Problem
It seems obvious that “measuring” means comparing a quantity with a reference sample. But beneath that apparent obviousness lies a very strong choice.
Deciding to measure a quantity means, first of all, deciding that it is worthy of being measured, and others are not. For millennia temperature was not: people said “hot” and “cold” without numbers, relying only on sensation. It became one only when Galileo and Fahrenheit linked thermal sensation to a measurable phenomenon: the expansion of a liquid inside a graduated tube. Before that step temperature was not a physical quantity; afterwards, it became one.
Hence the deeper point: a quantity exists only if there is a measurement procedure. It is not enough to have an intuitive idea of a quality of the world; a concrete, repeatable method is needed, one that translates that quality into a number. This is one of the deepest lessons of all modern physics, and it holds well beyond thermology. When in the following chapters we introduce energy, entropy, the field, we will each time have to ask ourselves which “little tube” — which experimental procedure — allows us to pass from sensation to number. Without that procedure, the quantity simply does not exist.
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Topics: Grandezze fisiche e misura
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