The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,54

yellow with purple stripes, and would offer exactly the same explanation. On the other hand, if you explain the colour of the sky in terms of light being scattered by dust in the upper atmosphere, and discover that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, then you will understand why short-wavelength blue light will dominate compared to the longer wavelengths of yellow and red. (The fourth power of a small number is very small indeed, and inverse proportionality means that small numbers are more important than big ones – just as one tenth is bigger than one hundredth.) Now you’ve learned something useful and informative, which you can apply to other questions.

However, this type of explanation only goes so far: it doesn’t explain where the dust came from, or more difficult things like why blue light looks blue. If you want complete explanations of anything at all, creation is the way to go. Theology really does have all the answers. Indeed, the myriad religions and creeds on the planet offer a huge choice of answers, any one of which will keep you happy if what you really want is a reason to stop asking why the sky is blue. Attributing it to a deity is just a roundabout way of saying ‘it just is’.

Asimov pointed out that when churches adopted lightning-conductors, they promoted science above theology. Following that way of thinking, we are trying to present scientific – or at least rational – explanations for origins, indeed for many other issues. Ponder Stibbons is the most rational of the wizards, yet even he is fighting an uphill battle. On the whole, though, he’s winning, explaining Roundworld without magic, even though magic – the mechanism behind most Discworld phenomena – is his default viewpoint.

Many, perhaps most, human beings are not rational in their beliefs. Essentially they believe in magic, the supernatural. They are rational in many other respects, but they allow what they want the world to be like to cloud their judgement about what it is like. In the run-up to the American Presidential elections in 2012, several Republican candidates who had previously accepted basic science ended up denying it. A prominent Republican supporter opposed any kind of regulation of the markets on the grounds that this was ‘interfering with God’s plan for the American economy’. More extreme figures on the political right oppose taking steps to mitigate climate change – not because they think it doesn’t exist, but because the quicker we wreck the planet, the sooner Christ’s second coming will happen. Armageddon? Bring it on!

One reason for trying rational approaches first is that most phenomena here on Roundworld have turned out not to be magical. More strongly, many that used to seem magical now make a lot of sense without any appeal to the supernatural: thunder, for instance – though not the American economy, which baffles even economists. So in this book, our explanations of origins will, so far as we can manage, stick to the rational, however complicated it is. But we do wonder whether Christian Scientists, who believe it to be sinful to transplant organs, or even to transfuse blood – because they have been taught that this defies God’s wishes – use lightning-conductors.

Even today, we understand less about thunderstorms than you might imagine.

Two decades ago, astronauts on the space shuttle Atlantis placed a satellite in orbit, the Compton Gamma Ray Observatory. Gamma rays are electromagnetic waves, like light, but of much higher frequency. Since the energy of a photon is proportional to its frequency, that makes them very energetic. CGRO was designed to detect gamma rays from distant neutron stars and remnants of supernovas, and it seemed clear that something was horribly wrong, because the observatory was reporting long bursts of gamma rays, emanating from … the Earth.

This was ridiculous. Gamma rays are produced when electrons and other particles are accelerated in a vacuum. Not in an atmosphere. So something was obviously going wrong with CGRO. Except – it wasn’t. The observatory was functioning perfectly. Somehow, the Earth’s atmosphere was generating gamma rays.

At first, these rays were thought to be generated about 80 kilometres up, well above the clouds. It had just been discovered that strange glowing lights, known as sprites and resembling huge jellyfish, existed at that height. They are thought to be an unexpected effect of lightning in thunderclouds below. At any rate, it seemed clear that sprites must be producing the gamma rays,