The Information: A History, a Theory, a Flood

by James Gleick

Landauer and Bennett were a double act: a straight and narrow old IBM type and a scruffy hippie (in Bennett’s view, anyway). The younger man pursued Landauer’s principle by analyzing every kind of computer he could imagine, real and abstract, from Turing machines and messenger RNA to “ballistic” computers, carrying signals via something like billiard balls. He confirmed that a great deal of computation can be done with no energy cost at all. In every case, Bennett found, heat dissipation occurs only when information is erased. Erasure is the irreversible logical operation. When the head on a Turing machine erases one square of the tape, or when an electronic computer clears a capacitor, a bit is lost, and then heat must be dissipated. In Szilárd’s thought experiment, the demon does not incur an entropy cost when it observes or chooses a molecule. The payback comes at the moment of clearing the record, when the demon erases one observation to make room for the next. Forgetting takes work. “You might say this is the revenge of information theory on quantum mechanics,” Bennett says. Sometimes a successful idea in one field can impede progress in another. In this case the successful idea was the uncertainty principle, which brought home the central role played by the measurement process itself. One can no longer talk simply about “looking” at a molecule; the observer needs to employ photons, and the photons must be more energetic than the thermal background, and complications ...