Book & Author
Professor Dr Abdus Salam: Unification of Fundamental Forces — The First of The 1988 Dirac Memorial Lecturers
By Dr Ahmed S. Khan
Chicago, IL
Professor Dr Abdus Salam (January 29, 1926 – November 21, 1996) was a prominent Pakistani theoretical physicist. During the 1950s and 1960s he explored the relationship between two of the four basic forces governing nature then known to scientists: the electromagnetic and weak forces. In 1979, the Nobel Prize in Physics was awarded jointly to Sheldon Lee Glashow, Abdus Salam and Steven Weinberg "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."
Paul Adrien Maurice Dirac (August 8, 1902 – October 20, 1984) was one of most significant physicists of the 20th century. He made pivotal contributions to the developments of quantum mechanics and quantum electrodynamics. Among his major discoveries was the formulation of Dirac equation which describes the behavior of fermions and predicted the existence of antimatter. In 1933, he shared the Nobel Prize in physics with Erwin Schrodinger "for the discovery of new productive forms of atomic theory." To honor his memory, Dirac's college, St John's of Cambridge, endowed annual lectures to be held at Cambridge University.
The book Unification of Fundamental Forces is an expanded version of the third Dirac Memorial Lecture, given in 1988 by the Nobel Laureate Professor Dr Abdus Salam. His lecture presents a summary of the progress in modern particle physics — from its inception at the turn of the century to the present theories – in quest of the unification of all the fundamental forces. In addition to Dr Salam’s lecture, two previously unpublished lectures by Paul Dirac, and Werner Heisenberg, are also included in the book’s four sections: 1. Unification of fundamental forces Abdus Salam; 2. History unfolding: an introduction to the two 1968 lectures by W. Heisenberg and P. A. M. Dirac by Abdus Salam; 3. Theory, criticism, and a philosophy Werner Heisenberg; and 4. Methods in theoretical physics Paul Adrian Maurice Dirac.
Professor Dr Abdus Salam’s lecture on “Unification of fundamental forces” consists of seven parts: 1. Introduction, 2. The unifying concepts of physics in the past, 3. The concept of elementarity and nuclear forces, 4. The unification of the weak nuclear force with electromagnetism, 5. The strong nuclear force as a gauge force and the standard model, 6. Beyond the standard model, and 7. Envoi.
In the foreword, John Taylor of University of Cambridge, commenting on the unification observes: “From time to time, science succeeds in unifying apparently diverse sets of phenomena. These unifications provide some of the most impressive achievements in the sciences. Unification, in this sense, means understanding how apparently different effects are really aspects of a single underlying thing. In the nineteenth century, for example, electricity and magnetism were unified. They are different, but they are intimately interconnected, and in general situations it is impossible to imagine one without the other. In physics, one of the unifications of the present century has been that of electromagnetism with the weak force. These are apparently totally different. Electromagnetism ranges across any distance, from atomic to astronomical.”
Further elaborating on unification, John Taylor notes: “The weak force, on the other hand, operates deep within the atomic nucleus (for example) to produce radioactive decay. This unification was mainly the work of three men, Sheldon Glashow, Abdus Salam and Steven Weinberg. They made use of earlier theoretical ideas of two British physicists Peter Higgs and Tom Kibble. Finally, a Dutch theoretician, Gerard t'Hooft, put the equations into a form in which any competent physicist could handle them in standard ways. The experimental implications of all this are at present being tested with the new high-energy colliders at CERN, Fermilab and Stanford. A whole new branch of physics is unfolding, just as the science of electromagnetism did in the last century.”
Reflecting on lectures by Salam, Dirac and Heisenberg, John Taylor states: “In this volume, Abdus Salam, one of the discoverers of the electro-weak unification, writes about unification in physics in the past and present, and describes hopes for the future. The whole enterprise takes place within the framework of quantum mechanics, of which one of the main architects was the great English physicist Paul Dirac. The volume is based upon a Dirac Memorial Lecture given at Cambridge University in 1988. Salam and other authors of the electro-weak theory operated squarely within the framework of quantum theory, which was laid down in the first half of this century. Two of the great founders of quantum theory were Werner Heisenberg and Paul Dirac. A lecture by each of these is included in this volume. These, like Salam's lecture, provide fascinating insights into the thoughts of creative theoretical physicists.”
Professor Dr Abdus Salam starts his lecture by paying tributes to Dirac: “I am honored to speak about P. A. M. Dirac whom we all loved and whom I so greatly admired…Paul Adrian Maurice Dirac was undoubtedly one of the greatest physicists of this or any century. In three decisive years,1925, 1926, and 1927, with three papers, he laid the foundations, first of the Theory of Quantum Mechanics, second of the Quantum Theory of Fields, and third - with his famous equation of the electron - of the Theory of Elementary Particles. (In the course of this lecture, I shall explain the relevant concepts of the Quantum Theory of Fields and the Dirac equation for the electron.)”
