



The mathematician Mark Kac divided geniuses into two classes. Ordinary ones whose achievements others will emulate, and magicians whose inventions are so astounding that it is hard to see how any human could have imagined them.

Paul Dirac was one of these magicians.

When he went to Cambridge in 1923, at the age of 21, the world of physics was in turmoil. Experiments had shown that classical physicists could not explain the behaviour of atoms. The old principles of Isaac Newton didn't seem to apply to the microscopic world.

The Dirac equation

Dirac soon developed his own widely-acclaimed theory of quantum mechanics. His theory included wave mechanics, the version of quantum mechanics developed by Erwin Schrodinger, and matrix mechanics, the theory put forward by Werner Heisenberg.

At the time, it seemed miraculous. Albert Einstein described Dirac's theory as "the most logically perfect of quantum mechanics".

Perfect as it was, the theory was only a reformulation of a newly-discovered branch of physics. In 1928, Dirac made a breakthrough. He combined the theories of quantum mechanics and Einstein's special relativity.

The resulting Dirac equation, still widely used today, was able to explain the mysterious magnetic and "spin" properties of the electron. Like so many great discoveries, it required an extraordinary leap of imagination.

The nuclear physicist Niels Bohr described Dirac as "the purest soul in physics".

But there was a conundrum. The equation had two solutions, one representing the electron, the other representing its opposite, a particle with negative energy and positive charge, that had never been seen or suspected before.

Dirac concluded that each electron had an "anti-particle". The two could be created or destroyed in matching pairs. Dirac had predicted the existence of anti-matter, which makes up, at least in principle, half the universe.



The purest soul in physics

Neils Bohr on Paul Dirac

Heisenberg judged this to be the supreme achievement of 20th Century physics.

A few months later, a Californian experimenter, Carl Anderson, discovered anti-matter for real, confirming Dirac's genius. The acts of creation and annihilation are now the mainstay of particle accelerators and high-energy physics.

For his achievement, Paul Dirac was awarded the Nobel Prize for Physics in 1933 at the age of 31.

Musing on who was the 20th Century's greatest English-speaking poet, Graham Farmelo, a physicist at London's Science Museum, nominated Paul Dirac for "his amazing ability to write down fundamental equations, the poems of science".

When Dirac himself was once asked how he had found the Dirac equation, he replied: "I found it beautiful."

Introvert

So, why is Paul Dirac such an unknown quantity outside the scientific world?

Certainly he lived in an era in which the cult of personality was far less pronounced than it is today. But then everyone has heard of Einstein.

The answer may lie in Dirac's pronounced shyness.

He was born in 1902 to an English mother and a Swiss father. He attended secondary school in Bristol where his father taught French.

Charles Dirac was famously strict and would only allow his son to speak in perfect French at the dining table. He was mainly quiet.

As someone who actively avoided any kind of attention, he wanted to refuse the Nobel Prize in 1933 in order to avoid the publicity.

He accepted it only when advised that, as the first person to refuse a Nobel Prize, the publicity would be even greater.

As with so many of his ilk, he had an unorthodox precision in his social interaction. So-called Dirac tales are frequently told in physicist circles.

For example, at a meeting in a castle, a guest remarked to him that, in a certain room, a ghost would appear at midnight. Dirac asked "Is that midnight Greenwich time or daylight saving time?"

After he retired from Cambridge in 1969, Paul Dirac moved to Florida State University in the United States. He died in Tallahassee, Florida in 1984.

While his achievements may mean little to non-scientists, the Dirac equation has had implications for us all.

As John Enderby, vice-president of the Royal Society puts it: "Without understanding the origin of spin, and the Dirac statistics, you wouldn't have mobile phones, computers or anything else that runs on electronics."

Paul Dirac's achievements are marked in the Royal Society's Summer Science Exhibition.