**Quantifying Classical Physical Information**

An amount of (classical) physical information may be quantified, as in information theory, as follows. For a system *S*, defined abstractly in such a way that it has *N* distinguishable states (orthogonal quantum states) that are consistent with its description, the amount of information *I*(*S*) contained in the system's state can be said to be log(*N*). The logarithm is selected for this definition since it has the advantage that this measure of information content is additive when concatenating independent, unrelated subsystems; e.g., if subsystem *A* has *N* distinguishable states (*I*(*A*) = log(*N*) information content) and an independent subsystem *B* has *M* distinguishable states (*I*(*B*) = log(*M*) information content), then the concatenated system has *NM* distinguishable states and an information content *I*(*AB*) = log(*NM*) = log(*N*) + log(*M*) = *I*(*A*) + *I*(*B*). We expect information to be additive from our everyday associations with the meaning of the word, e.g., that two pages of a book can contain twice as much information as one page.

The base of the logarithm used in this definition is arbitrary, since it affects the result by only a multiplicative constant, which determines the unit of information that is implied. If the log is taken base 2, the unit of information is the binary digit or bit (so named by John Tukey); if we use a natural logarithm instead, we might call the resulting unit the "nat." In magnitude, a nat is apparently identical to Boltzmann's constant *k* or the ideal gas constant *R*, although these particular quantities are usually reserved to measure physical information that happens to be entropy, and that are expressed in physical units such as joules per kelvin, or kilocalories per mole-kelvin.

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