THE CENTRAL FACTS FROM THE COURSES YOU ALWAYS MEANT TO TAKE, IN 25 LECTURES

Werner Heisenberg's glory moment came when he was still very young - just 24 - in the summer of 1925. He'd felt trapped at his university in Germany; his hay fever was bad and hecouldn't bear the obscure science he was supposed to accept - with images of atoms as rickety little solar systems, with electrons orbiting at fixed altitudes around the centre.

Who had ever seen such a thing? It was unwieldy, just a wind-up toy model, which you were supposed to accept on trust. And Heisenberg, bellowingly argumentative, hated having to take anything on trust.

He went to a North Sea resort - the ocean air cured his hay fever - and began to work out what would happen if you stopped bothering about these invisible insides of an atom.

Why not just list the sorts of energy that go in, list the different sorts of energy that come out, and simply find a rule, a system of calculations, for linking the two lists?

No one had dreamed of working like this before - which only encouraged Heisenberg to go forward - and after a final, stupendous night of calculations, he had it.

He climbed up on a high rock in his excitement, waiting for the dawn like a hero from a German romantic myth. He would be famous, and wouldbe able to command respect.

But then he came back to earth, back to his university, and although a few colleagues were on his side, gradually, furiously, he realised that the world's physicists were not convinced by what he'd done.

There had to be some hidden mechanism inside the atom, they thought, some pattern of moving electrons, which young Heisenberg perhaps hadn't been intelligent enough to find, which would properly explain those cold linking rules he had found.

Schrodinger was the worst culprit, a physicist almost the opposite in temperament to Heisenberg. He was from Vienna, which didn't help, as there's a type of educated Viennese who has a way of driving Germans nuts: observing from afar, being quietly superior. Heisenberg leapt up from the audience at one of Schrodinger's seminars, trying to protest at the misunderstandings of his theory, but he proved no match for the sarcastic older man.

If you've made a great discovery, and then been disbelieved and humiliated, what do you do next? Heisenberg went back to work, concentrating harder than ever.

Schrodinger and his other opponents insisted that any one particular electron inside an atom was moving in very exact ways. Fine: he would challenge them to see that individual electron moving.

First they'd have to shine a light wave or some other energy down on the atom to light up the electron in there. But a small blast of light wouldn't be enough to locate the electron clearly, while a large blast would be so strong as to jolt it out of position.

Heisenberg showed that you can measure either the electron's location or its velocity, but you can't measure both with full accuracy at once. You're always going to be a little bit unsure - uncertain - about the complete mix.

This is the Uncertainty Principle, published in 1927 and one of the key points of quantum mechanics. At the smallest underlying levels, it says, the universe is not rigidly linked up. There are gaps, inherent uncertainties, due not to some temporary limit of technology but to the fact that Nature really does work that way.

Now Heisenberg had fame. At first, he was content just to lord it up at the universities, but then he was called to greater honours and proudly led Nazi Germany's A-bomb project.

After the war he was hated, but his work had entered the bedrock of physics. Electrons are not only features of objects incredibly far away but suffuse our world: when light comes out of a bulb, it's because electrons are making sudden - quantum - jumps, in accordance with his rules.

The Uncertainty effects average out when you move up to larger objects.

For example, if your AA map tells you that the distance from London to Birmingham is 120 miles, it makes no difference to you if it turns out to be a millionth of a millimetre further. If, however, the effects on the atomic level, where the Uncertainty Principle does dominate, are channelled carefully enough, they can be useful. Computers, fibre-optic cables and CD players all depend on his rules.

Heisenberg lived until 1976, but he almost didn't make it through the war. In late 1944, he was giving a private seminar in neutral Switzerland, and an American assassin, the ex-athlete Moe Berg, was planted in the audience.

If Heisenberg showed evidence that his bomb project was on the right tracks, he was to be killed. But the talk was too technical for Berg to follow. His scrawled notes from the meeting survive in CIA archives: "As I listen, I am uncertain - see: Heisenberg's Uncertainty Principle - what to do to H ..."

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