Tag Archives: Dirac

Anticipating Antimatter

Collider exhibition curator Dr. Harry Cliff blogs on Dirac’s discoveries and anticipating antimatter.

It was 86 years ago on Saturday (8 February) that one of the most important scientific papers of the 20th century appeared in the Proceedings of the Royal Society. Written by the young British physicist Paul Dirac, it was simply titled A Quantum Theory of the Electron, and was nothing short of a theoretical triumph.

Paul Dirac

Paul Dirac. Image: Nobel Foundation

In it, Dirac had set out to solve a problem that was occupying some of the greatest minds in physics. To date, quantum mechanics had failed to explain the fine detail of atomic spectra – the discrete wavelengths of light emitted and absorbed as electrons hop between different energy levels in atoms. In particular the electron had to be given a strange property known as “spin” to explain the number of different energy levels.

Spin itself was a rather mysterious quantity. It suggested that that the electron behaved as if it was rotating rapidly on its axis, but a quick calculation showed that this couldn’t be true – the electron would have to be spinning faster than the cosmic speed limit, the speed of light, something forbidden by Einstein’s theory. It also had to be bolted on to quantum mechanics like a clumsy afterthought, without any explanation for its origin.

Dirac, for whom mathematical beauty in the laws of physics was almost a religious cause, was deeply dissatisfied with this awkward situation. He believed that the problem lay in combining the two pillars of modern physics, quantum mechanics (the theory of the very small) and relativity (the theory of the very fast).

He was after an equation describing the behaviour of electron that was consistent with both theories, and also explained the known properties of the electron. Rather surprisingly perhaps, the approach he took was to guess.

An educated guess mind, based on some properties he knew the correct equation must possess and also on his aesthetic desire for simplicity and beauty. Working methodically, he tried equation after equation, discarding them one by one until in late November 1927 he came upon a solution.

Dirac's equation

Dirac’s equation

The equation was perfect. Not only did it accurately reproduce the known energy levels of the hydrogen atom, the property of spin naturally appeared in the equation, without the need to be stuck on by hand afterwards. Spin itself now seemed to be an inevitable consequence of combining relativity and quantum mechanics.

St. John’s College, Cambridge, where Dirac discovered his famous equation.

St. John’s College, Cambridge, where Dirac discovered his famous equation. Image: Andrew Dunn

Dirac, though famously reserved, must have been jumping for joy (though perhaps only in his head). He had pulled off a coup so impressive that his German competitors, Jordan and Heisenberg were left stunned and deflated.

As news spread of Dirac’s success, the man himself was growing increasingly nervous about an odd feature of his equation, one that he had brushed under the carpet in his Royal Society paper.

The equation itself had four solutions, and each solution represented a state that the electron could be in. Two of these corresponded to the garden-variety electron with negative electric charge, but the other two described an electron with positive electric charge and negative energy.

This made no sense whatsoever. No one had ever seen a positively charged electron, and worse still, if these negative energy states existed then ordinary electrons should be able to fall into them, causing an electron to spontaneously switch its charge from negative to positive.

For all the success of the Dirac equation, these negative energy electrons could well have spelt its doom, and no-one was more acutely aware of this than Dirac himself. In fact, this “problem” turned out to be Dirac’s greatest contribution to physics.

It would take Dirac more than three years to understand the true meaning of this extra set of solutions. He had first thought that these negative energy, positively charged electrons might in fact be protons – the positively charged particles inside the atomic nucleus – but he soon realised that this would imply that protons should have the same mass as electrons, when in fact they are roughly 2000 times heavier.

What Dirac eventually reasoned was that these odd solutions actually represented a completely new type of particle, a sort of mirror image of the electron that he dubbed the “anti-electron”. Anti-electrons would look completely identical to ordinary electrons, but positively charged. He also reasoned that other particles like protons should also have anti-versions, and that when a particle met its anti-particle they would annihilate each other.

This must have seemed far-fetched at the time; after all, no one had ever seen an anti-particle. But Dirac was convinced by the beauty of his equation, and in one of the most stunning episodes in modern physics, was proven right just a year later, as Carl Anderson spotted an anti-electron in cosmic ray experiments.

It’s hard to overstate what Dirac had achieved. Through the power of sheer thought, he had predicted the existence of a completely new type of stuff, a stuff never before imagined by scientists. This stuff, what we now call antimatter, is just as real as the stuff you and I are made from, but for some reason doesn’t exist in large quantities in our Universe. This is in fact one of the greatest unsolved mysteries in physics, and one that physicists at the Large Hadron Collider are trying to solve.

Find out more about antimatter by watching this short video or by visiting the Collider exhibition before the 5th May 2014.

