Cambridge, Princeton, and Lahore

In 1946 Salam won a special scholarship to Cambridge, in remarkably fortuitous circumstances. The head of the Punjab government had some funds remaining that had been collected to aid the war effort, and had determined to use them to provide scholarships for overseas study, aimed specifically at the sons of small farmers. As it happened, Salam's father had acquired a small piece of land, making him eligible for the scheme. Moreover, at the last minute, Salam heard from St John's College, Cambridge, that another student had withdrawn, making it possible for him to take up his place immediately. The scheme was later cancelled, but leaving Salam as the sole beneficiary, a piece of good fortune that he saw as a special gift of Allah.

In Cambridge, Salam first took a first in the mathematical tripos in two years and added a first in natural sciences in his third year; he found this particularly demanding because of his lack of previous experience of experimental work. After graduating he returned to his home in Pakistan, where, on 4 August 1949, he married his cousin, Amtul Hafeez Hussain (b. 1927). There were three daughters and a son of the marriage. He obtained an extension of his scholarship to allow him to do some postgraduate work, but since it was insufficient to support a wife he had to return alone to Cambridge. Initially he joined the Cavendish Laboratory and worked under Samuel Devons, but he rapidly came to the conclusion that experimental work was not for him. He then turned to Nicholas Kemmer, who suggested that he work with another student, Paul Matthews, who was just then completing a PhD thesis on renormalization theory. Recently developed by Julian Schwinger, Sin-Itiro Tomonaga, and Richard Feynman, this was a way round the long-standing problem of infinities in quantum electrodynamics (QED): any attempt to go beyond the lowest order of approximation had previously yielded nonsensical infinite answers, and renormalization theory provided a way of eliminating these infinities and extracting finite and meaningful answers. Matthews had been trying to extend these ideas to meson theory, then thought to describe the strong nuclear forces. He suggested to Salam that he try to fill in a gap in the proof by Freeman Dyson that renormalization works, and was amazed at the speed with which Salam returned with a complete answer. This was the beginning of a long and fruitful collaboration and close friendship between Matthews and Salam.

Following a very productive year with Matthews in Princeton, USA, Salam returned to Pakistan as professor of mathematics at both Government College and the University of Lahore. He wanted to continue his research while at the same contributing to the development of his country, but he found poor facilities and no encouragement to do research. Finally in 1954 he decided to return to Cambridge as a lecturer and fellow of St John's. Another reason for his departure was the growing and violent antagonism towards the Ahmadiyya sect of Islam, to which he belonged. This sect, founded in 1889 by Mirza Ghulam Ahmad, who declared himself the Madhi, the rightful successor to Muhammad, was regarded by many orthodox Muslims as heretical.

Imperial College and Trieste

Two years later Salam was offered the chair of theoretical physics at Imperial College, London (in succession to Hyman Levy). He arranged that Matthews join him as a reader. Together they built at the college one of the best theoretical physics groups anywhere. Salam was already famous for his work on renormalization, and in 1959 he was elected as the youngest fellow of the Royal Society, at the age of thirty-two. Initially the group was part of the mathematics department, but in 1960 Patrick Blackett persuaded Salam to move into the rapidly expanding physics department which had the advantages of a fine new building and easy interaction with experimenters.

Salam continued his close connections with Pakistan. In 1961 he was appointed scientific adviser by President Ayub Khan. He believed very strongly in the central importance of science and technology in development, and set about implementing these ideas, instituting laboratories for nuclear and space research, and research institutes for wheat and rice. He also tried to tackle the problem of waterlogging and salinity caused by irrigation, but the recommended solutions were never fully implemented. In 1974 Salam resigned in protest when, under Zulfikar Ali Bhutto, the Ahmadiyya were officially declared non-Muslim and deprived of civil rights. Nevertheless, though living mainly in Britain and Italy, Salam retained his Pakistani nationality throughout his life.

Salam deeply regretted having to leave his native country to pursue his chosen career, and determined to do what he could to help others faced with a similar dilemma. His opportunity came when he was appointed to represent Pakistan on the general council of the International Atomic Energy Agency (IAEA). He conceived the idea of an international centre with which scientists from developing countries could be associated, while retaining their links with their own countries. More or less single-handed, and against the opposition of most of the major powers, he persuaded the IAEA to back the idea, provided funds could be found from elsewhere. He found a suitable home for the centre in the city of Trieste, then seeking a new role after being separated from its natural hinterland by the post-war division of Europe. The International Centre for Theoretical Physics (ICTP) received generous backing from the Italian government and the city of Trieste, and was formally inaugurated in 1964; it moved to a new building on the outskirts of the city in 1968. Increasingly from then on, Salam came to spend more of his time in Trieste, though he retained his part-time professorship at Imperial College until his retirement in 1994.

