David Deutsch (born 1953) is a physicist at Oxford University. He pioneered the field of quantum computers, and is a proponent of the many-worlds interpretation of quantum mechanics.
In his book The Fabric of Reality this interpretation, or what he calls the multiverse hypothesis, is one strand of a four-strand theory of everything, the other strands being Karl Popper's epistemology and philosophy of science, Richard Dawkins's refinement of Darwinian evolutionary theory, and Alan Turing's theory of computation especially as developed in Deutsch's idea of a universal quantum computer. His theory of everything is emergentist rather than reductive. It aims not at the reduction of everything to particle physics, but rather mutual support among multiverse, computational, epistemological, and evolutionary principles.
This emergentist posture allows computing theory's Turing principle to do serious work in Deutsch's world-view. In the strong form he favors it implies that a universal quantum computer, capable of rendering any physically possible environment, actually exists near the end of spacetime in every universe and is maintained by sentient beings with the knowledge required to increase its memory, computing cycles, and energy supply. In this he follows Frank Tipler in The Physics of Immortality, though he emphasizes the scientific component of Tipler's Omega Point hypothesis, the component that is justified by Popperian epistemology as implied by our best science. He is much less sympathetic to the non-scientific component, which provides rational reconstructions for traditional theological categories such as God, omniscience, omnipresence, benevolence, creation, and so on.
The Turing principle is also sometimes called the Deutsch principle by those who question whether Turing's work on the foundations of computing was aiming to disclose what could be computed tractably "in nature". There are tractability issues when, for instance, factoring and decryption problems are attacked with Turing-machine or classical-computation methods, problems that seem to be resolved by quantum-computing techniques such as Shor's algorithm. Turing universality isn't universality enough, Deutsch thinks. Turing's abstract computer needs to be replaced by the actual, physical, universal quantum computer derived from the Turing/Deutsch principle.
That principle is also sometimes called the Matrix principle, because Deutsch's conception of virtual reality figures in its statement: "It is possible to build a virtual reality generator whose repertoire includes every physically possible environment." Some cognitive psychologists think that Deutsch's view of the brain as a virtual-reality generating computer, adequate to rendering a humanly experienced environment, affords a sufficiently robust account of subjective experience or qualia, one consistent with a view of the mind/brain as a computer, to break down the impasse between qualia-phobes and qualia-freaks. A virtual-reality generator consists of an image generator to provide the subject with perceptual content from the several sensory modalities, perhaps in the forms of transducers connected directly to afferent nerves by use of neural implants, and a program to handle interaction between the subject's choices and the virtual environment. Nearer to the omega point this transhuman enhanced-biology scenario gives way to a posthuman condition, because biology becomes untenable. Gravitational shearing and other extreme forces call for more durable substrates for human psychology. The brain is replaced by sturdy computational equivalents in virtual realities, protected from the Big Crunch and pushed in the final moments by unlimited computational cycles affording their posthuman residents the subjective experience of immortality.
A quantum computer farms out computing problems to other universes in order to achieve tractability for solutions that otherwise get bogged down by exponentially increasing demands for more time and other computational resources. The apparent need on a realist conception of science to posit such collaboration inspires a pugnacious comment from Deutsch: "To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor's algorithm works." The challenge is meant to imply that a Turing machine is incapable in principle of doing what a quantum computer can do, since the latter's operations in executing Shor's algorithm require computational resources from other worlds. And generally, a quantum computer's operations include computational steps in other worlds that are not present in any Turing-machine's tape (in this world). Deutsch thinks this has implications for proof theory, which must abandon the model of an inspectable list of premises leading to a conclusion, in favor of a model of a process in which the relationship between premises and conclusion may be mediated by computations that are not inspectable (in this world).
Another important theme in the book is that basic ideas about the universe are either vindicated or undermined by the multiverse hypothesis. For instance, counterfactual conditionals refer to nearby parallel worlds when they stipulate what a thing would do under conditions that do not actually obtain; one-worlders implicitly collapse what things can do into what they actually do. (He acknowledges a kindred spirit in the philosopher David Lewis, whose modal realism handles counterfactuals in a similar fashion. He takes Lewis to have "postulated the existence of a multiverse for philosophical reasons alone." This is a contentious claim, since Lewis's realism about possible worlds extends to worlds that are not physically possible, such as the world where Harry Potter was schooled at Hogwarts, whereas Deutsch's multiverse includes all and only physically possible worlds. Also Lewis's possible worlds are disjoint, whereas Deutsch's parallel worlds interact through interference. On the other hand, Lewis recognizes overlapping worlds as a theoretical possibility, and Deutsch's universal quantum computer can render Harry Potter worlds to any desired degree of accuracy.)
Knowledge is a trans-universe structure, as one might expect because knowledge supports counterfactual implications, as revealed for instance in Robert Nozick's tracking account of knowledge. Nearby parallel worlds are united by a common history of knowledge acquisition, spelled out in broadly Popperian terms. The resulting epistemological niche lends stability and reliability to knowledge in each universe. Life is a similar trans-universe structure, molded by natural selection rather than rational criticism. What distinguishes genuine replicating DNA from junk DNA is that the former but not the latter is representative of a niche of replicators that extends across worlds. Indeed personal identity is inseparable from such a niche, which Deutsch picks out with the word "copies". A person is a set of copies in nearby parallel worlds. This comes out in his analysis of free will: I could have chosen otherwise is analysed as Other copies of me chose otherwise. And in the denouement to a dramatic chapter that rehearses interference experiments from a multiverse viewpoint, he writes of his copies, "Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."
Not only are persons spread out through worlds, but they, like everything else, are quantized through time in any given world. Time is a series of moments, and a person who exists at a moment exists there forever in four-dimensional spacetime, rather than being transformed continuously through the flow of time. Such change and flow are mythical, Deutsch argues. Since "other times are just special cases of other universes", this temporal granularity of personhood is a special case of being spread out through worlds. In addition to one's identically time-stamped copies at a moment across parallel worlds transversely, there are the differently time-stamped copies across parallel worlds longitudinally, linked by natural law so as to give the individual's experience of one world and a continuous self.
An intellectual descendant of David Hume via the paternity of Popper, Deutsch is not only a critic of induction but also a Humean about causation, to the degree that he rejects the idea of a causal power effecting a change, in favor of construing it as a multiverse regularity. So A causes B means something like After A-copies occur in many nearby parallel worlds, including the one in this world, B-copies occur. This regularity supports counterfactuals that accompany true causal claims, such as If A hadn't happened, B would not have taken place. There are affinities to Hume's constant-conjunction understanding of causation and Popper's deductive-nomological account.
Politically, Deutsch is known to be sympathetic to Libertarianism, and was a founder of the Taking Children Seriously movement. He is also an atheist.
He was awarded the Dirac Prize of the Institute of Physics in 1998, and the Edge of Computation Science Prize in 2005.
Popular publications
- The Fabric of Reality, ISBN 0140146903
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