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THE SECOND LAW OF THERMODYNAMICS . . . AND YOU

Scientism commits us to physics as the complete description of reality. But what exactly does it tell us about the answers to our persistent questions? Almost everything. The fundamental piece of physics that we need in order to answer the persistent questions is the second law of thermodynamics. Physics has accepted this law throughout all the developments of the discipline since its discovery in the nineteenth century. It is a law that much in physics—including all of relativity and quantum mechanics—has continued to support and that nothing in physics—be it ever so shocking—has challenged. Here we’ll take a little time to try to explain what the second law says and how it relates to the rest of physics and our inevitable questions about life, the universe, and everything in between.

The usual way to put physics across is to tell a story about how the second law was discovered in the 1800s. Start by making some noises about the age-long quest to understand heat; sketch the ancient physics of the four elements—earth, air, fire, and water; then invoke the “caloric” theory of heat as a very fine fluid that flows from hot things to cool ones; after that comes Lavoisier’s idea that heat is a substance that can mix with ice to produce water; next bring in Count Rumford from stage right (Germany) showing that horses going round and round turning huge drill-bits to bore cannon barrels would also boil buckets full of water left on top of the cannons; until at last William Thomson becomes Lord Kelvin for figuring out that in a gas, at least, heat is just the motion of molecules. But science isn’t stories, and no story can teach the second law of thermodynamics. Let’s do enough of the science to get a grip on it.

The second law tells us that in any region of space left to itself, differences in the amount of energy will very, very, very probably even out until the whole region is uniform in energy, in temperature, in disorder. Suppose you are trying to pump air into your flat bicycle tire. You attach the hose to the valve on the tire and start to pump. Notice the increasing resistance as you push the handle down. The sources of that resistance are the air molecules being pressed together into a small space at the bottom of the pump. And of course some are entering the tire through the narrow opening where the hose connects to the tire. If you keep pumping, the bottom of the pump gets warmer and warmer to the touch. This increased heat is the molecules moving around faster and faster, bumping into one another and into the container walls more and more as a result of your pushing them. The cause of this increase in warmth is the same as the cause of the increased resistance to the handle: the air molecules are bouncing against the sides and bottom of the pump faster and more frequently as you push them into a more and more confined space. If you closed off the hose to the tire, you would soon be unable to overcome the push-back of the air molecules in the small space at the bottom of the pump. They would be more than a match for the strength of your arms. In fact, if you let go of the handle at the bottom, it would rise a bit, being pushed up by the air molecules, allowing the space in which the molecules move to increase. The handle would not go up as high as it was at the beginning of your last downstroke, however. Some of your downstroke effort will have been wasted.

The reason some of your effort gets wasted is easy to see. Not all of the molecules you pushed down have hit the bottom and bounded straight back up at the piston head. Some of the molecules were not headed down to the bottom when you started to pump; some bumped into one another or headed to the sides of the pump instead. So some of that energy that your pumping added to each molecule will not be sent back toward the handle when you stop pressing. This energy is just wasted, and that is the second law of thermodynamics at work.

Whenever heat—that is,