Describing Dirac’s personality, Dr Salam observes: “When one met Dirac, one could see the complete and utter dedication of a great scientist. One could feel with him the pleasure of scientific creation at its noblest, and the highest personal integrity. He had a childlike simplicity about him. His lucidity and clarity of thought made him a legend. He was undoubtedly one of the greatest human beings.”
Discussing the history of unification, Dr Salam states: “The first name I would like to mention in this context is that of Al-Biruni who flourished in Afghanistan a thousand years ago. One might not think of modern Afghanistan as a likely place where high class physics could be done. Al-Biruni however, to my knowledge, was the first physicist to say explicitly that physical phenomena on the Sun, Earth and the Moon obey the same laws. This deceptively simple idea is the basis of all of science as we know it…”
Elaborating further on history, Dr Salam notes: “The next person to mention in this context is Isaac Newton. Around 1680 he asserted that the force of ‘terrestrial’ gravity (which makes apples fall to the ground, and which in Newton's view was a universal force) was the same as ‘celestial’ gravity (the force which keeps planets in motion around the Sun). Such a force is long-range. Its effects can be felt at any distance, though attenuated by the square of the distance between the two ‘gravitating’ objects concerned.”
Continuing with the history and referring to the unification of electricity and magnetism, Dr Salam observes: “The next unification of fundamental forces was postulated some 150 years later. I wish to recall Faraday and Ampere in the context of electromagnetism - the 'force of life' (so-called because all chemical binding is electromagnetic in origin, and so are all phenomena of nerve impulses) …. It is thus an environmental factor which distinguishes between electricity and magnetism – namely, whether or not the electric charge is in motion or not. This is the essence of the unification of electricity and magnetism…. This unification of the two disjoint fundamental forces of electricity and magnetism was the basis of electrical current technology of the nineteenth century which depended on the generation of electric currents by rotating a coil of wire between the two poles of a magnet. This is the basis of the electric motors and electric dynamos which led to the electric power stations. These followed on the remarkable unity of two of the disparate forces of nature.”
Describing the work of Maxwell, Dr Salam states: “…Classical electromagnetism found its culmination fifty years later in the work of Maxwell, who showed that if an electric charge is accelerated (i.e., its speed changes or there is a change of direction), it would emit energy in the form of electromagnetic radiation (radio waves, heat waves, light rays, X-rays, gamma rays which differ from each other in respect of their wavelengths only). This miraculous unification has been the basis of technology of the twentieth century, with radio, television and X-rays dominating our lives. Maxwell had unified optics with electromagnetism.”
Expounding on Einstein's unification of space and time and generalization of gravity, Dr Salam notes: “This finally leads us to Einstein who was responsible for a number of far-reaching unifying ideas. His Special Theory of Relativity (1905) places space and time on an equal footing. One consequence of this work was the time dilation formula which says that the faster a body moves, the longer its life is - as observed by a stationary observer. This particular phenomenon can be observed any day with the greatest precision possible at the CERN Laboratory at Geneva, where a particle like the muon with a definite lifetime (as measured by an observer riding with the particle) appears to us, the stationary observers, to live longer and longer as its speed approaches the velocity of light. Thus, the secret of longevity is to stay in motion!”
Commenting further on Einstein’s work, Dr Salam observes: “Another consequence of Einstein's special relativity theory is the well-known relationship between mass and energy embodied in the famous equation E = mc2 where c is the velocity of light, m is the mass of the moving particle and E is its energy. …Einstein went further in his general theory of relativity (1915). He geometrized physics in the sense that in his theory, the curvature of space and time determined gravity. Curvature is a geometric notion while gravity is one of the fundamental forces of nature. Einstein, by a stroke of genius, equated these two and thereby accomplished the geometrization of physics.”
Referring to the work of Friedman and Hubble, Dr Salam notes: “Einstein's theory of gravity revived the idea that space and time were curved. The next steps were taken by the Russian astrophysicist Friedmann, who considered the overall structure of the universe and found that an expanding universe could arise as a solution of Einstein's equations. This was experimentally confirmed by Hubble, who discovered that distant galaxies are receding from us, precisely in accordance with these ideas.”
Discussing the concept of Hot Bang, Dr Salam observes: “In 1965, Penzias and Wilson accidentally found a radiative background with temperature of 3° Kelvin ( - 270 °C) which apparently filled all of space. Extrapolating backwards from 2 x 105 years, we would (speculatively) arrive at a moment when the universe began, the moment of the so-called Hot Bang.”
Reflecting on the Unification of gravity and electromagnetism, Dr Salam states: “… a unification of general relativity and electromagnetism was a dream which Einstein lived with and worked on for much of his later life. He spent 35 years on this problem and in the end, we believe, did not succeed. This may explain why Dirac in 1968 remarked negatively about the possibility of unification of fundamental forces.”
