What is Gravitational redshift?
[[File:Gravitational red-shifting2.png|thumb|200px|The gravitational [[redshift]] of a light wave as it moves up and away from a gravitational field. The effect is greatly exaggerated in this image]] In [[physics]] and [[general relativity]], we have something called '''gravitational redshift'''. It is like this: Imagine you have [[light]] (photons), moving away from a place with very strong gravity, like a [[star]] or a [[black hole]]. As these [[photons]] move away, they look like they are losing energy. This makes them look like they have a lower [[frequency]] and longer [[wavelength]], which we call a redshift because this moves light to the red part of the [[spectrum]].<ref>{{Cite journal |last=Einstein |first=Albert |date=1907 |title=Relativitätsprinzip und die aus demselben gezogenen Folgerungen |trans-title=On the Relativity Principle and the Conclusions Drawn from It |url=http://www.soso.ch/wissen/hist/SRT/E-1907.pdf |journal=Jahrbuch der Radioaktivität |issue=4 |pages=411–462}}</ref><ref>{{Cite journal|last=Valente|first=Mário Bacelar|date=2018-12-06|title=Einstein's redshift derivations: its history from 1907 to 1921|url=https://revistas.pucsp.br/index.php/circumhc/article/view/36437|journal=Circumscribere: International Journal for the History of Science|language=en|volume=22|pages=1–16|doi=10.23925/1980-7651.2018v22;1-16|s2cid=239568887 |issn=1980-7651|doi-access=free}}</ref> But, if photons move towards a place with strong gravity, like towards a star or black hole, they look like they are gaining more energy. This is called '''gravitational blueshift''', and it is like moving light towards the blue end of the spectrum.<ref>{{Cite journal |last=Einstein |first=Albert |date=1907 |title=Relativitätsprinzip und die aus demselben gezogenen Folgerungen |trans-title=On the Relativity Principle and the Conclusions Drawn from It |url=http://www.soso.ch/wissen/hist/SRT/E-1907.pdf |journal=Jahrbuch der Radioaktivität |issue=4 |pages=411–462}}</ref><ref>{{Cite journal|last=Valente|first=Mário Bacelar|date=201