One of the most important physics Nobel Prize winners in the field of theoretical particle physics passed away – An obituary for Steven Weinberg
Hardly noticed by the German speaking press, one of the most important theoretical physicists of the second half of the 20th century died on 23 July 2021 at the age of 88. The Neue Zürcher Zeitung, the largest magazine in Switzerland, and the weekly magazine Der Spiegel, with the most important news website in Germany, seem to have completely ignored the death of Steven Weinberg, the 1979 Nobel Prize winner in physics. Are floods and the Tokyo Olympics so much more important than one of the greatest physicists of the 20th century who created a new foundation for his field?
In the late 1960s, Steven Weinberg, Sheldon Glashow and Abdus Salam succeeded (in separate papers) in presenting the quantum field theory of the weak nuclear force and relativistic quantum electrodynamics as two different sides of a single theory. Physicists today speak of the „theory of the electroweak force“. This was the first – and to date last – step towards representing the various forces in the universe within a global theory as different facets of a single universal force. For the third force, the strong nuclear force, convincing theories have already existed for some years now that bring it together with the other two. Physicists call such a theory a „Grand Unification Theory“ (GUT). However, the energies at which this unity reveals itself are still many orders of magnitude higher than what physicists achieve today in experimental particle accelerators, so all GUT-theories remain untestable. Thus, the standard theory of elementary particle physics still consists of two different basic models: the so-called SU(3) group of quantum chromodynamics for the strong nuclear force (SU(3) is a symmetry group describing the invariance of the underlying equations). The other is the combined SU(2)-U(1) group of the electroweak theory discovered by Weinberg and his colleagues. Probably the most significant GUT is the theory based on the SU(5) gauge invariance, but this still requires numerous new, previously undiscovered particles implied by this 24-dimensional invariance transformation matrix. And finally, how the gravitational force is to be integrated is still completely unknown (even though there are already theories for this as well, but they are still even a few more orders of magnitude away from anything we are currently achieving experimentally). The ultimative theory requires a unification of quantum theory and general relativity, from which theoretical physics today is still quite far away.
Thus, the electroweak force discovered by Weinberg and his colleagues more than 40 years ago, which is split into the electromagnetic and the weak nuclear force on lower energy scales, is actually the only – still very small – step so far on the dream path of theoretical physicists towards a unified theory for all forces in the universe, towards what they call the „Theory of Everything“ (TOE). Recently, physicists have been speculating whether they might even have recognised a fifth force, but the values pointing to it deviate only very slightly from those of the Standard Model, so that they could still with a relevant probability be a result of simple measurement errors (even if the measurements for it have now been made at several different places and appear quite consistent).
Weinberg also did important work in other areas of physics and wrote several authoritative textbooks on topics such as general relativity, cosmology, and quantum field theory. He was an early proponent of superstring theory, which in the 1980s represented a promising path to the Theory of Everything by unifying quantum field theory with general relativity (which it is no longer, due to its huge number of free parameters to be chosen).
Weinberg was a professor of physics at Harvard University from 1973 to 1983 and then went to the University of Texas at Austin, where he founded a theoretical physics group that today has eight full professors and is one of the leading research groups in this field in the US. In addition to the Nobel Prize, the highest award in physics, his research on elementary particles and cosmology has been honoured with numerous other prizes and awards, including the National Medal of Science in 1991 and the Benjamin Franklin Medal of the American Philosophical Society in 2004. For the latter, the citation said that he was „considered by many to be the most important theoretical physicist alive today“. His publications were generally highly regarded, as shown by the so-called “h-index” for physics papers. The Publication Institute Science News named him together with the two theoretical physicists Murray Gell-Mann (died in 2019) and Richard Feynman (died in 1988) as the leading physicist of his time (second half of the 20th century). In addition to the US National Academy of Sciences, he was elected to the British Royal Society as well as the American Philosophical Society and the American Academy of Arts and Sciences.
Weinberg also wrote books and articles for a wider public on various topics. His first, and to this day probably best known, popular science book The First Three Minutes: A Modern View of The Origin Of The Universe from 1977 describes the beginning of the universe with the Big Bang and presents an argument for its expansion, which is now firmly established in cosmology. As late as in 2015, at the age of 80, he wrote the book on the history of science To Explain the World: The Discovery of Modern Science. Numerous essays by him have appeared in The New York Review of Books and other journals. He was also an advisor to the US Arms Control and Disarmament Agency and the JASON group of defence advisors, president of the Philosophical Society of Texas and the Council of Scholars of the Library of Congress. He also worked for numerous other state boards and committees.
Weinberg was not least known for his distinctive reductionist view of the world. Physics should ultimately be able to clarify everything in the world from scratch, as he once summed up as follows:
All the explanatory arrows point downward, from societies to people, to organs, to cells, to biochemistry, to chemistry, and ultimately to physics.”