What is heat?

Heat is amazing. The energy you could in principle extract, by lowering the temperature of any amount of water by a barely perceptible one degree Celsius, would be enough to lift that same amount of water to a height of over four hundred meters. And the energy you could extract by condensing any amount of steam into liquid water would be enough to lift that same amount of water into space. This is why the Industrial Revolution was such a big deal. Harnessing heat, in fuel-burning engines that drive pistons or spin turbines with hot gases, is what has let us hair-challenged primates conquer the planet. Heat is magic. What is it?

Until the mid 19th century, people thought that heat was a special kind of fluid, like air or water but different, and invisible. They called it "phlogiston", or "caloric". Some considered that cold was a distinct fluid, "frigoric", while others argued that cold was simply absence of heat. These were by no means stupid or crazy theories. Electrical charge is a phenomenon which is about as basic and important as heat, and it really is carried by two different kinds of stuff, namely electrons and protons (as well as other much less common particles), which each carry opposite charge. Objects can become positively or negatively charged if they pick up excess protons or electrons. It was not silly to imagine that objects might become hot or cold by picking up excess caloric or frigoric.

But early physicists figured out that this was wrong, mainly from carefully observing how grinding metal keeps on making it hotter, even when the grinder and the metal are kept well apart from any other objects that might conceivably be able to inject a steady supply of caloric into them. They concluded that heat is actually some form of energy, and that the more familiar kinds of energy carried by moving objects can be converted into heat, through friction; while heat may in turn be converted into motion and useful work, in engines.

But then just what is the difference between heat and work, as forms of energy? It's not easy to get a straight answer even from a fully trained physicist, because the truth is that we're still not completely sure what heat is. If I have many bazillions of atoms all zipping around in a big box, bouncing rapidly off each other and the walls, making up a gas, then I can use statistical mechanics to say an awful lot about heat and pressure and temperature for this gas. But if I have one single atom, perhaps ionized and trapped in a strong electric field, I know that the concept of heat is not even relevant. With one atom, I can compute the motions of its nucleus and of its electrons, rather as I worked out the motion of solid objects in freshman physics. It does not even make sense to ask whether the atom is hot or cold. Usually no single atom has heat, but a billion atoms do. So heat is somehow an emergent property of large numbers of atoms together.

"Emergent property" is a fine bit of fashionable philosophical mumbo-jumbo, which spends rather too much time in the blogs of wild-eyed crackpots and tenured philosophers, to be comfortably welcome among respectable scientists. But in the case of heat, you can slurp an emergent property from a cup of coffee. Heat rules the world. It's quite concretely real. So what is it?

Well, we're working on it. There is ample precedent in perfectly well understood physics for new behavior to emerge in larger systems; it's just that in this particularly fundamental case of heat there are still some major obscurities in exactly how it works. But in just the past few years, atomic and optical physicists have gained the capability to make extremely precise and direct measurements on small samples of gas, with only hundreds to thousands of atoms. If heat emerges, we're soon going to be able to catch it in the act. Watch this space.