In the Summer of 1951, Professor G. Uhlenbeck gave his
famous course on Statistical Mechanics at Stanford, and fol-
lowing the lectures I had many conversations with him, over
lunch, about the foundations of the theory and current progress
on it. I had expected, naively, that he would be enthusiastic
about Shannon's work, and as eager as I to exploit these ideas
for Statistical Mechanics. Instead, he seemed to think that
the basic problems were, in principle, solved by the then
recent work of Bogoliubov and van Hove (which seemed to me
filling in details, but not touching at all on the real basic
problems)--and adamantly rejected all suggestions that there
is any connection between entropy and information.
His initial reaction to my remarks was exactly like my
initial reaction to Shannon's: "Whose information?" His
position, which I never succeeded in shaking one iota, was:
"Entropy cannot be a measure of 'amount of ignorance,' because
different people have different amounts of ignorance; entropy
is a definite physical quantity that can be measured in the
laboratory with thermometers and calorimeters." Although the
answer to this was clear in my own mind, I was unable, at the
time, to convey that answer to him. In trying to explain a
new idea I was, like Maxwell, groping for words because the
way of thinking and habits of language then current had to be
broken before I could express a different way of thinking.
Today, it seems trivially easy to answer Professor Uhlen-
beck's objection as follows: "Certainly, different people
have different amounts of ignorance. The entropy of a thermo-
dynamic system is a measure of the degree of ignorance of a
person whose sole knowledge about its microstate consists of
the values of the macroscopic quantities Xi which define its
thermodynamic state. This is a completely 'objective' quantity
in the sense that it is a function only of the Xi, and does not
depend on anybody's personality. There is then no reason why
it cannot be measured in the laboratory."
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