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Prof. David Deutsch

Prof. David Deutsch


launch 027.jpgThe Fabric of Reality

Transcript of a talk given on 17th February 2008 by Professor David Deutsch of the Centre for Quantum Computation, University of Oxford on the occasion of the opening of the Oxford University Chabad Society’s Samson Judaica Library.

‘The fabric of reality’ is a world view and my contention is that it should be the basic world view that is implied by our best ideas at present. Not that it’s not going to be improved upon, but it’s my opinion that our best ideas currently aren’t taken seriously enough, and The Fabric of Reality, the title of my first book, is my name for what you get when you do take them seriously.

It starts with the fact that we are human beings and therefore we are fallible: we make mistakes.

The first thing I ever published contained quite a big mistake. It was a review of a book called Advice to a Young Scientist by the biologist Peter Medawar, which I wrote for the Wolfson College magazine when I was a graduate student. They asked me to write it because I was a young scientist, so that I could give a sort of ‘reply’ to the book – and I think that may have gone to my head. Also, Medawar was a supporter of the philosophy of Karl Popper, and so was I. So, my review was generally favourable. But it ended with a snide remark. Medawar had said in the book, ‘I cannot give any … better advice than this: the intensity of your conviction that a hypothesis is true has no bearing on whether it is true or not.’ Well, of course! The history of science is littered with false ideas on which scientists would at one time have staked their lives. But saying that that’s the most important thing you could tell a young scientist seemed to me absurdly out of proportion. So I wrote that, and I accused him of being condescending to the eponymous ‘young scientist’. But as I’ll explain, I have realised that he was right.

While I was still at school, I had been deeply impressed by a popular book on cosmology by the physicist Dennis Sciama. Later I had the very good fortune to become a graduate student of his here at Oxford. His book was called The Unity of the Universe. In the course of giving a nice overview of the state of cosmology at the time, it made a powerful case that the laws of nature are neither arbitrary nor separable from each other but are part of – almost, a single idea.

I think the main reason I was impressed, is that by saying this, Sciama was contradicting something I’d always been told, and didn’t like – namely that, as human knowledge grows, explosively, it grows increasingly out of reach of any one human mind. And therefore there can never again be such thing as understanding the world, to the limits of human knowledge. One can only understand tiny facets of what is known. This is therefore deemed to be the age of specialisation – and that will always get worse from now on. But Sciama said: we can understand the world, not by memorising a heap of facts but by explaining it, through theory. That’s one reason I liked science: a single, comprehensible theory can cover an infinity of facts. As knowledge increases we find that the world is not a heap of facts but, on the contrary: great discoveries are often unifications, revealing simplicity where previously we had seen only complexity and arbitrariness.

By the time I came to Oxford, Sciama had actually abandoned most of the specific cosmological theories in that book. Many of them, for instance, had been about the Steady State theory, which had been superseded; there’d been a period of fundamental discovery in cosmology, as there is at the moment by the way. Wherever the theories in his book had been refuted by experiment, he was already enthusiastically investigating their successors. But his idea of the unity of the universe had not been refuted, but spectacularly vindicated by those discoveries and others. Soon one couldn’t do cosmology (the science of the biggest things we know of) without studying quantum physics (the science of the smallest). Apparently unrelated fields were being connected. And often, each field helped to understand others more simply than before.

That doesn’t necessarily mean more easily – because deep, simple theories are usually also counter-intuitive: meaning that they conflict with what Medawar warned against: the ‘intensity of our conviction’. They’re counter-intuitive just because the deeper a theory is, the further it is from everyday experience, so the more opportunity there is for our misconceptions about the world to seem self-evident truths.

My own work, developing the quantum theory of computation, was another unification. On the one hand quantum theory – one of the deepest theories in physics, which provides the mathematical framework and language within which the rest of physics is formulated. And on the other, the theory of computation – which had previously been self-evidently a branch of mathematics, not science at all. Once, I was giving a lecture, explaining that in my opinion this unification implies that there is nothing a priori, logically, special about the class of operations that we call computations – including mathematical proofs – and therefore, I said, the persuasive power of mathematical proofs comes only from our knowledge of physics. An eminent mathematician in the audience stood up and said that was the most foolish statement he had ever heard: knowledge of mathematics is by definition entirely independent of scientific knowledge. (Now that’s a mistake: mathematical truth is indeed independent of all empirical facts, but mathematical knowledge is derived by brains which are physical objects, and their operation obeys the laws of physics which are for these purposes ultimately expressed in the quantum theory of computation.) He wasn’t pleased at that unification. It conflicted with the intensity of his conviction on which he thought, mistakenly, his knowledge of mathematics rested.

Today, physicists routinely write, on the same sheet of paper, equations of quantum physics and computational algorithms. Two strands of knowledge that used to be studied in separate buildings are now seen as inseparable and referring to the same underlying reality.

I also kept noticing that those two strands were linked with two other fundamental strands of knowledge: one was the theory of evolution. And the other was Popper’s theory of the growth of knowledge itself (there’s something nicely self-referential there). So there are four strands of our most fundamental knowledge of the natural world. Taken together, they constitute the single world view which became the subject of my book The Fabric of Reality.

The four strands also have something more mundane in common: in the way they are received and the way they are used. For instance: one strand, quantum theory, which is the one that I am basically ‘coming from’, implies that the universe we see around us – this room, each other, stars, galaxies – is not the only universe that exists: there are countless others, and in microscopic ways, they affect each other, though they are largely independent. Yet if you ask 100 theoretical physicists whether quantum theory implies that, at least 90 will say no, some of them quite angrily. They’re not denying the equations. They’re denying that they imply this about reality – or indeed that they imply anything about reality. And much the same holds for the other three strands. All four of our best theories in those areas of human knowledge are counter-intuitive – but very successful pragmatically, so they can’t be straightforwardly dismissed. So people are tempted to use them only pragmatically, for prediction and analysis, and to refuse to think about the reality that brings those predictions about. Which leads to a loss of confidence in the existence of objective knowledge (knowledge that corresponds to reality). This sort of denial about reality, knowledge and truth, blighted philosophy and much of science during the 20th century, and they have by no means recovered.

If we want objective knowledge about the physical world, how can we heed Medawar’s warning, that the intensity of our convictions is no guarantee of truth? What should we do about the fact that the truth is often counter-intuitive, and can be expected to become more so, the more progress we make? Well, first of all, if you take seriously that we can be mistaken, that we are fallible, then that already implies that an objective reality exists: the very concept of error implies that there’s a truth. Then, it comes down to: how can we correct our errors? As the physicist Richard Feynman said, science is what we have learned about how to avoid fooling ourselves. Therefore, if we are to understand the physical world more than superficially despite our limitless fallibility, it must be through scientific theories and through reason and criticism, and not through our feelings, our convictions, nor even through common sense, because common sense is routinely superseded by deep scientific theories. But the good news is, as I say in the book, our best theories, the four strands, are not only truer than common sense, they actually make more sense than common sense, if they are understood in combination. Then, they portray a unified Fabric of Reality that is always still mysterious (because there is always still an infinity left to discover) – mysterious in the sense of illimitable – but also limitlessly comprehensible.

That’s my message.


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