The Science of Discworld II

collapse too quickly for an object to pass through it, unless it is held open by threading ‘exotic matter’ with negative energy through it. Nonetheless, none of this is forbidden by the current laws of physics. So what of the paradoxes? It turns out that the laws of physics forbid genuine paradoxes, although they permit many apparent paradoxes. A useful technique for understanding the difference is known as a Feynman diagram, which is a picture of the motion of an object (usually a particle) in space and time.
    For example, here is an apparent time travel paradox. A man is imprisoned in a concrete cell, locked from the outside, with no food, no water and no possibility of escape. As he sits in a corner in despair, waiting for death, the door opens. The person who has opened it is … himself. He has returned in a time machine from the future. But how (the paradox) did he get to the future in the first place? Well, a kind person opened the door and set him free …
    There seems to be something very odd about the causality in this story, but the corresponding Feynman diagram shows that it violates none of the laws of physics. First, the man follows a space-time path that puts him inside the cell and then removes him from it through an opened door. This time-line continues into his future until he encounters a time machine. Then the time-line reverses direction, heading into the past, until he encounters a locked cell. He opens it, and his time-line reverses again, propelling him into his own future. So the man follows a single zig-zag path through time, and at every step the laws of physics hold good. Provided his time machine violates no physical law, of course.
    If you try to ‘explain’ the grandfather paradox by this method, it doesn’t work. The time-line leading from grandfather to killer is severed when the killer returns; there is no consistent scenario, even in a Feynman diagram. So some stories of time travel are consistent with the laws of physics, and have their own kind of causal logic, albeit twisted; but other equally plausible stories are inconsistent with the laws of physics. You can rescue the Grandfather Paradox by assuming that changing the past in a logically inconsistent way switches you into a different alternate universe – say a quantum-mechanical parallel world. But then it wasn’t your grandfather that you killed, but the grandfather of an alternate you. So this ‘resolution’ of the Grandfather Paradox is a cheat.
    Faced with all this, the way that the wizards handle the complications of time travel seems quite reasonable!
    1 The superstition is common in the Black Country, in places like Wombourne and Wednesbury. Though that’s not why it’s called the Black Country. The thing about your Black Country is, it’s black. At least, it was black, with industrial grime and pollution, when it got its name. Some bits no doubt still are.
    2 Schrödinger pointed out that quantum mechanics often gives silly answers like ‘the cat is half alive and half dead’. His intention was to dramatise the gap between a quantum-level description of reality and the world we actually live in, but most physicists missed the point and derived complicated explanations of why cats really are like that. And why the universe needs conscious observers to ensure that it continues to exist. Only recently did they twig what Schrödinger was on about, and come up with the concept of ‘decoherence’, which shows that superpositions of quantum states rapidly change into single states unless they are protected from interaction with the surrounding environment. And the universe doesn’t need us to make it hold together, sorry. See The Science of Discworld , with a cameo appearance of Nanny Ogg’s cat Greebo.
    3 Discworld runs this far more sensibly. Heroes will have adventures.
    4 Recall that Yossarian is a pilot in Joseph Heller’s Catch-22 .
    5 We use this word because it’s standard in science fiction, but UK English would require ‘alternative’.
    6 Named after the physicist Enrico Fermi. See Evolving the Alien by Jack Cohen and Ian Stewart.

TWENTY-NINE
ALL THE GLOBE’S A THEATRE
    T HE ELVES DID NOT SPEND A LOT OF TIME in serious thought. They could control people who could do the thinking for them. They didn’t play music, they did not paint, they never carved stone or wood. Control was the talent, and it was the only