1Pour a cup of hot tea and set it down in a cold room. Go away for an hour. When you come back, the tea is as cold as the room.
2You have known this since you were three years old. Now ask the question no one asked for ten thousand years. Why does it never go the other way? Why does a cup of cold tea, left in a cold room, never draw heat out of the air and begin to steam? There is heat enough in the air of the room to boil it many times over. No law that Newton wrote forbids it. Yet in all the days of the world, it has not happened once.
3Matthew Reay saw it at his engine. Heat goes out, and does not come back. Sadi Carnot built his little book on it. But neither of them said it as a law, and Carnot's book had a crack in its foundation, as you have heard. He thought that heat ran through an engine like water through a mill, and all came out at the bottom.
4In the years after Carnot's death there lived in the English town of Manchester a brewer named James Joule. He let a weight on a cord sink slowly to the floor. As it sank, it turned a paddle in a tub of water. And the water grew warmer. It was a very little warmer, less than you could feel with your finger; but he had thermometers that could read the two-hundredth part of a degree, and he did it again and again, for years.
5Always it came out the same. So much falling weight, so much warming. Work could be turned into heat at a fixed price, like silver into copper. And if that was so, then heat was not a stuff at all. It was a kind of motion, hidden in the small parts of things, as Carnot had guessed in the notes that were burned.
6Now there were two truths, and they seemed to be at war. Carnot said: work comes from heat falling, and the heat comes through whole. Joule said: work and heat are one coin, and when you get work, heat has been spent. The cleverest men in Europe could not see how to keep both. The young Scot Thomson, who had hunted for Carnot's book through the shops of Paris, wrote that he was at a loss.
7The one who saw the way through was a German, the son of a country pastor in Pomerania, on the cold Baltic shore. His name was Rudolf Clausius. He was twenty-eight, a schoolteacher in Berlin, and no one had heard of him.
8He said: keep both, and give up only the fine water. When an engine runs, a part of the heat is turned into work, and is heat no more; that is Joule's truth. The rest falls to the cold place; that is Carnot's. The fall is still what drives the wheel. The Frenchman's great law stands: the work you can get depends on how far the heat falls, and never on what the engine is made of.
9Then he set down two laws for all heat everywhere.
10The first: the sum is never changed. Heat may become work and work heat, but nothing is made from nothing and nothing is lost into nothing. The whole store of the world stays the same for ever. People came to call that store energy.
11The second: heat does not pass of itself from a colder thing to a hotter one. It is the cup of tea, written as a law of nature. You can force heat uphill, as you can pump water uphill, but only by paying for it with a greater fall somewhere else.
12That was his first great work, and he might have rested on it. He did not. For fifteen years, in Berlin and then in the Swiss city of Zürich, he turned the second law over in his hands. The first law had a quantity in it, a thing you could count: the energy, which never changes. The second law had no quantity. It only said does not. He wanted to know what it was that changed when heat fell. Something was different about the world after the tea had cooled. Not the amount of heat; the room had gained exactly what the cup had lost. What, then?
13Think of it so. Before, there was a hot place and a cold place, and between them a fall, and in that fall you could have set a little engine and turned a wheel. After, all is one lukewarmness. Every bit of the heat is still there. But the fall is gone, and with it the power to do anything. Something has been spent that is not heat.
14Clausius found the way to reckon it. For every body, at every moment, he could write down one number. When heat falls from hot to cold, that number goes up. When an engine runs perfectly, with no waste, the number holds still. And there is nothing whatever, in all of nature, that makes the number go down for the whole of things taken together. In one small place you may push it down, but only by raising it more somewhere else.
15It was a new thing in the world, and it wanted a name. He went looking in old Greek. He held that the great quantities of science should have names from the dead languages, so that they would be the same word in every living one, and belong to no nation. He took the Greek for a turning, a change from one state to another. And he shaped the word, on purpose, to sound like energy, because the two belonged side by side.
16He called it entropy.
17Then he wrote the two laws again, as short as he could make them, to stand as the two great laws of the universe:
18"The energy of the universe is constant. The entropy of the universe tends to a maximum."
19Read the second of those again, slowly, and see why it frightened people.
20It says that every fall of heat is spent once and cannot be spent again. Every fire that burns, every river that runs, every breath a creature draws, uses up a little of the difference between hot and cold. The sun is pouring its heat into the cold dark, and the dark gives none of it back. So there must come a day, however far off, when the sun has no more to give. The stars will burn down. All things will come to one even lukewarmness, from end to end of everything. Not a wheel will turn. Not a leaf will move. The whole store of energy will still be there, every grain of it, and none of it will be worth anything.
21Thomson had already said as much. Within some span of years, he wrote, the earth must become unfit for people to live on. To many it seemed the cruellest thing that the new knowledge had brought. The old faiths had promised an end with a meaning in it. This was an end like a fire going out in an empty house.
