Roger Penrose is hardly the sort of man who would normally excite much popular interest, let alone controversy. The shy, somewhat rumpled and unfailingly polite Oxford professor, 58, has spent most of his career spinning theories in the most abstruse areas of mathematics and physics. His contributions to both disciplines have earned him a sterling reputation among his colleagues. But his pursuits have been so far removed from the everyday world that few people outside his fields of expertise were even aware of his existence.
Until last fall, that is, when Penrose brought out the book The Emperor's New Mind, an in-depth discussion of the relationship between artificial intelligence, consciousness and the laws of physics. Despite its complexity and intellectual rigor, the book quickly jumped onto the best-seller list. And just like his friend and sometime collaborator Stephen Hawking, the once obscure Penrose suddenly found himself showered with publicity. The professor was so unaware of how much the book was earning that he asked his editor whether there was enough to cover a few thousand pounds for a new car.
In the ensuing months, he has also found himself under attack. Reason: Penrose's central conclusion is that computers will never think because the laws of nature do not allow it. That angers many artificial-intelligence researchers. M.I.T.'s Marvin Minsky, one of the field's pioneers, is downright hostile. Says he: "Penrose is O.K. when he talks about mathematics, but most of his evidence argues against his conclusions. As far as I can tell, he is just plain wrong." Stanford psychologist and AI researcher David Rumelhart is somewhat milder: "He defines intelligence too narrowly by saying it depends on consciousness."
Nonetheless, Penrose's reasoning is powerful, and he delves extensively into such heavy topics as fractal geometry, number theory, quantum physics, entropy and cosmology to give readers the necessary background to understand his ideas. "I have to admit," he says, "that I had been looking for an excuse to write about many aspects of physics and mathematics anyway, and this gave me one."
Penrose's first major point is that the human mind can reach insights that are forever inaccessible to computers. The reason is that all digital computers operate according to algorithms, or sets of rules that prescribe how to solve problems. Yet there are problems that cannot be approached by any system of rules, a fact shown in the 1930s by the mathematician Kurt Godel. Godel's theorem establishes that in any mathematical system there must be certain propositions that are obviously true but that can never be proved within the rules of the system.
Mathematician Alan Turing made a related discovery in the 1950s when he used his Turing machine -- an imaginary, simple computer -- to prove that there are some mathematical problems that are solvable but that cannot be solved even in principle by a digital computer. Says Penrose: "The very fact that the mind leads us to truths that are not computable convinces me that a computer can never duplicate the mind" -- this despite the fact that the human brain is often described as a particularly complex computer.