Read Online Science of Discworld by Terry Pratchett - Free Book Online
Authors: Terry Pratchett
Tags: Non-Fiction
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like that. But according to the old lady who espoused the Hindu cosmology, and was asked the same question by a learned astronomer, there is a different answer: 'It's turtles all the way down!' The image of an infinite pile of turtles is instantly ludicrous, and very few people find it a satisfying explanation. Indeed very few people find it a satisfying kind of explanation, if only because it doesn't explain what supports the infinite pile of turtles. However, most of us are quite content to explain the origins of time as 'it's always been there'. Seldom do we examine this statement closely enough to realize that what it really says is 'It's time all the way back.' Now replace 'time' by 'turtle' and 'back' by 'down' ... Each instant of time is 'supported', that is, a causal consequence of, the previous instant of time. Fine, but that doesn't explain why time exists. What caused that infinite expanse of time? What holds up the whole pile?
All of which puts us in a serious quandary. We have problems thinking of time as beginning without a precursor, because it's hard to see how the causality goes. But we have equally nasty problems thinking of time as beginning with a precursor, because then we hit the turtle-pile problem. We have similar problems with space: either it goes on forever, in which case it's 'space all the way out' and we need somewhere even bigger to put the whole thing, or it stops, in which case we wonder what's outside it.
The real point is that neither of these options is satisfactory, and the origins of space and time fit neither model. The universe is not like a village, which ends at a fence or an imaginary line on the ground, neither is it like the distant desert which seems to vanish into eternity but actually just gets too far away for us to see it clearly. Time is not like a human lifespan, which starts at birth and ends at death, nor is it like the extended lifespan found in many religions, where the human soul continues to live indefinitely after death, and the much rarer belief (held, for example, by Mormons) that some aspect of each person was somehow already alive in the indefinite past.
So how did the universe begin? 'Begin' is the wrong word. Nonetheless, there is good evidence that the age of the universe is about 15 billion years, * so nothing — not space, not time — existed before some instant of time roughly 15 billion years ago. See how our narrativium-powered semantics confuses us. This does not mean that if you went back 15 billion and one years, you would find nothing. It means that you cannot go back 15 billion and one years. That description makes no sense. It refers to a time before time began, which is logically incoherent, let alone physically so.
This is not a new point. Saint Augustine made it very clearly in his Confessions of about 400, when he said: '… if there was no time before heaven and earth were created, how can anyone ask what you [God] were doing “then”? If there was no time, there was no “then” '.
Some have gone to greater extremes. In The End of Time , Julian Barbour argues that time does not exist. In his view, the 'real' laws of physics specify a static universe of timeless 'nows'. He argues that time is not a physical dimension in the way that space is. The apparent flow of time is an illusion, created because the collection of nows that corresponds to a human being makes that human experience its world as if it were an ordered series of events, happening one after the other. Apparent gaps of 'duration' between successive events are merely blanks where no 'now' exists. Thief of Time explores some remarkably similar issues from the Discworld viewpoint. But for this book, we will take a conventional attitude towards time, and consider it to be a real part of physics.
Cosmologists are pretty sure that it all began like this. The universe came into being as a tiny speck of space and time. The amount of space inside this tiny
All of which puts us in a serious quandary. We have problems thinking of time as beginning without a precursor, because it's hard to see how the causality goes. But we have equally nasty problems thinking of time as beginning with a precursor, because then we hit the turtle-pile problem. We have similar problems with space: either it goes on forever, in which case it's 'space all the way out' and we need somewhere even bigger to put the whole thing, or it stops, in which case we wonder what's outside it.
The real point is that neither of these options is satisfactory, and the origins of space and time fit neither model. The universe is not like a village, which ends at a fence or an imaginary line on the ground, neither is it like the distant desert which seems to vanish into eternity but actually just gets too far away for us to see it clearly. Time is not like a human lifespan, which starts at birth and ends at death, nor is it like the extended lifespan found in many religions, where the human soul continues to live indefinitely after death, and the much rarer belief (held, for example, by Mormons) that some aspect of each person was somehow already alive in the indefinite past.
So how did the universe begin? 'Begin' is the wrong word. Nonetheless, there is good evidence that the age of the universe is about 15 billion years, * so nothing — not space, not time — existed before some instant of time roughly 15 billion years ago. See how our narrativium-powered semantics confuses us. This does not mean that if you went back 15 billion and one years, you would find nothing. It means that you cannot go back 15 billion and one years. That description makes no sense. It refers to a time before time began, which is logically incoherent, let alone physically so.
This is not a new point. Saint Augustine made it very clearly in his Confessions of about 400, when he said: '… if there was no time before heaven and earth were created, how can anyone ask what you [God] were doing “then”? If there was no time, there was no “then” '.
Some have gone to greater extremes. In The End of Time , Julian Barbour argues that time does not exist. In his view, the 'real' laws of physics specify a static universe of timeless 'nows'. He argues that time is not a physical dimension in the way that space is. The apparent flow of time is an illusion, created because the collection of nows that corresponds to a human being makes that human experience its world as if it were an ordered series of events, happening one after the other. Apparent gaps of 'duration' between successive events are merely blanks where no 'now' exists. Thief of Time explores some remarkably similar issues from the Discworld viewpoint. But for this book, we will take a conventional attitude towards time, and consider it to be a real part of physics.
Cosmologists are pretty sure that it all began like this. The universe came into being as a tiny speck of space and time. The amount of space inside this tiny
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