Sunday, December 10, 2006

Poisonous Positivism

David Deutsch, professor of physics at the University of Oxford's Centre for Quantum Computation, pioneered the field of quantum information science by describing a universal quantum computer. He is critical of the
toxic effect logical positivism has had on science. Logical Positivism (also called Logical Empiricism) is a philosophical doctrine formulated in Vienna in the 1920s, according to which scientific knowledge is the only kind of factual knowledge and all traditional metaphysical doctrines are to be rejected as meaningless. Deutsch believes that, though it was intended to be a retreat from metaphysics, it was really a retreat from reality and explanation."In physics", he says, "it took the form of deciding as a matter of principle that science is not about discovering how the world really is, but instead must confine itself to predicting the outcomes of observations. When quantum mechanics came along it required a drastic revision of people's conception of the world. Many physicists responded by denying that physics is about the world at all, only about what we see."

"Logical positivism is a form of solipsism", states Deutsch, "If you say physics is only about predicting the outcomes of experiments, you can only really say it's about experiments that you personally do, because to you any other person is just another thing you're observing. But solipsism is a dead-end philosophy and when it comes to science it's a poison. It doesn't allow further progress from existing theories, and that's why I think applications of quantum theory, particularly quantum computation, were overlooked for decades. You could say people didn't really think the theory was true because they had rejected the idea of truth in science. Truth in science must mean correspondence to reality, or it means nothing."

The many-worlds interpretation of quantum mechanics can explain how a quantum computer could perform, in a few seconds or minutes, a computation of such magnitude that a classical computer, even if it consisted of all the matter in the universe and ran as long as the age of the universe, could not even come close to; for example, the factorisation of a 10,000-digit integer, the product of two very large primes. The quantum computer must use some process beyond the perceivable universe to arrive at the answer. "At that point logically, we have already accepted the many-worlds structure. The way the quantum computer works is: the universe differentiates itself into multiple universes and each one performs a different sub-computation. The number of sub-computations is vastly more than the number of atoms in the visible universe. Then they pool their results to get the answer. Anyone who denies the existence of parallel universes has to explain how the factorisation process works."

It used to be the case that many physicists were completely sceptical that quantum computing could ever be a possibility. Now the consensus is that it is possible, but will probably take decades, to build a quantum computer.

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