A Nobel Tradition

Content Developer Rupert Cole explores the most famous science prize of all, and some of its remarkable winners. 

Today, science’s most prestigious and famous accolades will be awarded in Stockholm: the Nobel Prize.

Before we raise a toast to this years’ winners in physics, Peter Higgs and Belgian François Englert, let’s take a look back at the man behind the Prize, and some of its winners.

Alfred Nobel

A Swedish explosives pioneer who made his millions from inventing dynamite, Alfred Nobel left in his will a bequest to establish an annual prize for those who have “conferred the greatest benefit to mankind”, across five domains: physics, chemistry, physiology or medicine, literature and peace. To this end, he allocated the majority of his enormous wealth.

Alfred Nobel. Credit: Science Museum / SSPL

Alfred Nobel. Credit: Science Museum / SSPL

When Nobel’s will was read after his death in 1896, the prize caused an international controversy. Unsurprisingly, Nobel’s family were not best pleased, and vigorously opposed its establishment. It took five years before it was finally set up and the first lot awarded – the 1901 physics accolade going to Wilhelm Rontgen for his 1895 discovery of x-rays.

Paul Dirac’s maternal mortification

When the phone rang on 9 November 1933, the exceptionally gifted yet eccentric Paul Dirac was a little taken back to hear a voice from Stockholm tell him he had won the Nobel Prize.

The looming press attention, which had always surrounded the Nobels, made the reclusive Dirac consider rejecting the award, until Ernest Rutherford – JJ Thomson’s former student and successor as Cavendish professor – advised him that a “refusal will get you more publicity”.

Under different circumstances Rutherford had been similarly “startled” when he found out he was to be given a Nobel – a physicist through and through, he was awarded the 1908 Prize in Chemistry, joking his sudden “metamorphosis into a chemist” was very unexpected.

Dirac shared the 1933 physics prize with Erwin Schrödinger – famed for his eponymous equation and dead-and-alive cat – for their contributions to quantum mechanics. Each was allowed one guest at the award ceremony held at the Swedish Royal Academy of Science. Schrödinger brought his wife, Dirac brought his mother.

Quantum theorists: Wolfgang Pauli and Paul Dirac, 1938. Credit: CERN

Quantum theorists: Wolfgang Pauli and Paul Dirac, 1938. Credit: CERN

Florence Dirac did what all good mothers do: embarrass her son in every way imaginable. The first incident came at a station café in Malmo, where in this unlikely setting an impromptu press conference took place.

Dirac, who had been described by the British papers as “shy as a gazelle and modest as a Victorian maid,” was asked “did the Nobel Prize come as a surprise?” Before he could answer, Dirac’s mother butted in: “Oh no, not particularly, I have been waiting for him to receive the prize as hard as he has been working.”

The next embarrassment came when Mrs Dirac failed to wake up when the train reached Stockholm. She was ejected by a guard, who had thrown her garments and belongings out of the carriage window. The Diracs arrived late, and meekly hid from the attention of the welcoming party – who had been wondering where they were.

The third and final maternal faux pas came at Stockholm’s Grand Hotel. The pair had been booked into the finest room – the bridal suite. Mrs Dirac, displeased, demanded a room of her own, which Dirac paid for out of his own pocket. It doesn’t matter if you’ve co-founded quantum mechanics, predicted antimatter and won the Nobel Prize; mothers will be mothers.

Peter’s Pride

Like other humble laureates before him, Peter Higgs wished to duck out of the press furore surrounding the Nobel. At the time of the announcement on the 8th October there was a nail-biting delay. The cause? The Nobel committee could not get hold of Higgs, who had turned his phone off and planned to escape to the Scottish Highlands.

As Peter Higgs revealed to me at the opening of the Collider exhibition at the Science Museum, if it was not for a dodgy Volkswagen beetle or public transport, Peter would have made it to the Highlands on Nobel day. Instead, he just laid low in Edinburgh.

Peter Higgs (right) with friend Alan Walker and the personalised bottles of London Pride at Collider opening. Credit: Science Museum.

Peter Higgs (right) with friend Alan Walker and the personalised bottles of London Pride at Collider exhibition opening. Credit: Science Museum.

At the Collider launch last month, we celebrated with Higgs in the appropriate way: over a personalised bottle of London Pride ale – the same beverage he chose in favour of champagne on the flight home from CERN’s public announcement of the Higgs boson discovery. So, when Englert and Higgs receive the honour today, let’s all raise two glasses: an English Ale and a Belgian Blonde!

For more on many of the Nobel prize-winning discoveries in physics history, including those of Dirac, Englert and Higgs, visit the Collider exhibition at the Science Museum.