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What shape is the world?

discworld

The world is flat and rests upon the backs of four elephants who, in turn, stand upon the back of a giant, space-faring turtle. This, to those familiar with the work of Terry Pratchett, is, of course, not Earth but the Discworld where many of his publications take place. Terry, along with Professors Ian Stewart and Jack Cohen, recently published The Science of Discworld IV. Not everything made it into the book and Ian Stewart was kind enough to give us a peek at the missing science of the Discworld.

You can’t trust scientists, when new evidence comes along, they change their minds.

Terry Pratchett, Jack Cohen and I recently completed the fourth book in the popular science series The Science of Discworld. The format of these books combines a fantasy short story, set in Terry’s Discworld, with scientific commentary about the real universe.

Our own universe, or ambiguously our planet, appears in the story as the Roundworld Project. This format has some very useful features for writing popular accounts of science. The main one is that since the scientific action always takes place inside Roundworld, the scientific chapters are genuine real-world science and any scientific topic might in principle be discussed.

The fourth, and most recent, book in the series hinges upon events in Discworld rather than Roundworld. On the Disc there is an ancient religion, Omnianism, which believes Discworld is round. Despite all the evidence, they do not believe in the turtle or the elephants. Seeing the Roundworld Project as an infringement of their theological property rights, they launch a legal case to gain custody. This scenario allows us to examine, in the science chapters, a variety of issues. In particular, we look at how scientists can try to infer the shape of their world, or the universe itself, when they are unable to get outside it and take a direct look. For the Earth, we can now do that, but scientists mapped out the entire planet long before we could get into orbit. We still don’t have that luxury for the universe as a whole, and it’s hard to see how we could ever get it.

The commentary discusses a key feature of science, one that many non-scientists fail to appreciate: science is mainly about inference from experiments and theories, not direct observation. The ancient Greeks inferred the spherical form of the Earth thousands of years ago, by logical inference from phenomena that they could observe, such as the planet’s shadow on the Moon during an eclipse and the way boats seemed to sink below the horizon as they headed out to sea. Today we infer the temperature at the centre of the sun from our understanding of nuclear reactions, but no person or instrument has ever been there to check. The majority of scientific knowledge rests on inference, not on direct observation. Indeed, that’s what theories are for.

I don’t want to give away the contents of the book, so I’ll focus here on a short passage that we cut out, at a late stage, because the chapter about the shape of the Earth was getting too long. It illustrates how science often pursues ideas that eventually turn out to be wrong. Critics often try to convince us that this tendency means that scientists haven’t got a clue, but actually it reveals their willingness to seek fresh evidence and adapt to it. As the saying goes “You can’t trust scientists, when new evidence comes along, they change their minds.”

The topic that we deleted concerned the short-lived hypothesis that the Earth might be expanding:

On the Beagle voyage, Charles Darwin observed fossil shells at high altitudes, showing that the Andes mountains had been uplifted though thousands of metres. He suggested that this might be explained if the Earth were expanding, but he quickly changed his mind, proposing instead that as some land rose, parts of the seabed descended to compensate. Several early geologists noticed the way the west coast of Africa resembles the east coast of South America, suggesting that those continents have moved apart. Roberto Mantovani explained this as the effect of the Earth expanding, so that its surface area increased.

Continental rocks of fixed area would necessarily break up, and the pieces would move apart. In 1889 and 1909, he proposed that originally the planet’s entire surface was covered in a shell of rock. Internal heating caused the planet to expand, and the expansion was fastest at the ‘rip zones’ down the middle of the oceans. This led to today’s geographical distribution of land and ocean.

Contrariwise, James Dwight Dana thought that the Earth contracted as time passed. This was caused by global cooling as our planet radiated its heat away into space. Dana’s theory seemed to explain some puzzling features of our planet’s geophysics, in particular mountains, which were wrinkles in its skin caused by shrinkage. Early in the 20th century, Eduard Suess explained earthquakes in terms of a contracting planet and Robert T. Hill wrote that

“…rocks are being folded, fractured and otherwise broken or deformed by the great shrinking and settling of the earth’s crust as a whole... The prehistoric convulsions of the earth before man inhabited this planet were terrific, almost inconceivable.”

The ring of fire round the Pacific Ocean, riddled with volcanoes, could be explained in a similar manner. The contracting Earth hypothesis is now considered disproved, by a variety of evidence. In particular we now know that radioactive elements inside the Earth provide a significant source of heat. And the occurrence of the same fossil species on either side of the Atlantic argues against contraction. Oddly, some geological (‘hermeological’?) features of the planet Mercury, namely cliffs, are currently explained by global cooling, and the same goes for comparable features on the Moon. Neither body has a source of internal heat that is comparable to the Earth’s.

Expanding Earth theories bear some resemblance to Alfred Wegener’s continental drift, and seek to explain the same features. But Wegener did not think the planet was getting bigger. The discovery that the Earth’s surface is composed of gigantic, moving tectonic plates provided a credible mechanism for Wegener’s idea. Before the plates were found, continental drift was generally held to be at least as absurd as an expanding or contracting Earth.

Perhaps ironically, today’s cosmologists have found strong evidence that the entire universe is expanding. It’s pretty convincing, but there are some loopholes and some inconvenient observations that cast a small amount of doubt on current proposals for how the expansion got started and how it subsequently developed. Maybe in the future an expanding universe hypothesis will seem just as absurd as an expanding Earth. I think that’s wildly unlikely, but because the expanding universe theory rests on inference from something that happened over 13 billion years ago, I wouldn’t rule it out completely.

Professor Ian Stewart is co-author of Science of Discworld IV with Terry Pratchett and Dr Jack Cohen. He is Emeritus Professor of Mathematics and recognised researcher at the University of Warwick.

A version of this article first appeared on the Warwick Knowledge Centre. Please visit warwick.ac.uk/knowledge
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