The Happiest Thought

In 1907 a patent clerk in Bern was writing a review article when something obvious occurred to him.

A man falling from a roof does not feel his own weight. While he falls, he is weightless — gravity has vanished for him completely.

Einstein called it the happiest thought of his life, because it suggested that gravity is not a force acting on things, but something about the frame you are in.

Eight years later that hunch had become a set of equations saying that mass bends space and time, and that light follows the bend.

Einstein finished his degree in 1900 and could not get an academic job. He applied across Europe and was refused.

A friend’s father found him a post at the Swiss patent office in Bern: technical expert, third class, assessing applications for electrical devices.

He was good at it, and it was undemanding enough to leave his mind free. He later called the office his worldly monastery.

In 1905 he published four papers in a single year: on the quantum nature of light, on Brownian motion, on special relativity, and the short one with the equation about energy and mass.

The first paper said light comes in discrete packets, which explained why shining light on metal knocks electrons out only above a certain colour. That is the paper that won him the Nobel Prize.

The second explained the jittering of pollen grains in water as the result of being hit by molecules, which settled the argument about whether atoms are real.

The third was special relativity: if the speed of light is the same for every observer, then time and distance cannot be.

The fourth was three pages long and pointed out a consequence: mass is a kind of energy, and the exchange rate is the speed of light squared.

Put yourself in a windowless box. Everything you drop falls to the floor at the same rate.

You could be standing on the Earth. Or the box could be in deep space, with a rocket accelerating it upwards at nine point eight metres per second squared. From inside, there is no way to tell.

Now cut the cable. Everything in the box floats, including you. You could be falling towards the Earth, or drifting in empty space with no gravity at all. Again, no way to tell.

If no experiment can distinguish gravity from acceleration, then they are not two things. Press the buttons and try it.

The equivalence principle has consequences you can work out on the back of an envelope.

Shine a torch straight across an accelerating box. While the light crosses, the box moves up, so the beam hits the far wall slightly low. From inside, the light looks bent.

But the inside of an accelerating box is indistinguishable from a gravitational field. So gravity must bend light as well.

The same argument says clocks lower in a gravitational field run slow. Einstein published the light-bending prediction in 1911 with a value that was half of the right answer.

The full theory, in 1915, doubled it.

It took Einstein eight years and a crash course in differential geometry to turn the happiest thought into mathematics.

What came out in November 1915 is one equation with two sides. On one side, the curvature of spacetime. On the other, the matter and energy in it.

Matter tells spacetime how to curve, and the curve tells matter how to move. A planet in orbit is not being pulled. It is going as straight as it can through a region that is bent.

The first thing he checked was Mercury. Its orbit drifts by forty-three arcseconds a century more than Newton allows, and the new equations produced that number exactly. He said it gave him palpitations.

A theory that only explains what is already known is doing half a job. General relativity made a prediction nobody had measured.

Starlight grazing the edge of the Sun should be deflected. Newtonian gravity, treating light as particles, gives about zero point eight seven arcseconds. Einstein’s theory gives one point seven five — exactly twice as much.

An arcsecond is the width of a coin seen from four kilometres away, so this needs careful photography. And you cannot photograph stars next to the Sun.

Unless the Moon gets in the way.

In 1919 Britain sent two expeditions to photograph a total eclipse: one to the island of Príncipe off West Africa, the other to Sobral in Brazil.

At Príncipe it rained, and cloud covered most of totality. Eddington got a usable result from one or two plates. At Sobral the main telescope produced blurred images, and the smaller backup instrument gave the better data.

On the sixth of November the Royal Society announced that the measurements favoured Einstein. The next morning he was world-famous, and he had not left Berlin.

The honest version is that it was a difficult measurement at the edge of what the equipment could do. Later eclipses, and then radio astronomy, confirmed it to far better precision.

The story travelled because of its shape. A German physicist’s theory had been confirmed by a British expedition, one year after four years of war between those two countries.

Newspapers made it a parable about science crossing borders, and Einstein became the first scientist to be a global celebrity.

He was uncomfortable with it and used it, campaigning for pacifism, for Zionism, and later against the rise of the Nazis.

The Nobel Prize came in 1921, carefully awarded for the photoelectric effect. Relativity was still considered too speculative to honour.

By the late 1920s Einstein was both the most famous scientist in the world and, in Germany, a target.

Nationalist physicists denounced relativity as Jewish physics. When the Nazis took power in 1933 he was abroad on a lecture tour. He resigned from the Prussian Academy, gave up his citizenship, and never returned.

His house was raided, his property seized, his books burned, and a Nazi magazine published his photograph with the caption not yet hanged.

He settled at the Institute for Advanced Study in Princeton and stayed for the rest of his life.

Einstein’s 1905 paper on light quanta is one of the founding documents of quantum theory. He then spent the rest of his life arguing that the theory was incomplete.

His objection was not that the mathematics failed — it worked perfectly — but that it describes only probabilities, and he believed physics should describe what is actually there.

In 1935 he and two colleagues published a thought experiment designed to show the theory was absurd. Experiments since the 1970s have shown that nature behaves exactly as the absurd version says, and that paper is now the foundation of research into entanglement.

He was wrong, and he was wrong in a way that produced a whole field.

In the summer of 1939 Leó Szilárd came to Einstein with a draft letter warning President Roosevelt that nuclear fission might make an extraordinarily powerful bomb, and that Germany was working on it.

Einstein signed. The letter helped start the American programme that became the Manhattan Project. He did no work on the bomb himself and was refused clearance to do so.

After Hiroshima he campaigned against nuclear weapons for the rest of his life and said that if he had known the Germans would fail, he would never have lifted a finger.

He died in Princeton in 1955, aged seventy-six, refusing surgery.

Here is the part that touches you. A GPS satellite carries an atomic clock in a weaker gravitational field than yours, so general relativity says it runs fast — by about forty-six microseconds a day.

It is also moving at fourteen thousand kilometres an hour, so special relativity says it runs slow, by about seven microseconds a day.

The two do not cancel. The net effect is a gain of roughly thirty-eight microseconds every day, which would put your position out by about eleven kilometres after twenty-four hours.

So the clocks are deliberately detuned before launch. Set the correction on and off and watch the error grow.

General relativity has now been tested for over a century and has not failed once.

In 2015 two detectors in America felt a ripple in spacetime from two black holes colliding more than a billion light years away, stretching a four-kilometre arm by less than the width of a proton.

In 2019 a network of radio telescopes photographed the shadow of a black hole, and its size and shape matched the equations.

The one place it still fails is where it meets quantum mechanics. Joining them is the unfinished problem he was working on when he died.

The chapter stills here are AI-generated; the 1921 portrait, the eclipse plate and the black hole image are real. Sources are below.