heat -- 10/23/19

Today's selection -- from Seven Brief Lessons on Physics by Carlo Rovelli.

What is heat?:

"Until the mid-nineteenth century, physicists at­tempted to understand heat by thinking that it was a kind of fluid, called 'caloric,' or two fluids, one hot and one cold. The idea turned out to be wrong. Eventually James Maxwell and the Austrian physicist Ludwig Boltzmann understood. And what they understood is very beautiful, strange, and profound -- and takes us into regions that are still largely unexplored.

"What they came to understand is that a hot sub­stance is not one that contains caloric fluid. A hot sub­stance is a substance in which atoms move more quickly. Atoms and molecules, small clusters of atoms bound together, are always moving. They run, vibrate, bounce, and so on. Cold air is air in which atoms, or rather molecules, move more slowly. Hot air is air in which molecules move more rapidly. Beautifully simple. But it doesn't end there.

"Heat, as we know, always moves from hot things to cold. A cold teaspoon placed in a cup of hot tea also be­comes hot. If we don't dress accordingly on a freezing cold day, we quickly lose body heat and become cold. Why does heat go from hot things to cold things and not vice versa?"It is a crucial question because it relates to the nature of time. In every case in which heat exchange does not occur, or when the heat exchanged is negligible, we see that the future behaves exactly like the past. For exam­ple, for the motion of the planets of the solar system heat is almost irrelevant, and in fact this same motion could equally take place in reverse without any law of physics being infringed.

Ludwig Boltzmann

"As soon as there is heat, how­ever, the future is different from the past. While there is no friction, for instance, a pendulum can swing forever. If we filmed it and ran the film in reverse, we would see movement that is completely possible. But if there is friction, then the pendulum heats its supports slightly, loses energy, and slows down. Friction produces heat. And immediately we are able to distinguish the future (toward which the pendulum slows) from the past. We have never seen a pendulum start swinging from a stationary position, with its movement initiated by the energy obtained by absorbing heat from its supports. The difference between past and future exists only when there is heat. The fundamental phenomenon that distinguishes the future from the past is the fact that heat passes from things that are hotter to things that are colder.

"So, again, why, as time goes by, does heat pass from hot things to cold and not the other way around?

"The reason was discovered by Boltzmann and is sur­prisingly simple: it is sheer chance.

"Boltzmann's idea is subtle and brings into play the idea of probability. Heat does not move from hot things to cold things due to an absolute law: it does so only with a large degree of probability. The reason for this is that it is statistically more probable that a quickly moving atom of the hot substance collides with a cold one and leaves it a little of its energy, rather than vice versa. En­ergy is conserved in the collisions but tends to get dis­tributed in more or less equal parts when there are many collisions. In this way the temperature of objects in con­tact with each other tends to equalize. It is not impossi­ble for a hot body to become hotter through contact with a colder one: it is just extremely improbable.

"This bringing of probability to the heart of phys­ics, and using it to explain the bases of the dynamics of heat, was initially considered to be absurd. As fre­quently happens, no one took Boltzmann seriously. On September 5, 1906, in Duino, near Trieste, he commit­ted suicide by hanging himself, never having witnessed the subsequent universal recognition of the validity of his ideas."

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Carlo Rovelli


Seven Brief Lessons On Physics


Riverhead Books


Copyright 2014 by Riverhead Books


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