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Reverse Engineering

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Brian Hayes

Let's Get Physical

Landauer, who died in 1999, championed the slogan "Information is physical." He meant, first of all, that information cannot exist in the abstract but has to be embodied somehow—in electrons, photons, chalk marks, neural excitations. But the slogan can also be taken as a call to heed the laws of physics when dealing with information, just as one must with matter and energy. In this respect irreversible computers are notorious scofflaws. They make bits appear out of nothing and then disappear into the void again when they're no longer needed. They obey no conservation laws.

A reversible computer is a better-behaved device, more at home in the universe we live in. As Toffoli wrote in 1982: "Computation—whether by man or by machine—is a physical activity. If we want to compute more, faster, better, more efficiently, and more intelligently, we will have to learn more about nature. In a sense, nature has been continually computing the 'next state' of the universe for billions of years; all we have to do—and, actually, all we can do—is 'hitch a ride' on this huge ongoing computation, and try to discover which parts of it happen to go near to where we want."

© Brian Hayes

(See Brian Hayes' weblog,, for two related items, "Swapping lies" and "Newton and Notwen". An extended bibliography is also available.)

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