Followup to: Angry Atoms
After yesterday’s post, it occurred to me that there’s a much simpler example of reductionism jumping a gap of apparent-difference-in-kind: the reduction of heat to motion.
But there was a time when the kinetic theory of heat was a highly controversial scientific hypothesis, contrasting to belief in a caloric fluid that flowed from hot objects to cold objects. Still earlier, the main theory of heat was "Phlogiston!"
Suppose you’d separately studied kinetic theory and caloric theory. You now know something about kinetics: collisions, elastic rebounds, momentum, kinetic energy, gravity, inertia, free trajectories. Separately, you know something about heat: Temperatures, pressures, combustion, heat flows, engines, melting, vaporization.
Not only is this state of knowledge a plausible one, it is the state of knowledge possessed by e.g. Sadi Carnot, who, working strictly from within the caloric theory of heat, developed the principle of the Carnot cycle – a heat engine of maximum efficiency, whose existence implies the second law of thermodynamics. This in 1824, when kinetics was a highly developed science.
Suppose, like Carnot, you know a great deal about kinetics, and a great deal about heat, as separate entities. Separate entities of knowledge, that is: your brain has separate filing baskets for beliefs about kinetics and beliefs about heat. But from the inside, this state of knowledge feels like living in a world of moving things and hot things, a world where motion and heat are independent properties of matter.
Now a Physicist From The Future comes along and tells you: "Where there is heat, there is motion, and vice versa. That’s why, for example, rubbing things together makes them hotter."