The Science of Discworld Revised Edition

throughout space. But suppose it isn’t? Ordinary matter is clumpy, not uniform. David Santiago has pointed out that if quintessence is clumpy too, then Einstein’s Equations predict that the universe could contain ‘anti-Black Holes’ that repel matter instead of swallowing it. These are not quite the same as hypothetical White Holes, time-reversed Black Holes, which spit matter out. However, it’s not yet clear that anti-Black Holes can be stable. Ordinary matter is clumpy because gravity is an attractive force – it
likes
to create clumps. Antigravity is a repulsive force, and by analogy it ought to destroy clumps. If that argument is right, then anti-Black Holes are unstable, and would not be able to form in the first place. They would be mathematical solutions of Einstein’s Equations, but not ones that could be physically realised. Until somebody does the necessary calculations, we can’t be sure.
    1 ‘Most civilizations’ is admittedly not the same as ‘most people’. ‘Most people’ through the history of the planet have not needed to concern themselves with what shape the world is, provided it supports, somewhere, the next meal.
    2 This word, meaning ‘fifth essence’, originally referred to a fifth ‘element’ after earth, air, fire and water. On Discworld this role is played by surprise.

ELEVEN
NEVER TRUST A CURVED UNIVERSE

    PONDER STIBBONS HAD set up a desk a little separate from the others and surrounded it with a lot of equipment, primarily in order to hear himself think.
    Everyone
knew
that stars were points of light. If they weren’t, some would be visibly bigger than others. Some were
fainter
than others, of course, but that was probably due to clouds. In any case their purpose, according to established Discworld law, was to lend a little style to the night.
    And everyone knew that the natural way for things to move was in a straight line. If you dropped something, it hit the ground. It didn’t
curve
. The water fell over the edge of the world, drifting sideways just a tiny bit to make up for the spin, but that was common sense. But inside the Project, spin was everything. Everything was bent. Archchancellor Ridcully seemed to think this was some sort of large-scale character flaw, akin to shuffling your feet or not owning up to things. You couldn’t trust a universe of curves. It wasn’t playing a straight bat.
    At the moment Ponder was rolling damp paper into little balls. He’d had the gardener push in a large stone ball that had spent the last few hundred years on the university’s rockery, relic of some ancient siege catapult. It was about three feet across.
    He’d hung some paper balls of string near it. Now, glumly, he threw others over it and around it. One or two did stick, admittedly, but only because they were damp.
    He was in the grip of some thought.
    You had to start with what you were certain of.
    Things fell down. Little things fell down on to big things. That was common sense.
    But what would happen if you had two big things all alone in the universe ?
    He set up two balls of ice and rock, in an unused corner of the Project, and watched them bang into each other. Then he tried with ball of different sizes. Small ones drifted towards big ones but, oddly enough, the big ones also drifted slightly towards the small ones.
    So … if you thought that one through … that meant that if you dropped a tennis ball to the ground it would certainly go
down
, but in some tiny, immeasurable way the world would, very slightly, come
up
.
    And that was
insane
.
    He also spent some time watching clouds of gas swirl and heat in the more distant regions of the Project. It was all so … well, godless.
    Ponder Stibbons was an atheist. Most wizards were. This was because UU had some quite powerful standing spells against occult interference, and knowing that you’re immune from lightning bolts does wonders for an independent mind. Because the gods, of course,
existed
. Ponder wouldn’t even attempt to deny it. He just didn’t
believe
in them. The god currently gaining popularity was Om, who never answered prayers or manifested himself. It was easy to respect an invisible god. It was the ones that turned up everywhere, often drunk, that put people off.
    That’s why, hundreds of years before, philosophers had decided that there was another set of beings, the
creators
, that existed independently of human belief and who had actually built the universe. They certainly