The Science of Discworld Revised Edition

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, 2 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 speck grew rapidly, and time began to elapse so that ‘rapidly’ actually had a meaning. Everything that there is, today, right out into the furthest depths of space, stems from that astonishing ‘beginning’. Colloquially, the event is known as the Big Bang. The name reflects several features of the event – for example, that tiny speck of space/time was enormously hot, and grew in size exceedingly rapidly. It was like a huge explosion – but there was no stick of cosmic dynamite, sitting there in no-space with its non-material fuse burning away as some kind of pre-time pseudo-clock counted down the seconds to detonation. What exploded was – nothing. Space, time, and matter are the
products
of that explosion: they played no part in its cause. Indeed, in a very real sense, it
had
no cause.
    The evidence in favour of the Big Bang is twofold. The first item is the discovery that the universe is expanding. The second is that ‘echoes’ from the Big Bang can still be detected today. The possibility that the universe might be getting bigger first appeared in mathematical solutions to equations formulated by Albert Einstein. Einstein viewed spacetime as being ‘curved’. A body moving through curved spacetime deviates from its normal straight line path, much as a marble rolling on a curved surface does. This deviation can be interpreted as a ‘force’ – something that
pulls
the body away from that ideal straight line. Actually there is no pull: just a bend in spacetime, causing a bend in the body’s path. But it looks as if there’s a pull. Indeed this apparent pull is what Newton called ‘gravity’, back in the days when people thought it really did pull bodies together. Anyway, Einstein wrote down some equations for how such a bendy universe ought to behave. They were very difficult equations to solve, but after making some extremely strong assumptions – basically that at any instant of time space is a sphere – mathematical physicists worked out a few answers. And this tiny, very special list of solutions, the only ones their feeble methods could find, told them three things that the universe could do. It could stay the same size forever; it could collapse down to a single point; or it could start from a single point and grow in size without limit.
    We now know that there are many other solutions to Einstein’s