Science of Discworld III

whenever a choice has to be made, the universe splits into a series of ‘parallel worlds’ in which each of the choices happens. Yes, in this world you got up, had cornflakes for breakfast, and walked to work. But somewhere ‘out there’ in the vastness of the multiverse, there is another universe in which you had kippers for breakfast, which made you leave the house a minute later, so that when you walked across the road you had an argument with a bus, and lost, fatally.
    What’s wrong here is not, strangely enough, the contention that this world is ‘really’ a sum of many others. Perhaps it is, on a quantum level of description. Why not? But it is wrong to describe those alternative worlds in human terms, as scenarios where everything follows a narrative that makes sense to the human mind. As worlds where ‘bus’ or ‘kipper’ have any meaning at all. And it is even less justifiable to pretend that every single one of those parallel worlds is a minor variation on this one, in which some human-level choice happens differently.
    If those parallel worlds exist at all, they are described by changing various components of a quantum wave function whose complexity is beyond human comprehension. The results need not resemble humanly comprehensible scenarios. Just as the sound of a clarinet can be decomposed into pure tones, but most combinations of those tones do not correspond to any clarinet.
    The natural components of the human world are buses andkippers. The natural components of the quantum wave function of the world are not the quantum wave functions of buses and kippers. They are altogether different, and they carve up reality in a different way. They flip electron spins, rotate polarisations, shift quantum phases.
    They do not turn cornflakes into kippers.
    It’s like taking a story and making random changes to the letters, shifting words around, probably changing the instructions that the printer uses to make the letters, so that they correspond to no alphabet known to humanity. Instead of starting with the Ankh-Morpork national anthem and getting the Hedgehog Song, you just get a meaningless jumble. Which is perhaps as well.
    According to Max Tegmark, writing in the May 2003 issue of Scientific American , physicists currently recognise four distinct levels of parallel universes. At the first level, some distant region of the universe replicates, almost exactly, what is going on in our own region. The second level involves more or less isolated ‘bubbles’, baby universes, in which various attributes of the physical laws, such as the speed of light, are different, though the basic laws are the same. The third level is Everett’s many-worlds quantum parallelism. The fourth includes universes with radically different physical laws – not mere variations on the theme of our own universe, but totally distinct systems described by every conceivable mathematical structure.
    Tegmark makes a heroic attempt to convince us that all of these levels really do exist – that they make testable predictions, are scientifically falsifiable if wrong, and so on. He even manages to reinterpret Occam’s razor, the philosophical principle that explanations should be kept as simple as possible, to support his view.
    All of this, speculative as it may seem, is good frontier cosmology and physics. It’s exactly the kind of theorising that a Science of Discworld book ought to discuss: imaginative, mind-boggling,cutting-edge. We’ve come to the reluctant conclusion, though, that the arguments have serious flaws. This is a pity, because the concept of parallel worlds is dripping with enough narrativium to make any SF author out-salivate Pavlov’s dogs.
    We’ll summarise Tegmark’s main points, describe some of the evidence that he cites in their favour, offer a few criticisms, and leave you to form your own opinions.
    Level 1 parallel worlds arise if – because – space is infinite. Not so far back we told you it is finite, because the Big Bang happened a finite time ago so it’s not had time to expand to an infinite extent. 7 Apparently, though, data on the cosmic microwave background do not support a finite universe. Even though a very large finite one would generate the same data.
    ‘Is there a copy of you reading this article?’ Tegmark asks. Assuming the universe is infinite, he tells us that ‘even the most unlikely events must take place somewhere’. A copy of you is likelier than many, so it must