The Science of Discworld Revised Edition

circumference of this spherical Discworld at 4,000 miles (6,400 km). Unfortunately for this theory, it was known on other grounds that Discworld is some 10,000 miles (16,000 km) from rim to rim. Still, you can’t let an awkward fact get in the way of a good theory, and Ratonasticthenes went to his grave believing that it was a small world after all.
    His error was to interpret perfectly good observational data in terms of a flawed theory. Scientists repeatedly return to established theories to test them in new ways, and tend towards testiness with those priests, religious or secular, who know the answers already – whatever the questions are. Science is not about building a body of known ‘facts’. It is a method for asking awkward questions and subjecting them to a reality-check, thus avoiding the human tendency to believe whatever makes us feel good.
    * * *
    From the earliest times, humans have been interested not just in the shape of the world, but in the shape of the universe. To begin with, they probably thought that these were the same question. Then they worked out, using roughly the same sort of geometry as Eratosthenes, that those lights in the sky were a
very
long way away. They came up with an amazing range of myths about the sun-god’s fiery chariot and so on, but after the Babylonians got the idea of making accurate measurements, their theories started to lead to surprisingly good predictions of things like eclipses and the motion of the planets. By the time of Ptolemy (Claudius Ptolemaeus, AD 100–160) the best model of planetary motion involved a series of ‘epicycles’ – the planets moved as if they were rotating round circles whose centres rotated round other circles whose centres rotated round …
    Isaac Newton replaced this theory, and its more accurate successors, with a
rule
, the law of gravity; it describes how each body in the universe attracts every other body. It explained Johannes Kepler’s discovery that planetary orbits are ellipses, and in the fullness of time it explained a lot of other things too.
    After a few centuries of stunning success, Newton’s theory ran into its first big failure: it made incorrect predictions about the orbit of Mercury. The place in its orbit at which Mercury came closest to the sun didn’t move
quite
the way Newton’s law predicted. Einstein came to the rescue with a theory based not on attractive forces, but on geometry – on the shape of spacetime. This was the celebrated Theory of Relativity. The theory came in two flavours: Special Relativity and General Relativity. Special Relativity is about the structure of space, time, and electromagnetism; General Relativity describes what happens when you throw in gravity too.
    The main point to appreciate is that ‘Relativity’ is a silly name. The whole point of Special Relativity is
not
that ‘everything is relative’, but that one particular thing – the speed of light – is unexpectedly
absolute
. The thought experiment is well known. If you’re travelling in a car at 50 mph (80 kph) and you fire a gun forwards, so that the bullet moves at 500 mph (800 kph) relative to the car, then it will hit a stationary target at a speed of 550 mph (880 kph), adding the two components. However, if instead of firing the gun you switch on a torch, which ‘fires’ light at a speed of 670,000,000 mph (186,000 mps or 300,000 kps), then that light will not hit the stationary target at a speed of 670,000,050 mph. It will hit it at 670,000,000 mph,
exactly the same speed as if the car had been stationary
.
    There are practical problems in staging that experiment, but less graphic and dangerous ones have indicated what the result would be.
    Einstein published Special Relativity in 1905, along with the first serious evidence for quantum mechanics and a ground-breaking paper on diffusion. A lot of other people – among them the Dutch physicist Hendrik Lorentz and the French mathematician Henri Poincaré – were working on the same idea, because electromagnetism didn’t entirely agree with Newtonian mechanics. The conclusion was that the universe is a lot weirder than common sense tells us, although they probably didn’t use that actual word. Objects
shrink
as they approach the speed of light, time slows down to a crawl, mass becomes infinite … and nothing can go faster than light. Another key idea was that space and time are to some extent interchangeable. The traditional three dimensions of space plus a