X-Message-Number: 11569 From: Date: Sun, 18 Apr 1999 00:37:36 EDT Subject: Digital vs Analog vs Deutsch In his book THE FABRIC OF REALITY, David Deutsch expresses some strong opinions that seem rather dubious to me. Even though my credentials are far inferior, today I offer the following commentary on one of his assertions of irrefutable evidence for the many-worlds interpretation of quantum mechanics. On p. 217 he says, "The argument of Chapter 2, applied to any interference phenomenon, destroys the classical idea that there is only one universe. Logically, the possibility of complex quantum computations adds nothing to a case that is already unanswerable. But it does add psychological impact….To those who still cling to a single-universe world-view, I issue this challenge: explain how Shorr's algorithm [for factoring large numbers] works……If the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed?" The answer is simple, and Deutsch himself apparently agreed with it on p. 209, where he refers to quantum-mechanical objects as special-purpose computers. The "who, how, where" are the same as in any analog computation. The experimenter or observer did it, in his lab, by devising a correspondence between a physical event and a computation. For those who have forgotten, an analog computation relates numbers to physical observables. As a primitive example, suppose you want to add 2 + 3. One way to do it would be to put 2 ml of water in a graduated cylinder, pour that into a second graduated cylinder, put 3 ml more water into the first cylinder, pour that into the second cylinder, and then read off the water level in the second cylinder. Lo and behold, 5 ml! You have performed an analog computation. There are still many mysteries in water, and many mysteries in glass, but there is no mystery in using glass cylinders and water to perform analog computations. On a slightly higher level, you could calculate a definite integral by measuring the charge accumulation on a capacitor during a certain time interval, or in many other ways. If a quantum computer is just another kind of analog computer, then it does not rely for its explanation on many-worlds or any other particular interpretation of quantum mechanics. The interpretation is simply irrelevant. If quantum effects occur, then devising appropriate correspondences will allow "quantum computation"--no mystery there, as far as I can see. As an aside, we might reflect on the hazy boundary between "analog" and "digital" computation. If the universe is quantum and discrete, then in a sense all events, including all computations, are digital. At the same time, if physical objects are used for computation, then from that point of view all computations are analog. I was going to stop here, but as long as I'm gnawing on Deutsch I may as well mention another defect in his book as it seems to me. He claims that interference effects prove the many-worlds hypothesis, and talks about photons interfering with each other, and discusses some well known experiments. But interference is most easily understood as a wave phenomenon, and nowhere does he so much as mention waves, let alone some way of reconciling the famous wave/particle dualism. One could, of course, ignore the question, and simply accept that particles interfere with each other according to certain mathematical rules--but Deutsch himself insists on explanations, not mere predictions or instrumentalism. Unless I have been totally oblivious, he has provided no explanation for interference. Robert Ettinger Cryonics Institute Immortalist Society http://www.cryonics.org Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=11569