OAS NOVEMBER 20013 ASTRONOMY EZINE VOL 2 | Page 21

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Notice that all of the above calculations have been done assuming Newton’s law of gravity. Newton’s
law of gravity is not actually correct, it has been superseded by Einstein’s, which we call the theory
of General Relativity. To do the calculations properly we should use this theory, but it is rather complicated. No, it is very complicated. But to illustrate the basic idea, Newton’s laws are fine. It is surprisingly often said that Einstein’s work led to the prediction of black holes. This is not true, they
had been suggested by a geologist by the name of John Michell in 1783. But we do need Einstein’s
work to do the calculations properly.
Any radiation being emitted from inside of the event horizon will never get to us, the gravitational
pull from the black hole stops it from escaping. How do we therefore even know that black holes exist? I will answer that question in a future blog, along with some discussion of what happens as
matter crosses the event horizon of a black hole, and what might be right at the centre of a black
hole.

Image of Sgtr A* the black hole at the centre of the Milkyway, as seen by the Chandra X-Ray Observatory
http://en.wikipedia.org/wiki/File:Chandra_image_of_Sgr_A.jpg

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