(ebook Pdf) Stephen Hawking A Brief History Of Time

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.All the matter in the star will be compressed into a region of zero volume, so the density of matterand the curvature of space-time become infinite.In other words, one has a singularity contained within a regionof space-time known as a black hole.At first sight, Penrose s result applied only to stars; it didn t have anything to say about the question of whetherthe entire universe had a big bang singularity in its past.However, at the time that Penrose produced histheorem, I was a research student desperately looking for a problem with which to complete my Ph.D.thesis.Two years before, I had been diagnosed as suffering from ALS, commonly known as Lou Gehrig s disease, ormotor neuron disease, and given to understand that I had only one or two more years to live.In thesecircumstances there had not seemed much point in working on my Ph.D. I did not expect to survive that long.Yet two years had gone by and I was not that much worse.In fact, things were going rather well for me and Ihad gotten engaged to a very nice girl, Jane Wilde.But in order to get married, I needed a job, and in order toget a job, I needed a Ph.D.In 1965 I read about Penrose s theorem that any body undergoing gravitational collapse must eventually form asingularity.I soon realized that if one reversed the direction of time in Penrose s theorem, so that the collapsebecame an expansion, the conditions of his theorem would still hold, provided the universe were roughly like aFriedmann model on large scales at the present time.Penrose s theorem had shown that any collapsing starmust end in a singularity; the time-reversed argument showed that any Friedmann-like expanding universemust have begun with a singularity.For technical reasons, Penrose s theorem required that the universe beinfinite in space.So I could in fact, use it to prove that there should be a singularity only if the universe wasexpanding fast enough to avoid collapsing again (since only those Friedmann models were infinite in space).During the next few years I developed new mathematical techniques to remove this and other technicalconditions from the theorems that proved that singularities must occur.The final result was a joint paper byPenrose and myself in 1970, which at last proved that there must have been a big bang singularity providedonly that general relativity is correct and the universe contains as much matter as we observe.There was a lotof opposition to our work, partly from the Russians because of their Marxist belief in scientific determinism, andpartly from people who felt that the whole idea of singularities was repugnant and spoiled the beauty ofEinstein s theory.However, one cannot really argue with a mathematical theorem.So in the end our workfile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/b.html (8 of 9) [2/20/2001 3:14:24 AM]A Brief History of Time - Stephen Hawking.Chapter 3became generally accepted and nowadays nearly everyone assumes that the universe started with a big bangsingularity.It is perhaps ironic that, having changed my mind, I am now trying to convince other physicists thatthere was in fact no singularity at the beginning of the universe as we shall see later, it can disappear oncequantum effects are taken into account.We have seen in this chapter how, in less than half a century, man s view of the universe formed over millenniahas been transformed.Hubble s discovery that the universe was expanding, and the realization of theinsignificance of our own planet in the vastness of the universe, were just the starting point.As experimentaland theoretical evidence mounted, it became more and more clear that the universe must have had abeginning in time, until in 1970 this was finally proved by Penrose and myself, on the basis of Einstein s generaltheory of relativity.That proof showed that general relativity is only an incomplete theory: it cannot tell us howthe universe started off, because it predicts that all physical theories, including itself, break down at thebeginning of the universe.However, general relativity claims to be only a partial theory, so what the singularitytheorems really show is that there must have been a time in the very early universe when the universe was sosmall that one could no longer ignore the small-scale effects of the other great partial theory of the twentiethcentury, quantum mechanics.At the start of the 1970s, then, we were forced to turn our search for anunderstanding of the universe from our theory of the extraordinarily vast to our theory of the extraordinarily tiny.That theory, quantum mechanics, will be described next, before we turn to the efforts to combine the two partialtheories into a single quantum theory of gravity.PREVIOUS NEXTfile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/b.html (9 of 9) [2/20/2001 3:14:24 AM]A Brief History of Time - Stephen Hawking [ Pobierz całość w formacie PDF ]

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