Thirty-eight minutes on the simplest question I know: why does anything happen at all? The narrator is Lily, the synthetic voice that carried Single Player, The Third Fraction and Subject To. My own cloned voice comes in for a minute after the cold open. The series is scored: listen for the sound that stops when the power does.
On Monday the twenty-eighth of April last year, a little after half past twelve, Spain and Portugal lost their power. All of it, from Lisbon to the Pyrenees, in under a minute. A radio journalist on his motorbike knew it was serious when he saw that every traffic light had stopped. He remembered a car standing in the road with its doors open and its radio turned up as loud as it would go, and people gathered round it to listen.
I run a company that writes software for the edge of the power grid, so a day like that one is what I think about for a living. What held my attention was what had not happened. There was no storm and no fire. It was not an attack. There was no shortage of anything: the sun laid exactly as much energy on the roofs of Madrid at 12:34 as it had at 12:32, and petrol sat in the tanks under every filling station, out of reach, because a petrol pump is an electric pump. Energy was everywhere that afternoon and almost nothing could be done with it. When the engineers went looking for the part that had failed, they could not find one. Nothing was broken.
So what had stopped? Answering that took me much further back than the grid, and it became three episodes. This is the first.
What you pay for
Start with your electricity bill. One kilowatt-hour brings about ten litres of cold tap water to the boil. Afterwards the water warms the kitchen, the kitchen warms the street, and if you could follow every scrap of that energy you would find it all still there, spread thin and not one bit smaller. You did not use up any energy when you boiled the kettle. Nobody ever has. Then what did you pay for?
The answer begins with a coal bill. About two hundred and sixty years ago the university in Glasgow handed a broken model steam engine to its instrument maker, James Watt. The engines of the day worked like a syringe: steam lifts the plunger, a squirt of cold water collapses the steam, and the air pushes the plunger down. Every stroke heated the barrel and chilled it again, and a full-size engine turned about one shovel of coal in two hundred into lifted water. Watt did the chilling in a second vessel, kept the barrel hot, and cut the coal bill by two thirds. That left a question no engine builder could answer. Could somebody cut it again, and again? Is there a limit?
Six hundred copies
The man who answered it never built an engine. Sadi Carnot was the son of the engineer who organised the armies of the French Revolution. As a boy of four, his brother said, he shook his fist at Napoleon for splashing the ladies in a rowing boat. In June 1824, an army officer on half pay, he published a thin book at his own expense: six hundred copies of Reflections on the Motive Power of Fire.
It throws away the engine and asks what is the least you need to get work out of heat. His answer: “Wherever there exists a difference of temperature, motive power can be produced.” A difference. He explained it with the picture everybody then knew. The work a waterfall can do depends on how much water comes down and on how far it falls. Heat is the same: what counts is the amount, and what he called “the height of its fall”, the gap between the hot place it leaves and the cold place it arrives. So there is a limit, and two temperatures set it. That is why every power station that runs on heat stands by a river, the sea or a row of cooling towers. The towers are the bottom of the waterfall. Take away the cold end and the engine does not run better. It stops.
The book got one kind review and then ten years of silence. Eight years after it came out, cholera reached Paris. That April, Carnot wrote to a cousin that he feared the disease little. In August he died of it, at thirty-six, in a private asylum outside the city; his family kept the asylum out of every account of his life for more than a century. His belongings were burned, as the custom was, and nearly all his papers with them. Twenty-three loose sheets survived. In them he had already written that heat is “simply motive power, or rather motion which has changed form”, and that motive power is “never either produced or destroyed”. He had the conservation of energy, alone, on a sheet nobody saw.
The idea survived by a thread. A railway engineer redrew the book as graphs. Years later a young man of twenty from Glasgow, William Thomson, read the engineer’s paper and went round the bookshops of Paris asking for the original. They handed him a book on a social question by the other Carnot, the brother in politics. No bookseller in the city where it was printed had heard of the book on fire.
The brewer’s number
Carnot’s waterfall has the height right and the water wrong. In his book all the heat arrives at the bottom, as all the water in a river reaches the sea. A ship’s doctor, Robert Mayer, was the first to see otherwise, looking at the bright red blood of sailors he bled off Java. He sent the idea to the leading physics journal and never got a reply. The number came from James Joule, a brewer’s son near Manchester, who let lead weights fall in a cellar to turn a paddle in a can of water, twenty times over, and read the thermometer by eye. In modern units: let a litre of water fall a little over four hundred metres, catch all its motion at the bottom, and it is one degree warmer.
