14th September 2006, manchester.ac.uk: General Relativity survives gruelling pulsar test – Einstein at least 99.95% right!] Citat: "...the double pulsar system should lose energy causing the two neutron stars to spiral in towards each other by precisely the amount that we have observed – thus our observations give an indirect proof of the existence of gravitational waves [as predicted by Einstein]..."</ref>
 
Men der er stadig nogle fysiske observationer, som endnu ikke er forklaret tilfredsstillende med Einsteins generelle relativitetsteori:
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