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24 maio 2016

Claude Shannon, o pai da Teoria da Informação

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As is sometimes the case with encyclopedias, the crisply worded entry didn’t quite do justice to its subject’s legacy. That humdrum phrase—“channel capacity”—refers to the maximum rate at which data can travel through a given medium without losing integrity. The Shannon limit, as it came to be known, is different for telephone wires than for fibre-optic cables, and, like absolute zero or the speed of light, it is devilishly hard to reach in the real world. But providing a means to compute this limit was perhaps the lesser of Shannon’s great breakthroughs. First and foremost, he introduced the notion that information could be quantified at all. In “A Mathematical Theory of Communication,” his legendary paper from 1948, Shannon proposed that data should be measured in bits—discrete values of zero or one. (He gave credit for the word’s invention to his colleague John Tukey, at what was then Bell Telephone Laboratories, who coined it as a contraction of the phrase “binary digit.”)

“It would be cheesy to compare him to Einstein,” James Gleick, the author of “The Information,” told me, before submitting to temptation. “Einstein looms large, and rightly so. But we’re not living in the relativity age, we’re living in the information age. It’s Shannon whose fingerprints are on every electronic device we own, every computer screen we gaze into, every means of digital communication. He’s one of these people who so transform the world that, after the transformation, the old world is forgotten.” That old world, Gleick said, treated information as “vague and unimportant,” as something to be relegated to “an information desk at the library.” The new world, Shannon’s world, exalted information; information was everywhere. “He created a whole field from scratch, from the brow of Zeus,” David Forney, an electrical engineer and adjunct professor at M.I.T., said. Almost immediately, the bit became a sensation: scientists tried to measure birdsong with bits, and human speech, and nerve impulses. (In 1956, Shannon wrote a disapproving editorial about this phenomenon, called “The Bandwagon.”)

Although Shannon worked largely with analog technology, he also has some claim as the father of the digital age, whose ancestral ideas date back not only to his 1948 paper but also to his master’s thesis, published a decade earlier. The thesis melded George Boole’s nineteenth-century Boolean algebra (based on the variables true and false, denoted by the binary one and zero) with the relays and switches of electronic circuitry. The computer scientist and sometime historian Herman Goldstine hyperbolically deemed it “one of the most important master’s theses ever written,” arguing that “it changed circuit design from an art to a science.” Neil Sloane, a retired Bell Labs mathematician as well as the co-editor of Shannon’s collected papers and the founder of the On-Line Encyclopedia of Integer Sequences, agreed. “Of course, Shannon’s main work was in communication theory, without which we would still be waiting for telegrams,” Sloane said. But circuit design, he added, seemed to be Shannon’s great love. “He loved little machines. He loved the tinkering.”


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