Is this guy serious? Yes! David Deutsch wants to understand the world and he believes that the only way to do that is to take our best theories seriously. He believes these best theories to be quantum mechanics in physics, specifically the Hugh Everett many worlds interpretation, the Darwinian theory of evolution, the theory of computation as pioneered by Turing, Church, Godel and others, and last of all the epistemological theories of Karl Popper. These are his four strands of the "fabric of reality". They tell us how we know and what we know about reality.
It is easy not to take theories seriously. In the Introduction to Copernicus’ "On the Revolutions of the Heavenly Spheres" it is written: "...for it is not necessary that these hypotheses should be true, or even probable; but it is enough if they provide a calculus which fits the observations." The Introduction was probably written by Osiander, a Lutheran theologian, and not by Copernicus. All branches of the church wanted to maintain their grip on reality.
So, today it is fashionable among scientists to say that quantum mechanics is a superb tool for calculating what happens. But it does not tell us what reality actually is. For Deutsch, the single-photon-at-a-time interference experiments demonstrate that reality cannot be explained in terms of the single photon that we see. There must be other photons that interfere with it and these photons are in the parallel universes which we don’t see. Or rather, we only see them by the weak interference effects. "[The quantum theory or parallel universes] is the explanation - the only one that is tenable - of a remarkable and counter-intuitive reality." If the multiverse actually exists, it shockingly changes our view of existence. If many people have not yet come to terms with evolution, they are going to have an even more difficult time coming to terms with this!
Deutsch’s principal interest is quantum computing. In his view, parallel universes are the only sensible way to understand and explain quantum computers. Shor’s algorithm will factor large numbers on a quantum computer in a reasonable time. Factoring the same number on a regular computer would be totally prohibitive in the resources required. But factoring a 250 digit number with Shor’s algorithm requires 10^500 parallel universes (in the parallel universe view). So Deutsch throws out the challenge: "Explain how Shor’s algorithm works." When 10^500 or so times the actual resources seen to be present were necessary to factor the number, where did those resources come from? This argument will be even more convincing when a quantum factorization engine actually does factor a 250 digit number.
In epistemology Deutsch is a Popperian. Our knowledge advances by solving problems using the method of critical discussion. When we were in high school we were taught the inductive method of science. Make your observations. Line them up and "induce" a theory that explains them. Confirm the theory by a few more observations. Presto! You have a scientific theory! It never made any sense to me. I could never figure out how one was to "induce" a theory. Nor could I figure out what I was supposed to observe, all by my lonesome self. So we were quite happy for this introduction to Karl Popper’s thought. Needless to say we rushed right out to buy "The Myth of the Framework", a collection of Popper’s essays.
Deutsch’s discussion of computation and virtual reality is more difficult to follow. Part of his theory of knowledge is that we can understand the universe because it possesses a "self-similarity". Part of the universe can render a much larger part in virtual reality. He writes: "The fact that virtual reality is possible is an important fact about the fabric of reality. It is the basis not only of computation, but of human imagination and external experience, science and mathematics, art and fiction." "A single, buildable physical object can mimic all the behaviours of any other physically possible object or process. This is what makes reality comprehensible." This is Deutsch’s version of the Turing Principle. We ourselves are excellent examples of virtual reality generators.
Deutsch envisions knowledge as a physical structure that extends over a swath of universes. Think of a DNA sequence in a gene and the same sequence in a piece of junk DNA in the same organism. The piece that is in the gene will be the same in a large number of parallel universes. The same segment in the junk DNA will vary greatly over the same swath of parallel universes. The knowledge-bearing material would be like a crystal in the multiverse. The other material would be random.
His view of mathematics is just as startling. Abstract mathematical entities actually exist as a part of reality. Why? Because they "kick back"; they go their own autonomous way irrespective of our wishes. But our proofs about mathematical objects are never completely certain. We can only carry out the proofs by physical means and the proofs are no more certain than our knowledge of physical law is certain.
Everybody loves the mystery of time and Deutsch is as intrepid here as with his other topics. He calls time the first quantum concept. Our common idea that time flows is not only wrong but doesn’t make any logical sense at all. The multiverse comes to the rescue again. At the most fundamental level, "Other times are just special cases of other universes." According to Deutsch it is the multiverse that makes many of our traditional views involving time possible. For example, the future is actually open because various outcomes actually occur in various parallel universes. The multiverse also neatly resolves the various paradoxes of time travel into the past. There is no paradox because the travel is to a different parallel universe.
Deutsch believes that our four principal theories are readily used by scientists, but not taken seriously as a description of reality. And they are seldom twined together to give the fabric of reality. Only by taking the theories seriously will they be subjected to the critical discussion that they deserve and the path will be open to move ahead.
David Deutsch deserves praise for his contributions to the ideas of physics and science. On his web site he claims to be a supporter of children's rights. At the same time he appears to be a supporter of forces that are today turning children into victims. I speak of Palestinian teenagers who are used as human shields in Israeli military operations against a civilian population. I speak of the more than 20% of Palestinian children who, according to a UN report, are suffering from acute malnutrition. I think of the Palestinian teenager from Jenin who said on television: "I have never seen, in my whole life, a single good day." I speak of the 2000 excess deaths per week of children in Iraq as a result of our economic blockade. These children seem to be in some parallel universe outside the ken of human compassion.
This review may have too much simplified David Deutsch’s thought. I may even have made a complete hash of it! My only consolation is that in some universe I am sure to have done a better job!
Reviewed by David Park
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