Four Hundred Years

Galileo was born in 1564. Turing died in 1954. Three hundred and ninety years.

In that time eleven people, who mostly never met, worked out a way of settling arguments about the world that does not depend on who is asking.

Measure it. Write down how you measured it. Predict something that has not happened yet. And when the measurement disagrees with you, say so in public.

This is the map of the whole series. Every bar on it is one episode.

The method is not complicated, and no single person invented it.

Measure something and write the number down. Publish how you measured it, so a stranger can repeat it and disagree. Use it to predict something nobody has seen yet. And when the measurement contradicts you, lose the argument in public.

That last move is the hard one. It is also the one that makes the whole thing work.

Before about 1600, the usual way to settle a question about nature was to find out what an authority had said about it. After 1600, slowly, it became normal to go and look.

Press the buttons and move the marker along the century.

The readout tells you who is alive at that moment and what has just happened.

Some of the overlaps are startling. Newton was born at the end of the year Galileo died, by the calendar England was still using.

Darwin and Mendel were alive at the same time for sixty years, three days’ travel apart, and neither ever read the other’s most important work.

Einstein was alive for every single day of Turing’s life.

Galileo did not invent the telescope, but he built a much better one and pointed it at Jupiter.

Four points of light shifted position night after night. They were going round Jupiter, not round the Earth, and that was enough to show the Earth is not the only centre.

He also saw mountains on the Moon, spots on the Sun, and the phases of Venus, which Ptolemy’s system cannot produce.

For publishing it he was tried in 1633 and spent the rest of his life under house arrest.

Newton was born at the end of 1642, the year Galileo died.

His move was to notice that the apple and the Moon are doing the same thing. The Moon is sixty Earth-radii away, so if the force weakens as the square of the distance, its acceleration should be three thousand six hundred times smaller than the apple’s.

He worked out what that acceleration actually is from the length of the month, and the numbers matched.

The Principia of 1687 turned the sky into arithmetic. It took Europe about fifty years to accept it.

Lavoisier’s instrument was the balance, and his contribution was to insist on it.

Burn something in a sealed vessel and weigh the whole vessel before and afterwards. The mass does not change. Burning is not something leaving the substance; it is something in the air joining it.

He named that something oxygen, and in 1789 published the first recognisably modern list of elements.

Five years later the Revolution executed him for having been a tax farmer. He was fifty.

Mendeleev lined up the sixty-odd known elements by atomic weight and found that their properties repeat.

The important decision was what to do when the pattern did not fit. Instead of forcing it, he left the square empty and described the element that would eventually fill it: its weight, its density, the colour of its oxide.

Gallium turned up in 1875, scandium in 1879, germanium in 1886. He had predicted a weight of 72.64 for the last one; the measured value is 72.63.

A classification that makes predictions has stopped being a filing system.

Darwin’s argument needs only three observations.

More offspring are born than can possibly survive. Individuals differ from each other. Some of those differences are passed on.

If all three are true, then the variants that happen to survive will be over-represented in the next generation, and the species will drift. Nobody has to be steering.

He had the argument by 1838 and published in 1859, pushed into print by a letter from Alfred Russel Wallace with the same idea in it.

On the Galápagos today, average finch beak depth measurably shifts from one drought to the next.

Darwin had a problem he could not solve: if inheritance blends, variation halves every generation and selection has nothing left to work on.

Three hundred kilometres away, a monk counting peas had the answer. Mendel grew about twenty-eight thousand plants and found that the second generation splits three to one.

That is what you get if inheritance comes in discrete particles that can hide in one generation and reappear in the next.

He published in 1866. It was read by almost nobody, and the result sat unused for thirty-four years.

Marie Curie measured the activity of uranium ore with an electrometer and found it was stronger than the uranium in it could account for.

Something else was in there. She and Pierre spent four years processing tonnes of pitchblende to get a tenth of a gram of radium chloride, enough to measure an atomic weight.

Her real claim was not the new elements. It was that the radiation is a property of the atom itself — which means an atom can fall apart.

Two Nobel Prizes, in physics and in chemistry, and a dose that eventually killed her.

Einstein’s starting point was that a person in free fall does not feel their own weight, so gravity and acceleration cannot be told apart locally.

Follow that through and gravity stops being a force and becomes the shape of spacetime. Things in free fall go straight; the straight lines are bent.

The theory was finished in 1915 and made a prediction nobody could argue with: starlight passing the Sun should bend by a specific amount, twice the Newtonian value. The 1919 eclipse measured it.

The satellites above you apply the correction continuously. Without it, GPS would drift about eleven kilometres a day.

Wegener put together coastlines that fit, rock belts that continue across oceans, the same fossil plant on five continents, and coal in Antarctica.

The conclusion was that the continents were once joined and have moved apart. The evidence was strong and the reaction was hostile, because he could not say what force moves a continent.

He died on the Greenland ice in 1930, aged fifty, with the idea still rejected.

In the 1960s the sea floor turned out to be spreading, carrying the continents with it, and the magnetic stripes on either side of the ridges are mirror images. He had been right for half a century.

In January 1913 a clerk in Madras posted nine pages of formulas to a Cambridge mathematician he had never met, with no proofs attached.

Hardy later said they must be true, because nobody would have had the imagination to invent them.

Their joint work on partitions produced a formula that is accurate enough to pin down an exact integer: the number of ways to write a hundred as a sum of positive integers is one hundred and ninety million, five hundred and sixty-nine thousand, two hundred and ninety-two.

Ramanujan died in 1920 at thirty-two, leaving notebooks that mathematicians were still proving results from seventy years later.

Turing answered the last great question of the series, which was about the limits of proof itself.

To show that no fixed procedure can decide every mathematical statement, he first had to define a procedure — so he described a machine with a tape, a head and a table of rules.

Write a machine’s rules onto the tape as data, and one machine can imitate any other. That is a computer, and the description is software.

Then he showed that no machine can decide, in general, whether a program will stop.

Three years later he was at Bletchley Park. In 1952 his own government convicted him for being gay.

Here is the same eleven, measured a different way: the years between making the claim and the argument being effectively over.

Einstein waited four years, because he had made a prediction that an eclipse could check. Mendeleev waited six, for gallium to be dug up.

Mendel waited thirty-four, because nobody read him. Wegener waited fifty, because he had no mechanism. Darwin’s natural selection waited roughly eighty for genetics to catch up with it.

And Galileo waited two hundred and six, because the question had stopped being about astronomy.

Press a name and the readout gives the two dates.

The last thing worth saying is that none of these people worked alone, and none of them started from nothing.

Newton had Galileo’s falling bodies and Kepler’s orbits. Mendeleev had Lavoisier’s list of elements. The theory of evolution only became quantitative when Mendel’s peas were rediscovered thirty-four years after he published. Turing had Hilbert’s question and Gödel’s answer.

Press a name and the readout says what each of them was standing on.

Eleven episodes follow this one. The chapter stills are AI-generated; the portraits and documents are real, credited where they appear, and the sources are listed in every step.