![]() These two features mean that for a sufficiently large star, the gravitational attraction between particles, can dominate over all other forces, and lead to gravitational collapse. The second advantage is gravity is always attractive, unlike electric forces, which can be attractive or repulsive. The earth is held in orbit by the Sun, 93 million miles away, and the Sun is held in orbit around the centre of the galaxy, about 26 thousand light years away. Although it's by far the weakest of the known forces of nature, it has two crucial advantages over other forces. ![]() To understand them, we need to start with gravity. Black holes are stranger than anything dreamed up by science fiction writers, but they are firmly matters of science fact. It is said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. ![]() I am pleased to be here at Oxford today, to give the inaugural Roger Penrose lecture, in honor of my long time friend and colleague. This is to allow correct pronunciation and timing by a speech synthesiser. You may not reproduce, edit, translate, distribute, publish or host this document in any way with out the permission of The Stephen Hawking Estate. Note that there may be incorrect spellings, punctuation and/or grammar in this document. This lecture is the intellectual property of Professor S.W.Hawking.
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