Describing the importance of Dirac’s famous equation, Dr Salam observes: “… Dirac's famous equation which was first written down in 1927 for electrons. This equation can equally describe elementary entities like free electrons or free quarks or even free composite entities like protons and neutrons…The first important point about the equation is that while the only inputs to it were Einstein's special theory of relativity and quantum mechanics, the equation did succeed in describing the correct ‘intrinsic spin’ as well as the ‘handedness’ of electrons….The second point about the Dirac equation is that it predicts that each particle has an antiparticle - with the same mass and intrinsic spin but opposite electric charge (if any)…As I said earlier, Dirac came to his equation by attempting to unify quantum mechanics and special relativity. He found that as well as describing positive energy particles, his equation described ‘negative’ energy objects also…Thus it was that Dirac was able to predict the existence of a new particle, the positron – the antiparticle of the electron. “
Elaborating further on Dirac’s equation, Dr Salam states: “The equation of Dirac and its successful reinterpretation is one of the greatest triumphs of twentieth century physics. It led to an unbounded adulation of Dirac illustrated by my next story (something I witnessed myself at the 1961 Solvay Conference…) At the 1961 Solvay Conference I was sitting next to Dirac, waiting for the session to start, when Feynman came and sat down opposite. Feynman extended his hand and said, ‘I'm Feynman.’ Dirac extended his hand and said, ‘I'm Dirac.’ (Apparently, this was the first time they formally introduced themselves, at least during that conference.) There was silence, which from Feynman was rather remarkable. Then Feynman, like a schoolboy in the presence of a master, said to Dirac: ‘It must have felt good to have invented that equation’ and Dirac replied, ‘But that was a long time ago.’ There was silence again. To break this, Dirac of all people asked Feynman: ‘What are you working on yourself?’ and Feynman answered, ‘Meson theories.’ Dirac said, ‘Are you trying to invent a similar equation?’ Feynman said, ‘That would be very difficult.’ And Dirac said in an anxious voice, ‘But one must try.’ At that point the conversation finished because the meeting had started.”
Describing nuclear forces, Dr Salam recalls how he learned fundamental forces of physics in 1934 at his school in Jhang: “ I still remember the school at Jhang in Pakistan (Jhang is my birthplace). Our teacher spoke of gravitational force. Of course, gravity was well-known, and Newton's name had penetrated even to a place like Jhang. Our teacher then went on to speak of magnetism; he showed us a magnet. Then he said, ‘Electricity! Ah, that is a force which does not live in Jhang, it lives only in the capital city of this province, Lahore 100 miles East.’ (And he was right. Electricity came to Jhang five years later.) And the nuclear force? ‘That was a force which lived only in Europe. It did not live in India (or Pakistan), and we were not to worry about it.’ But I still remember he was very keen to tell us about one more force - the capillary force. I always wondered why he was so insistent on calling the capillary force ‘a fundamental force of nature.’ I think I know the reason now. He was teaching us the force laws according to Avicenna. Avicenna was not only a great physicist of distinction but also a great physician. Surely for a physician there is no force more important than the one which makes blood rise in the smaller capillaries. He (and my teacher) regarded it as one of the fundamental forces of nature though we do not think so today.”
In his concluding remarks, Dr Salam observes: “In quantum theory, all forces - whether of gauge or non-gauge variety - are produced by an exchange of particles, which I shall call ‘messengers’…Physics would change its paradigm once again with the fundamental entities no longer appearing as point particles but as tiny strings…Could strings really be the Theory of Everything (TOE) combining all the known source particles, the quarks and the leptons, plus the messengers which we know of, and the Higgs, plus their interactions? If so, would they represent the culmination of one's endeavors to unify the fundamental forces of nature? These are questions which time alone will resolve.”
Dr Abdus Salam ends his lecture by quoting the Qur’an: “…would like to leave you with my final thought..:
Though all the trees on Earth were pens
and the Sea was ink,
Seven Seas after it to replenish it,
Yet the Words of thy Lord would not be spent;
Thy Lord is Mighty and All wise.
(The Qur’an, XXXI: 27)”
The lectures by prominent physicists — Dirac, Heisenberg and Salam — in “Unification of Fundamental Forces” offer an interesting insight into their approach to research and the developments in particle physics viz a viz unification of fundamental forces — with the ultimate goal to develop suitable starting equations from which the whole of atomic physics can be deduced. Professor Dr Abdus Salam presents a clear introduction to the major themes of particle physics and cosmology. General readers, students and researchers will find this book an interesting read. Hopefully, Science — via its progress towards unification of fundamental forces — will play a role in unifying people too!
(Dr Ahmed S. Khan - dr.a.s.khan@ieee.org - is a Fulbright Spiciest Scholar- 2017-2022).
