Quanta Magazine

Steven Weinberg, who passed away on July 23, was a giant in theoretical physics during the second half century. He believed that theoretical physicists could examine every phenomenon in the universe, from the smallest scale to the largest, if they only had the fundamental principles and quantum mechanics of relativity. His groundbreaking and innovative work changed our understanding of all aspects of fundamental physics.Weinberg was a master in quantum field theory. This branch of physics is based on applying quantum mechanics to the electromagnetic fields. It sees a photon as a quantized field excitation and a particle called the photon. He was instrumental in bringing quantum field theory to new heights in the description and understanding of nature.Weinberg's work was dominated by the themes of unification or symmetry. This led to his breakthrough in electroweak unification which revealed an invisible unity between two of four universes fundamental forces. The electromagnetic and weak interactions appear completely different at first glance. We perceive electromagnetic waves as light in our daily lives, while radioactivity's weak force operates on subnuclear levels. Weinberg saw that the forces should interact at extremely high energies. This is described in Yang-Mills theory, which has a special property called gaugesymmetry. The Higgs mechanism is able to generate masses for elementary particles like the electron, W and Z particles, and mediates short-range weak interactions. It also makes the long-range photons massless. His 1967 model for realizing his vision was based on many detailed predictions. It has been confirmed triumphantly by experiments that began in the 1970s and continued into the 1980s. The Higgs particle was discovered in 2012. Weinberg was awarded the 1979 Nobel Prize in Physics along with Sheldon Glashow, Abdus Salam and others for this work. It is a key pillar of the Standard Model in particle physics.Ironically, Weinberg's efforts in this area were not typical of his oeuvre. He tended to be more interested in general properties of the laws and not specific models. His unmistakable style was to start any discussion with general principles and then develop a system of arguments, one after the other, with an almost inevitability. He loved mathematics but was primarily interested in using it to describe the world. Weinberg asked both simple and profound questions. Quantum field theory is the best way to describe nature. What are the limitations of quantum field theory in describing elementary particles' properties and interactions?Weinberg, in this vein, reimagined quantum fields theory from a new perspective. He argued that special relativity, quantum mechanics, and the idea of particles were the most important starting points. He was a pioneer in studying long-range forces such as gravity and electromagnetism. These were mediated by massless particles like the photon or graviton. These elementary particles have an intrinsic angular momentum or spin. This can be expressed in quantized units. Photons have spin 1, while gravitons have spin 2. Weinberg demonstrated that quantum mechanics and special relativity put restrictions on interactions between massless particles. The theories that describe spin 1 particles must use gauge symmetry equations, while those for spin 2 must have the properties and strength of the graviton. This allowed Albert Einstein to derive a deeper understanding of the equivalence theory, which was his basis for general relativity. There are no other options. The long-range forces that we see in nature exhaust the special relativity or quantum mechanics.Weinberg's contribution to quantum field theory was another important one. It transformed our understanding of how quantum field theory describes the world. He proposed the concept of effective field theories. This theory, which focuses on the idea of particle interactions at different energy scales, is based on the principles of quantum mechanics. While the dominant interactions between particles can be described by a finite number interaction strengths, the fingerprints of unknown Physics at higher energies are systematically encoded within an infinite number of ever smaller interactions. Ken Wilson also contributed to the development of effective field theory.Weinberg was, above all, a great unifier. Weinberg disliked Einstein's view of gravity, which he saw as the curvature space-time that gives gravity a privileged place in defining the area where all other phenomena operate. He felt this created an artificial barrier that prevented researchers from making deeper connections between gravity with the rest of Physics. He was inspired to create general relativity by using particle physics methods in his first tour de force textbook, Gravitation and Cosmology. He realized that particle physics and Cosmology were not compatible, as high-energy collisions of elementary particles occurred in hot and dense early universe conditions shortly after the Big Bang. This led him to provide the theoretical tools necessary to bring about a golden age in explorations of early-universe cosmicology.Weinberg's fascination with cosmology led to him to consider the famous cosmological constant problem. Violent quantum mechanic fluctuations, which are present in all vacuums, should cause empty space to have an immense energy density. This would make space-time highly curvilinear, in stark contrast to the flat, large universe we see. This vacuum energy or cosmological constant is so small. Weinberg proposed a radical solution to this problem in 1987 using a minimal anthropic principle. If the vacuum energy were larger than a certain minute size, an accelerated universe expansion would result in galaxies being destroyed before they have a chance to form. This would leave a structureless universe without people asking about the size the cosmological constant. Weinberg claimed that this predicted a small but non-zero size of the vacuum energy. Astronomers found that the universe was expanding at an accelerating rate in 1998. The simplest explanation is the existence of vacuum energy at a size similar to Weinberg's argument. Weinberg avoided all the boring discussions about the anthropic principle and instead focused on using anthropic reasoning to accurately predict nature.Weinberg was not only one of the most influential theorists of his time, but also the foremost public intellectual in fundamental physics. The First Three Minutes, Weinberg's first book about cosmology and Big Bang, became a classic. It was a huge success and was a great influence for professional and general researchers. This book was a great resource for many physicists. Weinberg eloquently discussed the concept of beauty in physics in Dreams of a Final Theory. He stressed that it was not a capricious aesthetic judgment, but a reflection of both the rigidity of physical laws, and the greater sense of certainty associated with how they explain the world.Weinberg has been the intellectual hero of my physics career since I was a graduate student. His books on quantum field theory were a blessing. His perspectives on the inevitability and effectiveness of quantum field theories formed the basis of my view of the world. I read through his books meticulously and carried them with me everywhere. Weinberg's notation was too detailed, which made it difficult to read his text. I had to translate his ideas into my own notation, and this made them stick. One interesting side effect of my years of transcribing Weinberg was that I can still see and hear the words of his books when I think about fundamental aspects of field theory. I vividly recall reading his paper on anthropic explanations for the cosmological constant. It was a frightening experience that left me paralysed for over a month. This point of view took me many years before I was able to accept it and even incorporate it into my own research.Weinberg was the first person I met in person. Although he was kind and supportive about my work, I became uncharacteristically aloof in his presence. I said the phrase "You taught us many times more than I can recall" during a technical discussion. This feeling of reverence was something I never fully felt around him.The highest goal theoretical physicists can aim for is to discover the simplicity of nature's inner workings. Weinberg was the best at this in the last 60 years. Weinberg was also a deep and compassionate thinker who showed us how to find that which, as his words put it, elevates life above the level folly and gives it the grace of tragedy.