Steven Weinberg, who was born on this day in 1933, towered over theoretical physics in the second half of the 20th century. He strongly believed that, armed only with the fundamental principles of relativity and quantum mechanics, theoretical physicists can examine *all *phenomena in the universe — from the smallest to the largest scales. His work transformed our understanding of every aspect of fundamental physics in startlingly deep and original ways.

Weinberg was a master **of quantum field theory**, a branch of physics that applies the rules of quantum mechanics to the electromagnetic field, and considers a particle — the photon —as a “quantized” excitation of the field. He was instrumental in propelling quantum field theory to astonishing new heights in the description of nature, particularly to his famous breakthrough that unified two seemingly disparate fundamental forces: **electromagnetism** and the **weak force** (the fundamental force that governs the radioactive decay of atoms and that participates in both nuclear fusion and nuclear fission).

**Weinberg’s Work on Electroweak Unification**

We see electromagnetic waves as light in everyday life, while the weak force operates on the unseen subnuclear level. Weinberg saw that at very high energies, these two forces should be intertwined, described by what’s known as the Yang-Mills theory, whose equations have a special property called gauge symmetry. He also knew that this commonality between electromagnetism and the weak force is obscured by the so-called Higgs mechanism, which generates masses for elementary particles such as the electron and the W and Z particles (particles that mediate short-range weak interactions). In his seminal 1967 paper, Weinberg came up with a model that encapsulates his vision of this **electroweak unification**, a model whose detailed predictions have been triumphantly confirmed by experiments beginning in the 1970s and capped off by the momentous discovery of the Higgs particle in 2012. For this work, Weinberg won the 1979 Nobel Prize in Physics along with Sheldon Glashow and Abdus Salam.

**The Anthropic Principle of Cosmology**

Not content with doing pathbreaking research on the subatomic, Weinberg also pondered on bigger questions of cosmology. We know from Einstein that gravity is caused by the curvature of space-time. Weinberg wanted to understand what causes space-time to be curved in the first place. This led him to ponder the infamous **cosmological constant problem** which linked (rather improbably) quantum mechanics to the gigantic curvature of space-time. Violent quantum mechanical fluctuations, Weinberg postulated, which are present everywhere in the vacuum, should endow empty space with an enormous energy density. This would cause space-time to be highly curved, in stark disagreement with the large, nearly flat universe we observe.

But why is this vacuum energy, or “cosmological constant,” so small? In 1987, Weinberg proposed a radical approach to this problem using a minimal version of the “anthropic principle.” He reasoned that perhaps the vacuum energy could take on different values, and that if it were bigger than a specific minute size, an accelerated expansion of the universe would rip apart galaxies before they had a chance to form, leading to a structureless, empty universe, one devoid of people wondering about the size of the cosmological constant. Therefore, there must be a tiny but nonzero size for the vacuum energy, a prediction that is consistent with an astronomical discovery in 1998 that the expansion of the universe is accelerating, with the simplest explanation being the presence of vacuum energy at just about the size suggested by Weinberg’s argument. To be sure, the anthropic principle that Weinberg espoused has been hotly debated, but Weinberg stuck his ground, largely avoiding the many boring diatribes surrounding his conjecture, contenting himself with having pragmatically used anthropic reasoning to make a correct prediction about nature.

**A Theoretical Physicist Remembers Weinberg**

Nima Arkani-Hamed of the Institute for Advanced Study, Princeton, remembers Weinberg as a mentor:

“*I first met Weinberg in person in the early 2000s. Although he had been kind and encouraging to me about my work, I became uncharacteristically bashful in his presence. In the midst of a technical discussion, I uttered the phrase “as you taught us” many more times than I can remember. I never entirely shook this feeling of reverence around him.* *Discerning the fundamental simplicity behind nature’s inner workings is the highest goal a theoretical physicist can aspire to. No one in the past 60 years did this better than Weinberg. He was also a profound and humane thinker, who taught us all how to seek out that which, as he put it, “lifts human life a little above the level of farce and gives it some of the grace of tragedy.” His example will forever serve as an inspiration, and as a model for a life deeply lived.”*

**Weinberg, the Science Popularizer**

In addition to being one of the greatest theorists of his era, Weinberg was also the preeminent public intellectual of fundamental physics. His first popular book — *The First Three Minutes*: *A Modern View of the Origin of the Universe*, about cosmology and the Big Bang — became an instant classic and proved profoundly influential for both the general public and professional researchers.