Abdus Salam

Abdus Salam shared the 1979 Nobel Prize in Physics with Steven Weinberg and Sheldon Glashow.

In the previous century, James Clerk Maxwell brought about the first great unification in physics when he showed that the electric and magnetic forces are manifestations of a single force – the electromagnetic force, responsible for light.

In 1968 Salam, Weinberg, and Glashow independently produced a second great unification, uniting the electromagnetic force with the weak nuclear force to produce the electroweak force.

Beginnings

Mohammad Abdus Salam was born on January 29, 1926 in Santokdas, Punjab, British India (now Pakistan). His father was Chaudhry Muhammad Hussain, an education official, and his mother was Hajira Nabi Baksh. Abdus was the eldest of their eight children.

Abdus attended school in the town of Jhang. From an early age he showed enormous talent in mathematics and science. He also developed a lifelong love of poetry in English, Persian, and Punjabi.

At age 14, he amazed the people of Jhang when he achieved the highest marks ever seen in Punjab University’s entrance examination. Nearly the whole town turned out to celebrate his achievement. Not surprisingly, he was given a university scholarship. He graduated with a Master’s degree in Mathematics in 1946, age 20.

Cambridge and Important Prizes

Salam was awarded a three year scholarship to study for a Bachelor’s degree at St. John’s College, Cambridge, UK. After two years he graduated with first class honors in Mathematics. A year later, he graduated with first class honors in Physics.

Salam stayed at Cambridge for graduate studies. In 1950, a year after starting his PhD, he was awarded the university’s Smith’s Prize for the most outstanding pre-doctoral contribution to physics at the university.

Salam completed his PhD thesis in 1951: Developments in quantum theory of fields. This was a rather brilliant work: in addition to making his name as a physicist, it resulted in him winning a share of the highly prestigious Adams Prize for mathematical sciences in 1956. To be eligible for the prize, candidates had to be under forty years of age and living in the UK. Previous winners included James Clerk Maxwell, J. J. Thomson, and Subrahmanyan Chandrasekhar.

Pakistan and the UK

In 1951, age 25, Salam returned to the city of Lahore, in now independent Pakistan, to teach Mathematics at Government College. He was filled with hope that he could inspire a new generation of young people to become scientists and modernize society. The following year, he was appointed Head of Mathematics at Punjab University.

In 1953, Lahore suffered riots in which a very large number of Ahmadis were murdered. Ahmadis consider themselves to be Muslim, but are seen as heretics by many other Muslims. Salam, who was an Ahmadi, decided to leave Pakistan and return to Cambridge.

Salam spent four years as a lecturer at Cambridge until, in 1957, age 31, he was appointed Professor of Theoretical Physics at Imperial College, London.

In 1959, he was elected a Fellow of the Royal Society.

The Electroweak Force

The Initial Problem

At a physics conference in Seattle in 1956, Salam became very excited listening to Chen-Ning Yang describe parity violation – Yang and his colleague Tsung-Dao Lee would be awarded the 1957 Nobel Prize in Physics for this work.

Yang described how parity, a property physicists believed was always conserved – like energy, momentum, and electric charge – need not be conserved in weak nuclear interactions, such as beta decay.

The implication was that Nature could tell the difference between left and right. Why, Salam wondered, was left-right symmetry violated in weak interactions, but conserved in electromagnetic interactions?

He convinced himself, correctly, that the answer lay in a gauge theory – the first of which had been Maxwell’s formulation of electrodynamics.

Predicting the W and Z Bosons

Salam worked on the problem with his PhD student Ronald Shaw and, in 196l-62, Steven Weinberg spent a year with Salam at Imperial College.

The scientists produced a theory saying that while the electromagnetic force was transmitted by massless photons, there must exist particles with mass involved in a unified force – the electroweak force. They theorized the existence of W and Z bosons.

W and Z bosons, they said, are the particles with mass that take part in beta decay of an atomic nucleus.

The net outcome of beta decay is that a neutron turns into a beta particle (a high energy electron), a proton, and an electron antineutrino.

