Science of Discworld III

cosmology, what we normally think of as theuniverse is finite. It started as a single point in the Big Bang, and then expanded at a finite rate for about 13 billion years, so it has to be finite. Of course, it could be infinitely finely divisible, with no lower limit to the sizes of things, just like the mathematician’s line or plane – but quantum-mechanically speaking there is a definite graininess down at the Planck length, so the universe has a very large but finite number of possible quantum states.
    The ‘many worlds’ version of quantum theory was invented by the physicist Hugh Everett as a way to link the quantum view of the world to our everyday ‘sensible’ view. It contends that whenever a choice can be made – for example, whether an electron spin is up or down, or a cat is alive or dead – the universe does not simply make a choice and abandon all the alternatives. That’s what it looks like to us, but really the universe makes all possible choices. Innumerable ‘alternative’ or ‘parallel’ worlds branch off from the one that we perceive. In those worlds, things happen that did not happen here. In one of them, Adolf Hitler won the Second World War. In another, you ate one extra olive at dinner last night.
    Narratively speaking, the many worlds description of the quantum realm is a delight. No author in search of impressive scientific gobbledegook that can justify hurling characters into alternative storylines – we plead guilty – can possibly resist.
    The trouble is that, as science, the many-worlds interpretation is rather overrated. Certainly, the usual way that it is described is misleading. In fact, rather too much of the physics of multiple universes is usually explained in a misleading way. This is a pity, because it trivialises a profound and beautiful set of ideas. The suggestion that there exists a real universe, somehow adjacent to ours, in which Hitler defeated the Allies, is a big turn-off for a lot of people. It sounds too absurd even to be worth considering. ‘If that’s what modern physics is about, I’d prefer my tax dollars to go towards something useful, like reflexology.’
    The science of ‘the’ multiverse – there are numerous alternatives, which is only appropriate – is fascinating. Some of it is even useful.And some – not necessarily the useful bit – might even be true. Though not, we will try to convince you, the bit about Hitler.
    It all started with the discovery that quantum behaviour can be represented mathematically as a Big Sum. What actually happens is the sum of all of the things that might have happened. Richard Feynman explained this with his usual extreme clarity in his book QED ( Q uantum E lectro D ynamics, not Euclid). Imagine a photon, a particle of light, bouncing off a mirror. You can work out the path that the photon follows by ‘adding up’ all possible paths that it might have taken. What you really add is the levels of brightness, the light intensities, not the paths. A path is a concentrated strip of brightness, and here that strip hits the mirror and bounces back at the same angle.
    This ‘sum-over-histories’ technique is a direct mathematical consequence of the rules of quantum mechanics, and there’s nothing objectionable or even terribly surprising about it. It works because all of the ‘wrong’ paths interfere with each other, and between them they contribute virtually nothing to the overall sum. All that survives, as the totals come in, is the ‘right’ path. You can take this unobjectionable mathematical fact and dress it up with a physical interpretation. Namely: light really takes all possible paths, but what we observe is the sum, so we just see the one path in which the light ‘ray’ hits the mirror and bounces off again at the same angle.
    That interpretation is also not terribly objectionable, philosophically speaking, but it verges into territory that is. Physicists have a habit of taking mathematical descriptions literally – not just the conclusions, but the steps employed to get them. They call this ‘thinking physically’, but actually it’s the reverse: it amounts to projecting mathematical features on to the real world – ‘reifying’ abstractions, endowing them with reality.
    We’re not saying it doesn’t work – often it does. But reificationtends to make physicists bad philosophers, because they forget they’re doing it.
    One problem with ‘thinking physically’ is that there are