The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,73
technology. The interesting items are those that differ from the Ohm’s law picture: the witches, the spacecraft.
Ohm’s law is intimately involved with stories about electricity distribution. By describing these problems, and their solutions, we can show how leaving the law to ‘work its will’, but changing the context, can completely alter the situation. From there we can go from Feynman’s position – that laws determine the context as well as the content of natural events – to a more progressive view.
Electricity distribution to households is made difficult by the resistance of the cables, which causes a lot of electrical energy to be dissipated from the transmission lines as heat. Ohm’s law implies that the same amount of power can be transmitted, with lower losses, by making the voltage higher and the current lower. However, this would supply homes with very high-voltage electricity, and accidents would be fatal.
The trick is to use alternating current, back and forth fifty or sixty times a second. Transformers can change the voltage of alternating current, so it can be high for transmission and then reduced to not-very-lethal values when it gets to our homes. Today we could stick to direct current, using modern electronics to change the voltage, but that option wasn’t available when the distribution system was being created. We’ve now invested so much in alternating current systems that we can’t easily change them, even if that turned out to be a good idea. This trick dodges the Ohm’s law problem of resistance, hence energy loss. Even now, more than a third of the energy can be lost in long transmission lines, but that’s still far more efficient than the 70% loss delivered by the low-voltage direct current systems of the 1920s. By changing the parameters, by going to low-current high-voltage alternating current, we can to some extent change the rules.
Too many physicists seem to have a mind-set that considers physics to be all of reality, simply because it is concerned with all the basic structure of matter. In The Character of Physical Law, Feynman says:
The same kinds of atoms appear to be in living creatures as in non-living creatures (sic); frogs are made of the same ‘goup’ as rocks, only in different arrangements. So that makes our problems simpler; we have nothing but atoms, all the same, everywhere.
In the same book, he says:
Probably the most powerful single assumption that contributes most to the progress of biology is the assumption that everything animals do the atoms can do, that the things that are seen in the biological world are the results of the behaviour of physical and chemical phenomena, with no ‘extra something’.
Like Feynman, we don’t think that there’s an ‘extra something’, an élan vital (‘life force’) that drives life. No, it’s much simpler than that. Organisms have evolved, and whereas at the outset of life they were very limited, mostly ‘doing what their atoms did’, as Feynman would have it, they acquired new properties, like cell division. They got a workable heredity, they acquired eyes and the nervous systems to use them. They moved up out of physico-chemical systems, just as we’ve moved up out of the law of gravity. Organisms have new tricks, exploiting new contexts. For instance: birds, despite being heavier than air, can fly.
We’re not saying that what birds do is inconsistent with the ‘fundamental’ physical laws for the matter out of which they are made. That would be very close to making Descartes’s error, postulating that mind and matter are two different things. In fact, flight is entirely consistent with physical law. The force of gravity, acting on the atoms that make up the bird, must be counteracted by the lifting forces generated by the wings as they move through the air. If not, the bird won’t fly. Ditto jumbo jets. Our point is that flight is not something that you can naturally deduce from the fundamental laws. Molecules can’t fly, but birds – made from molecules – can. A molecule can fly if it’s included in a bird. Context makes a big difference. Life has acquired a multitudinous list of complex systems, each resulting from natural selection, to lift organisms out of ancient incompetences into new competences.
The goup in a frog isn’t a bit like the goup in a rock. The atoms may be much the same, but the different arrangements, to use Feynman’s words, completely change how you expect frog goup to behave. Similarly, the atoms in a person or a penguin or a packet of soap