The W boson is an intermediate particle in beta decay. The neutron first decays into a W boson. In the case of beta decay, the W boson is negatively charged, written W^–. It hangs around for an incredibly short amount of time: 3 × 10^-25 seconds, before decaying into a proton, beta particle, and antineutrino.

Z bosons carry no electric charge, but carry momentum. They are observed in very high energy situations.

The fact that the force carriers of the electroweak force – the W and Z bosons – have mass while photons have no mass was a major problem, because in an SU(2) gauge theory (particle physics jargon for the gauge framework Salam and others were working within) bosons must have no mass.

Peter Higgs solved the problem of bosons with masses in 1964 with the Higgs mechanism, predicting the Higgs boson. (Higgs was awarded the 2013 Nobel Prize in Physics for his work.)

W and Z Bosons are Discovered

Z boson interactions, called weak neutral currents, because Z bosons carry no charge, were observed at CERN in 1973.

W bosons were observed in 1983.

Taken together, the SU(2) gauge theory of the weak interaction, plus the electromagnetic interaction, plus the Higgs mechanism produces the Glashow-Weinberg-Salam model, finalized in 1968. The model successfully unified the electromagnetic and weak forces into the electroweak force.

Nobel Prize

Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics:

“for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current”.

Scientific Achievements

Salam’s best known contributions to science include:

• two-component neutrino theory and the prediction of the inevitable parity violation in weak interactions
• gauge unification of weak and electromagnetic interactions-the unified force is known as the “electroweak” force, named by Salam
• predicted existence of weak neutral currents and W particles and Z particles before their experimental discovery
• symmetry properties of elementary particles and unity symmetry
• renormalization of meson theories
• gravity theory and its role in particle physics including two tensor theory of gravity and strong interaction physics
• supersymmetry theory including formulation of superspace and formalism of superfields

Other Work

During the early 1960s, Salam played significant roles in establishing the Pakistan Atomic Energy Commission (PAEC) and Pakistan’s Space and Upper Atmosphere Research Commission (SUPARCO).

Salam was passionate about promoting science in developing countries. He recalled there had once been a golden age of Islamic science, when for hundreds of years Islamic scientists had led the world. He urged Muslims to regain the spirit of free enquiry that existed in those times.

Salam was the founder of the Third World Academy of Sciences (TWAS) and the International Centre for Theoretical Physics (ICTP).

He donated all the money he received from his Nobel Prize to setting up a fund in Pakistan to promote educational opportunities.

Personal Life

In 1949, age 23, Salam married his 22-year-old cousin Amtul Hafeez Hussain. The couple had three daughters and a son. According to Islamic law, a man may have more than one wife. In November 1967, Salam married the biophysicist Louise Johnson, with whom he had a son and a daughter.

In 1974, Salam was saddened when Pakistan’s government passed a law declaring Ahmadis to be non-Muslims. In 1980, after he received the Nobel Prize, he was invited as an honored guest to the Quaid-e-Azam University in Pakistan’s capital city, Islamabad. The ceremony was cancelled because extremists threatened to break Salam’s legs if he turned up at the university.

In the mid-1980s, Salam began suffering from a degenerative neurological disorder. Eventually he was confined to a wheelchair. He retired from his chair at Imperial College in 1994, age 68.

Abdus Salam died peacefully, age 70, at home in Oxford on November 21, 1996. He was buried four days later in the Ahmadi city of Rabwah, Pakistan.

He was survived by his children and his wives.

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Further Reading
F. J. Dyson
Abdus Salam, 1926–1996
Biographical Memoirs of the American Philosophical Society, Vol 143, No. 2, June 1999

T. W. B. Kibble
Salam, Muhammad Abdus 1926-1996
Oxford Dictionary of National Biography, 07 January 2016
https://doi.org/10.1093/ref:odnb/64072

Acknowledgements
Image of Salam courtesy of the Nationaal Archief, the Dutch National Archives, and Spaarnestad Photo.