Despite considerable initial scepticism in the scientific community, the ICTP was a great success, providing a lifeline for many thousands of scientists from developing countries. It was soon recognized as a world-class institute. It gradually broadened its scope, from high-energy physics to mathematics, materials science, and much else, and it organized workshops on more environmental topics such as soil science and oceanography. Similar institutes were set up in other subjects in several different countries. In 1997 it was formally renamed the Abdus Salam International Centre for Theoretical Physics.

Salam married twice. His second wife, whom he married on 11 November 1967, was Louise Napier Johnson (1940–2012), then a demonstrator in the zoology department of Oxford University, and later the David Phillips professor of molecular biophysics and a professorial fellow of Corpus Christi College, Oxford. There was one son and one daughter of the marriage.

Salam's research

The basic aim of Salam's research was to find the theories describing the interactions of matter at the most fundamental level. By the early 1950s quantum electrodynamics had provided an excellent and increasingly well-verified theory of electromagnetic forces, and the search was on for similar theories of the strong nuclear forces that bind protons and neutrons in atomic nuclei, and of the weak forces responsible for the beta decay of radioactive nuclei.

Two main themes of Salam's work were symmetries and unification. As more and more fundamental particles were discovered, physicists tried to bring some order into the subject by grouping them together into multiplets related by various symmetries. For example, so far as the strong interactions are concerned, protons and neutrons behave similarly, and it seems reasonable to regard them as two different states of a particle termed the nucleon. The symmetry between the two (isospin symmetry) is not exact, because protons and neutrons do behave differently in regard to electromagnetism, since the proton has an electric charge while the neutron does not. Gradually, other approximate symmetries were discovered, which laid the foundations for the later formulation of the theory of the strong forces, quantum chromodynamics (QCD).

Salam played a significant role in this work, but his major contribution in the 1960s was in unification. He saw the newly discovered approximate symmetries not so much as important in themselves, but rather as clues to the structure of the underlying theory. From a very early stage he was convinced that the solution was to be found in a gauge theory. In gauge field theories, particle interactions are related in a special and intimate way to the basic symmetries. The best-known gauge example is quantum electrodynamics, but in a paper published in the Physical Review in 1954, Yang and Mills proposed a similar theory, based on isospin symmetry, as a possible theory of strong forces. Essentially the same theory was proposed independently by a student of Salam's, Ronald Shaw, though his work was published only as a Cambridge University PhD thesis.

The main focus of Salam's interest was on the weak forces. Over a decade, from the late 1950s, especially with his collaborator John Ward, Salam sought a theory that would describe them and, more ambitiously, a unified theory of weak and electromagnetic forces. There were serious obstacles to be overcome. Superficially, there are big differences between the two. In most field theories, forces between pairs of particles can be understood in terms of the exchange of other particles between them; in quantum electrodynamics these exchanged particles are photons, which have zero rest-mass. However, the weakness and very short range of the weak interactions show that in that case the exchanged particles (called W bosons) must be heavy, which is unnatural in a gauge theory. Another mysterious aspect of the weak interactions is parity non-conservation, or violation of mirror symmetry, first suggested by Lee and Yang in the Physical Review in 1956 and discovered experimentally by Wu and others the following year. How, then, could the theory be unified with QED, in which parity is strictly conserved?

It took more than ten years to solve these problems. To understand the large mass of the W particle, the key idea was 'spontaneous symmetry breaking', as first suggested in the context of the mass of the photon in a plasma by Philip Anderson, writing in the Physical Review in 1963. The resolution of the parity problem came only with the realization that another type of exchanged particle, besides the W, was needed, the Z boson, a suggestion that came from Sheldon Glashow writing in Nuclear Physics in 1961. But it still took several more years to put these two ideas together into a viable unified theory, which Salam called the 'electroweak' theory. It was first written down by Steven Weinberg (in Physical Review Letters, in 1967), and independently by Salam (in Elementary Particle Theory, edited by N. Svartholm, and published in Stockholm in 1968). The theory was triumphantly vindicated a few years later by the observation of Z-mediated interactions at the Conseil Européen de Recherches Nucléaires laboratory in Geneva. For this work, Glashow, Salam, and Weinberg shared the 1979 Nobel prize for physics.