22Clausius himself did not preach on it. He was a plain man who disliked show. He had named a thing truly, and that was his work. But what he did afterward tells you more than a sermon would. Listen.
23In the summer of his forty-ninth year, war broke out between the German lands and France. He was by then a famous professor in the city of Bonn, on the Rhine, with a wife named Adelheid and a house full of small children. He was far too old to carry a rifle. He could have stayed at his desk, and no one would have thought the worse of him.
24He gathered his students instead. He made of them a company to carry the wounded off the field, and he led them himself into France.
25One of them was a student of medicine named Emil, nineteen years old, who had never seen a dead man. In the middle of August the company came up behind the army, near a village called Gravelotte, on the day of the greatest battle of that war. All afternoon they heard it, like a wagon of iron rolling on a wooden bridge. At dusk the guns stopped. Then they were sent in.
26There were twenty thousand German soldiers lying on those slopes that night, and thousands of French among them. The students went out in pairs with stretchers and lanterns. They went to whoever called. Emil said afterward that he had expected screaming, and that it was the quiet ones who were hard to pass, for those were the ones who could not be saved, and each of them watched the lantern go by.
27He worked all night beside the professor. That night he said little. He lifted his end of the stretcher. Toward morning they sat down for a quarter of an hour at a fire that some gunners had left, and someone had made coffee in a tin, and they drank it with their hands shaking.
28Two men lay near the fire who had been set down there because there was no more room at the dressing station. One wore the dark blue of Prussia and one the red trousers of France. They had been brought in from the same ditch. The German was dead. The Frenchman was dying, and knew it, and asked for nothing.
29Emil put his hand on the dead man's forehead, as he had been taught, to be sure. He drew it back quickly.
30"He is still warm," he said.
31"Yes," said Clausius. "For an hour or two."
32"And then he will be as cold as the ground."
33"As cold as the ground, and no colder. And the ground a very little warmer. That is all the difference there will be. Nothing is lost, you understand. Every particle of the heat that was in him will still exist." He was silent a moment. "I have written that sentence a hundred times. I do not think I knew until tonight what it leaves out."
34The boy looked at him.
35"A man is not the heat that is in him," said the professor. "A stove has as much. He is a place where heat was doing something. I can count the heat. I can count the entropy. I cannot tell you where to put my finger on what stopped."
36Then he got up, for there was shouting from the dark beyond the fire, and took the front poles of the stretcher. They went back out. The Frenchman lived until the sun was up. Emil sat with him at the end, because no one else was free to, and the man held on to his sleeve.
37In that same campaign, at that same work, Rudolf Clausius was wounded in the leg. It never mended rightly. He walked in pain for the rest of his life, and in his later years he took to riding a horse, because he could not go far on foot. His country gave him the Iron Cross.
38He had not gone to France to prove anything. But see what he did there, this man who had told the world that all things run down. He went into the worst running-down that people know how to make, where the warm were being turned to cold by the thousand in a day. He could not stop it. He carried out of it those he could reach, one at a time. Each was a small fire that he did not let go out that night. He knew better than any man alive that they would go out in the end. He carried them all the same.
39Five years after the war, his wife Adelheid died giving birth to their sixth child.
40He was fifty-three. He had six children, the eldest not yet grown and the youngest an hour old, and a leg that would not bear him. His fellow scholars expected that he would find a housekeeper and return to his great work. Younger men, in Scotland and in Vienna, were taking his entropy and turning it in ways he had not thought of, and letters came from all over Europe asking what he made of them.
41He answered few of those letters. He brought up his children. He heard their lessons himself in the evenings. He sat at the head of the table and saw that the little ones ate. Those who wrote the history of his science said, with some regret, that Clausius did little that was new in his last years, and fell behind. They did not ask what he had judged to be more pressing.
42Picture one winter evening in that house. The smallest girl would not eat her soup, and sat over it until it was cold. Then she was hungry, and wanted it warm again, and it was not. She asked her father why it could not get warm by itself, since it had got cold by itself.
43The others laughed. He did not. He put down his spoon.
44"That is the best question in physics," he said. "I have spent my life on it."
45"Then why?"
46"I can tell you that it never does. I can tell you how to count what has changed. I gave the count a name, a Greek one. People think that because I named it I understand it." He shook his head. "A name is not a reason. I know that heat falls. I do not know why. Why one way, and never the other? Someone will have to find that out. I think it will be someone who is not afraid to think about very small things."
47He sent the soup back to the kitchen to be warmed at the fire, which is the only way.
48He was right about the small things. The reason was hidden in them. To find it, people first had to believe a thing that no one had ever seen: that the tea, and the cup, and the hand that holds it, are made of grains too small for any eye, and that the grains never stop moving. A Scotsman with a dark beard and a taste for odd jokes was already thinking about those grains. He had asked himself what would happen if a creature small enough to see them stood at a little door.