author : David Deutsch

Science in the 20th century has become focussed on the what, with scant regard for the why. Deutsch wants explanation put back into our way of doing science -- science is our way of understanding the world, not just tersely describing what it does. This book is his attempt to describe what An Explanation of Everything might look like (as contrasted with physicists' quest for a theory of everything, by which they mean one single equation to describe fundamental physics). This explanation is structured around four theories central to modern science:

• Hugh Everett's Many Worlds interpretation of quantum theory
• Richard Dawkins' approach to Darwin's Theory of Evolution
• Karl Popper's philosophy of science
• Alan Turing's Theory of Universal Computation

He argues these theories should be 'taken seriously'; that is, scientists should be exploring their (possibly extreme) logical consequences, not just applying them in narrow domains.

Despite the fact that they are not taken seriously, usually because the consequences are disliked, there are no better alternatives. Deutsch argues that this results in some people supporting worse alternatives, leading to a lot of wasted effort.

I like the rigorously rational view Deutsch takes:

He also has little time for those who artificially handicap themselves:

He makes it clear that what we like to think of as purely abstract computation is in fact deeply grounded in physics, with the physics affecting the kinds of computers we can build, and therefore the kinds of models of computation we devise. Analogue classical physics gives us a fundamentally different model of computation.

[This focus on what we mean by proof in 'finite proof' contrasts rather nicely with Lavine's focus on what we mean by finite.] Similarly, the discrete 'classical' physics of Turing machines also gives us an incorrect model of computation.

Quantum computation is fundamentally different from classical computation. Deutsch states quite clearly that there are quantum programs that cannot be run on a classical Turing machine.

This statement confused me when I first read it: I have listened to quantum computing researchers describe their emulations of quantum computations on classical computers, they just require exponentially increasing resources (either processors, or time). These two statements seem incompatible. But then a few pages later I came across a paragraph that affects his meaning of 'in principle' in the quote above.

Now, a universal computer must, by definition, be able to emulate any other computer using only a 'similar' amount of resources (where 'similar' has a technical meaning that excludes 'exponentially more'). But a 'Universal' Turing Machine does need exponentially more resources than a quantum computer in order to emulate one, and so is not truly 'Universal' by this definition.

So Deutsch, being firmly grounded in physical law and the universe we are living in, argues that there are certain computations that cannot be performed classically in a tractable time, because there are insufficient resources in our single universe, but that can be performed quantumly, when the resources of exponentially many parallel universes can be brought to bear. There are computations that require exponential resources on classical computers, and so are intractable, but are are tractable on quantum computers. One such kind of computation is that of calculating the state of a multi-particle quantum system itself.

Deutsch weaves together his four strands, and comes up with some rather interesting, and sometimes startling, conclusions. In particular, his use of the Turing principle to define a universal virtual reality renderer, and to use that to infer consequences for the laws of physics, in particular, for time travel, is quite ingenious. And one has to admire the author who can conclude that his view

a mere 15 pages after describing Tipler's omega-point argument that it is possible to perform an infinite amount of computation in a universe with a particular configuration, inferring that we are in such a universe, and further inferring

This is an excellent book, with some fascinating ideas. It is particularly nice to find a real practicing physicist who is willing to come out and admit that explanation is what it's all about. Most of the book is solid scientific extrapolation, but his description, in the last chapter, of a universe full of beings who must continue to evolve and grow in knowledge for ever, is particularly exhilarating. (I found it made an optimistic contrast to Chaitin's slightly gloomy view of the place of randomness in increasing knowledge.)

I do have one area where I have some confusion, however, and would have liked more explanation. That is of the Many Worlds interpretation itself. It certainly does give an intuitive explanation of how quantum computers work (or rather, where they do all their work), but I am less convinced that it is the inevitable explanation of quantum interference experiments. (Deutsch is a much better physicist than I am, however.) Cramer's Transactional interpretation, in particular, seems to offer equally plausible explanations of these experiments, without being so profligate with universes. I would also have liked a little more detail of what the Many Worlds interpretation is: before reading this I had a vague picture of a universe branching at every decision point; Deutsch talks of reams of pre-existing identical universes subsequently evolving in different ways depending on the choice.

Despite my area of doubt and uncertainly, I highly recommend this book. It is very well written (the dialogue between Deutsch and the crypto-inductivist is particular fun), brings together some important ideas, avoids the excesses of Penrose and Tipler (whilst exploiting their good parts), and gives a view onto a humane and rational explanation of the world.