100 incredible years of physics – nuclear physics

“He was the first person to understand that atoms contain tiny, dense nuclei in their centres, and he predicted the existence of the neutron, the other constituent of a nucleus, long before it was discovered,” Al-Khalili continues. “I think he can rightly be regarded as the father of nuclear physics.”

Over a century on from former IOP President Rutherford’s famous alpha scattering experiment at the University of Manchester that exposed the nucleus, it is worth reflecting on the huge progress nuclear physics has made, as well as the challenges the discipline has faced. 

One of the greatest accomplishments towards understanding the nucleus in this early period was made by a former student of Rutherford’s, IOP Honorary Fellow James Chadwick. Al-Khalili refers to Chadwick as a “wonderful scientist who also had a big impact on society”. 

Though he developed the UK’s nuclear power programme in the 1940s and 50s and was head of the British team that worked on the Manhattan Project during World War II to build the first atomic bomb, Chadwick is perhaps best known for his 1932 discovery of the neutron. In the same year, IOP Honorary Fellows John Cockroft (IOP President 1954–56 and 1960–62) and Ernest Walton split the atomic nucleus for the first time and Mark Oliphant achieved the first artificial fusion reaction. Nuclear fission would follow six years later in experiments led by IOP Honorary Fellow Otto Hahn and Lise Meitner. 

Given these experimental landmarks, it is no surprise Al-Khalili refers to the 1920s and 30s as the “the golden age of nuclear physics”. But huge strides were also made towards the theoretical understanding of the nucleus in these early decades. In 1920, Arthur Eddington (Physical Society President 1930¬–32) correctly predicted that nuclear fusion powered stars. Then, in 1935, Japanese physicist Hideki Yukawa proposed a new force, which he called the strong force, to bind neutrons and protons in the atomic nucleus.

Things became clearer when Maria Goeppert Mayer published the nuclear shell model in 1950. “Mayer showed that even inside the crowded dense nucleus, where you’d think protons and neutrons are pretty much stuck in place because it’s so dense, they still arrange themselves in shells, just like electrons do around the nucleus” explains Al-Khalili. “Without the shell model, we wouldn’t be able to understand many nuclear reactions, nuclear lifetimes, nuclear shapes or nuclear behaviour, so the shell model was certainly very instrumental.”

Research since Mayer’s insight has largely focused on creating heavier elements that extend the periodic table, and gaining a deeper understanding of unstable nuclei and thereby the structure of matter. It was the latter that Al-Khalili focused on when he made his proudest achievement in nuclear physics in the mid-1990s. 

“I was developing mathematical models of neutron-rich isotopes of light nuclei,” he recalls. “Nuclear physicists had discovered a new phenomenon called the neutron halo, which is what you get when you keep adding more and more neutrons to a nucleus until the last one or two are very weakly bound and tend to float outside the nuclear core.” With colleagues at the University of Surrey, Al-Khalili discovered that neutron halos were about 50% bigger than anyone else had thought. 

“There was that ‘Eureka!’ realisation that I had discovered something that was going to lead to papers, lots of talks at conferences and people referencing my work. It wasn’t going to change the world, but I had discovered something new about the building blocks of matter. I remember sitting in front of my computer screen in my office – I thought, before I go and tell anyone I’m going to savour the few minutes that I know this before anyone else does.”

Around the same time, Al-Khalili’s career was starting to branch out into new territory under the influence of his mentor, experimental physicist and IOP Fellow William Gelletly. “It was probably Bill’s encouragement in the early 1990s that really gave me the confidence to get more involved in science communication,” he says. This encouragement led Al-Khalili to accept an invitation to be the IOP Schools and Colleges Lecturer for 1997, an annual role delivering physics lectures to 14–18 year old students at schools around the country. 

From heroes to villains and back again

Now, as much an award-winning author and broadcaster as a nuclear and quantum physicist, Al-Khalili is uniquely positioned to explain the impact of nuclear physics on society. He describes how in the mid-20th century nuclear physics offered so much hope and promise for understanding the world around us and benefiting society, that nuclear scientists were placed on a pedestal. “I remember my mother saying when I was a boy that if you really want the hero in a novel to be exotic in some sense, he’d be the tall, dark, handsome nuclear scientist – it was this great hope.”

However, hope slowly turned to fear as the nuclear arms race gathered pace and the world started to live under a constant threat of annihilation brought by the proliferation of these nuclear weapons during the Cold War. “I understand the concerns about nuclear weapons,” adds Al-Khalili. “After all, I remember as a student in the 1980s being a member of CND going out on marches.” 

When combined with accidents at nuclear power plants, like Three Mile Island, Chernobyl and Fukushima, it is understandable that public attitudes to nuclear physics changed for the worse. ‘Nuclear’ became a dirty word. It prompted some nuclear research groups to take ‘nuclear’ out of their names, says Al-Khalili: “They became the ‘sub-atomic physics group’ – somehow ‘sub-atomic’ was less scary.” Even a hugely beneficial technology like nuclear magnetic resonance (NMR) was renamed magnetic resonance imaging (MRI) to make it more palatable for the public. 

Today, though, attitudes are starting to shift once again – this time, at least in part out of necessity. “The fact is that the climate crisis we’re going through now is a far, far greater threat than the risk of something going wrong in a nuclear power station,” opines Al-Khalili. “Mistakes were made with old style reactors but we don’t build reactors like that anymore, and we don’t generate the sort of waste that we used to generate before, so I feel nuclear fission still has a role to play in our energy needs in the 21st century.”

Still much to learn

Turning the page on a tumultuous century of nuclear physics filled with both highs and lows, what’s in store for the next 100 years? 

“On the theoretical side, the big challenge is coming up with a unified theory of the atomic nucleus,” explains Al-Khalili. “We have different mathematical models for different nuclei of different sizes, so there are completely different approaches to describe how protons and neutrons connect together.” To develop such a unified theory, nuclear physicists today are using advanced computational techniques that start from the fundamental nucleon–nucleon force, the glue that holds nuclei together.

Meanwhile, on the experimental side, Al-Khalili highlights the synthesis of heavier and heavier elements as one of the greatest challenges for the next generation of nuclear physicists. The 92 naturally occurring elements have been supplemented by 26 new ones over the past 70 years. But creating even heavier elements presents vast challenges: “We have to fuse heavy nuclei together – bring them together too hard and they’ll break up, too gently and they won’t even stick – so a very big challenge is to understand how heavier nuclei synthesise.” 

Al-Khalili hopes that future nuclear physicists who help to solve these issues will be working in the UK, building on a rich history that began in Rutherford’s laboratory at the University of Manchester and continues in no small part through the IOP’s leadership in nuclear physics, as exemplified by the 2012 IOP Review of Nuclear Physics Research, which led to the establishment of a new nuclear theory group at the University of York to add to the two established groups at Manchester and Surrey universities. “There’s a small band of nuclear researchers, both theorists and experimentalists, in a dozen or so universities around the country,” Al-Khalili says. “But they certainly punch above their weight.” 

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