The success of the electroweak unification encouraged many people to try to extend these ideas to incorporate other interactions. Writing in the Physical Review, D (Particles and Fields) for 1973, Salam, with Jogesh Pati, proposed one of the first grand unified theories, unifying strong, electromagnetic, and weak interactions. This theory made the very remarkable prediction that the proton is unstable. Whether this is true or not was still, at the end of the twentieth century, not known. Salam was also, writing in Nuclear Physics, B (Particle Physics) for 1974, with John Strathdee, one of early advocates of supersymmetry, which allows the unification of very different types of particles.

Science and technology in developing countries

Salam was a passionate advocate of the importance of science and technology in development. He argued that only by building up an indigenous science base could the balance between the rich North and the poor South start to be addressed. As scientific adviser in Pakistan he pressed for 1 per cent of the gross national product to be devoted to this aim. He hoped to achieve the same kind of advance that was seen in Japan at the time of the Meiji restoration. But he failed to persuade his government to devote to science and technology anything approaching this figure. When he won the Nobel prize, President Zia ul Haq invited him to visit Pakistan, and awarded him the country's highest honour, the order of Nishan-e-Imtiaz. Salam then offered to donate his Nobel prize money of $66,000 to a fund to provide scholarships for young Pakistanis to study abroad, provided that the government itself would allocate $1 million to the fund. He was again disappointed by the meagre response, and in the end set up a fund independent of the government.

Salam was also very active on a wider international front. The award of the Nobel prize gave him enormous prestige throughout the developing world, particularly in Islamic countries—he was the first Muslim to win one of the science prizes. He received many honours and took every opportunity to argue the case for science in the ‘third world’, in particular urging on his co-religionists a return to the spirit of free enquiry that marked the golden age of Islamic science, when for 500 years the Arabs had led the world. He was fond of telling the story of Michael the Scot, who travelled to Spain in 1217 to study at the great Arab universities of Toledo and Córdoba and who translated Aristotle's works into Latin from Arabic texts. Salam wanted to perform a similar task in reverse, trying to inspire the Islamic world with the value of new knowledge and new ways of thinking.

One result of Salam's endeavours was the founding in 1981 of the Islamic Science Foundation, though with a much smaller budget than he would have liked. He was also the moving spirit behind the formation of the Third World Academy in 1983, with its base in Trieste and Salam as its first president. This was both active and influential, through its fellowship and associateship programmes, research grants, and prizes.

Later years

In the mid-1980s Salam developed a degenerative neurological disorder, somewhat similar to Parkinson's disease, which made his life increasingly difficult. It was eventually diagnosed as progressive supranuclear palsy. Despite this affliction he remained cheerful and continued to travel and to work for as long as he could. Characteristically he branched out into new fields of research, writing papers on a new theory of high-temperature superconductivity with S. Randjbar-Daemi and J. Strathdee in Nuclear Physics, B (Particle Physics) in 1990, and on the problem of the origin of chirality in biology in the Journal of Molecular Evolution in 1991.

Towards the end of his life Salam was confined to a wheelchair, and he found communication difficult. He gave up the post of director of the ICTP in 1994, becoming instead its first president. In the same year he retired from his chair at Imperial College. He died peacefully at his home in Oxford on 21 November 1996, at the age of seventy. He was buried on 25 November at Rabwah in Pakistan, the traditional centre of the Ahmadiyya community. The last prayers for him were attended by 35,000 people. Both his wives survived him, as did his six children.

Salam was a recipient of many honours, including the Atoms for Peace award (1968), the Nishan-e-Imtiaz (1979), an honorary KBE award (1989), membership of twenty-four academies, forty-five honorary doctorates from twenty-eight countries, the first Edinburgh medal and prize (1989), the Cataluña international prize (1990), and the Royal Society's premier award, the Copley medal (1990). He was scientific secretary of the Geneva conferences on the peaceful uses of atomic energy in 1955 and 1958, chairman of the United Nations advisory committee on science and technology in 1971–2 and of the UNESCO advisory panel on science, technology, and society in 1981, and a member of the South Commission. He was a warm and generous man, and an inspiration to many aspiring scientists throughout the world.