WHEN we consider some of the most striking phenomena permitted by the laws of physics – from human reasoning to computer technologies and the replication of genes – we find that information plays a central role. But, on the face of it, information is profoundly different from the basic entities that physical sciences use to describe reality. Neither quantum mechanics nor general relativity, the most fundamental theories in physics, provide a meaning for information or even a way of measuring it. And it has a "counterfactual" character: a message cannot carry information unless a different message is also possible.
Statements about information were therefore long regarded in physics as second-class, non-fundamental approximations. Information itself was considered an a priori abstraction, like Euclid's perfect triangles and circles, whose physical instantiations are inevitably approximate.
Yet there have long been clues that information is a fundamental physical quantity, obeying exact laws. Consider ...
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