The Science of Discworld Revised Edition

hydrocarbon molecule. Because of the way our mental modules evolved, we think of beginnings as being analogous to how a day begins, or how a hike across the desert begins; and we think of becomings in the same way that a polar bear’s tooth becomes a carved amulet, or a live spider becomes dead when you squash it.
    That is: beginnings start from somewhere (which is where whatever it is
begins
), and becomings turn Thing One into Thing Two by pushing it across a clearly defined boundary (the tooth was not carved, but now it is; the spider was not dead, but now it is). Unfortunately the universe doesn’t work in such a simple-minded manner, so we have serious trouble thinking about how a universe can begin, or how an ovum and a sperm can become a living child.
    Let us leave becomings for a moment, and think about beginnings. Thanks to our evolutionary prejudices, we tend to think of the beginning of the universe as being some special time, before which the universe did not exist and after which it did. Moreover, when the universe changed from not being there to being there, something must have
caused
that change – something that was around before the universe began, otherwise it wouldn’t have been able to cause the universe to come into being. When you bear in mind that the beginning of the universe is also the beginning of space and the beginning of time, however, this point of view is distinctly problematic. How can there be a ‘before’ if time has not yet started? How can there be a cause for the universe starting up, without space for that cause to happen in – and time for it to happen?
    Maybe there was something else in existence already … but now we have to decide how
that
got started, and the same difficulties arise. All right, let’s go the whole hog: something – perhaps the universe itself, perhaps some precursor – was around forever. It didn’t
have
a beginning, it just was, always.
    Satisfied? Things that exist forever don’t have to be explained, because they don’t need a cause? Then what caused them to have been around forever?
    It now becomes impossible not to mention the turtle joke. Stephen Hawking tells it at the start of
A Brief History of Time
, but it goes back a lot further. According to Hindu legend, the Earth rides on the back of four elephants, which ride on a turtle. But what supports the turtle? In Discworld, Great A’Tuin needs no support, swimming through the universe unperturbed by any thought about what holds it up. That’s magic in action: world-carrying turtles are
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