Today, we wonder what makes a science. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.
Thermodynamics is an odd science. Most people have only a vague notion of what it is, and I sometimes wonder if it really is a science. It came into being after we had steam engines, because people felt a need to explain just how their new engines worked.
At first look, it might seem that we could derive all of thermodynamics from atomic physics. Yet we've never quite managed to do that. It retains a life of its own. Like Euclidean geometry, it's an arrangement of logical deductions, resting on a few axioms -- or laws. Those laws can be framed in different ways, but they always embody two primal facts. One is that energy is conserved. The other is that everything moves toward more probable (and less orderly) states. And it's woe betide anyone who tries to invent an engine without remembering those two essential truths.
Energy conservation seems obvious if we understand how energy is stored and traded on the atomic level. All we have to do is keep track of all those tiny energies. But wait a minute! When we do that, we already assume that thermal energy is made of those pieces. Maybe thermodynamics is just as basic as atomic physics.
The other matter, the tendency toward disorder, is a lot harder to deduce from atomic behavior. It's always taken a certain amount of logical arm waving. But all that's moot, because thermodynamics exists without any reference to atoms. Even though its axioms are the result of atomic behavior, they express general truths that apply to vast aggregates of atoms -- aggregates large enough to be experienced by our human senses.
We'll always turn to thermodynamics when we need to make calculations about real systems on a human scale. To see why, I'll try explaining the heating value of a fuel in terms of atoms:
All molecules are held together by powerful electronic forces. It takes energy to pull them apart. If we burn carbon in oxygen, we dismantle the O2 molecule and get two oxygen atoms. Then those oxygen atoms combine with one carbon atom to form carbon dioxide, CO2. It takes more energy to pull CO2 apart than it does O2, so energy is released when carbon burns in oxygen.
Well, there will not be a test; all that gets far too messy. When we have to design something, atomic scenarios are little help. Somewhere down there, in that invisible world we never see, are atoms, photons, quarks -- maybe even strings. But to build on the human scale, we need physical laws that no more recognize atoms than our own eyes do.
I guess that's why I like thermodynamics. It's what we might call an anthropomorphic science -- a science of the forest, not just the trees. The sciences of the atomic trees also serve us very well in may arenas, make no mistake. But they ultimately do so by helping us to better understand the great forest of those things that we can touch and see.
I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.
These ideas are developed at greater length in: J. H. Lienhard, How Invention Begins: Echoes of Old Voices in the Rise of New Machines. (New York: Oxford University Press, 2006): Chapter 6.
For a discussion of the relation between thermodynamics and physics, see C. L. Tien & J. H. Lienhard, Statistical Thermodynamics (London: Taylor & Francis, 1979) Chapters 1-3. For a discussion of the derivation of the Second Law of thermodynamics, the tendency toward disorder, from atomic physics, see Chapter 11.
An over-centered wheel perpetual motion machine (PMM). This prototypical PMM was "invented" in India ca. AD 1100 and resurrected countless times thereafter. Like all PMMs, it violates the laws of thermodynamics and has never been made to run. Detail of the Hvalsøy Church adapted from Jansen (op. cit.). Originally given by A. Roussell, Farms and churches in Greenland, Meddr Grønland, Vol 89, No. 1. 1941.