Look at what that does to the waterfall. At the foot of the fall the water is all still there, and so is every bit of the energy. What is gone is the height.
Counting the ways
The conservation of energy cannot be the whole truth, and the proof is in your kitchen. A cup of coffee cools and the room warms by a whisker. Run it the other way, the coffee taking its heat back from the room, and the books balance just as well. It has never happened. Something that is not conserved tells the coffee which way to go. A German physicist named it entropy, and could say that it rises. He could not say what it is.
Ludwig Boltzmann could. Take atoms literally and entropy turns into counting. Let a drop of milk fall into black coffee and do not stir. There are few ways to arrange the specks of milk as a drop and unimaginably many ways to arrange them as brown coffee. Nothing pushes the milk outwards. The specks are knocked about blindly from one arrangement to the next, and nearly all arrangements are the mixed kind. A difference evens out because evened-out is what almost every arrangement looks like.
Boltzmann had to fight for atoms, and he won the argument in public. He was also ill for many years. On the fifth of September 1906, on holiday at Duino with his wife and youngest daughter, he took his own life. No note has survived. You will read that he died because nobody believed in atoms. That is not true, and it is not fair to him: by then most physicists were on his side. We do not know why he died. His stone in Vienna carries one line: S equals k log W. W is the number of ways.
The staircase
Counting says why a difference evens out. It does not say why there was a difference to begin with. Follow the question back and you reach the start of the universe, which was almost perfectly smooth, and under gravity smooth is the rare state, the drop of milk. Why it began that way, nobody knows. The stars are what that first difference looks like now, and the nearest one is the largest difference in our neighbourhood.
We do not live on the Sun’s energy. The Earth sends back to space almost exactly what it takes in. What changes is the form: for every fierce packet of sunlight that arrives, about twenty packets of gentle warmth leave. We live on the difference. The Moon gets the same light and sends it all back too, in one step, and nothing happens there. On Earth the same fall goes down a staircase: the sea warms, water rises, rain falls on a hill and runs to a river; a leaf builds sugar and something eats the leaf. On every step, something gets done. We do not slow the fall. We put steps in it, and we live on the steps.
Now the bill. Spend a kilowatt-hour as work and it lifts a tonne of water higher than the Eiffel Tower. Spend it as heat and the same tonne is less than one degree warmer. From the water on the tower you can have nearly all of it back. From the lukewarm water, a perfect engine returns about one part in seven hundred. The energy is all there. The height is gone. That is what you pay for: energy with its whole fall still ahead of it. Electricity is the top step of the staircase, the one we build ourselves.
That is what stopped at 12:33 on that Monday. Not the energy. The top step, and everything that stands on it. It was not a gentle night: people died, and the third part says who.
Half an hour earlier, at three minutes past noon, something had begun that almost nobody in Spain knew about. It showed on the instruments of the people who watch the grid: a rocking, one end of the country against the other. They calmed it. A quarter of an hour later a second one began, far wider, one beat every five seconds. Every spinning machine in Spain and Portugal had started to sway against the rest of Europe. Why a country should sway against a continent is part two.
Sources, for readers who want the originals: Carnot, Reflections on the Motive Power of Heat, in R. H. Thurston’s 1890 edition, which carries his brother Hippolyte’s Life and the posthumous notes; the Archives de la Côte-d’Or, “Une lettre inédite de Sadi Carnot (1832)”; the Dictionary of Scientific Biography on Carnot, Mayer and Joule; William Thomson, Popular Lectures and Addresses, vol. II (1894), for the Paris bookshops, and his “On a Universal Tendency in Nature to the Dissipation of Mechanical Energy” (1852); the Science Museum on Watt’s separate condenser; the Stadtarchiv Heilbronn’s chronology of Robert Mayer; J. Young, “Heat, work and subtle fluids: a commentary on Joule (1850)” (Phil. Trans. R. Soc. A, 2015); Clausius’s paper of 1865; Boltzmann, Populäre Schriften (1905), and D. Flamm, “Ludwig Boltzmann: A Pioneer of Modern Physics”; Feynman, Lectures on Physics, vol. I, ch. 4; Lineweaver and Egan, “Life, gravity and the second law of thermodynamics” (Physics of Life Reviews, 2008); Red Eléctrica’s incident report of 18 June 2025; and Reporters Without Borders, “Radios never go silent: the day Spain clung to the airwaves”.
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