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A String Theorist's Memoir

ABSTRACT: This is a slightly expanded collection of my posts on Threads, all about string theorists Joseph Polchinski's memoir: Memories of a Theoretical Physicist ‒ A Journey across the Landscape of Strings, Black Holes, and the Multiverse.

Renowned string theorist Joseph Polchinski wrote a memoir after he was diagnosed with brain cancer, which deprived him of the capability of doing physics and eventually claimed his life. I had an early glance at his memoir when it was released as a preprint in 2017, and was amused (sorry for the lightness of the tone, as both he and his readers were still hopeful about his recovery at that time) by a typo that misspelled "brain cancer" as "brane cancer" ("brane" was a major contribution of his to string theory).

The memoir was eventually published as a book (Memories of a Theoretical Physicist ‒ A Journey across the Landscape of Strings, Black Holes, and the Multiverse, in 2022), which I have recently finished reading. The book was foreworded by Polchinski's longtime friend and colleague Andrew Strominger who mentioned, among other anecdotes, that Polchinski's textbook String Theory "sold many times more copies than there are string theorists on the planet", and I felt a little proud that I had contributed a copy to the "many times more" part. In addition to that, the book form of the memoir also featured a collection of physics explanation boxes written by Polchinski's student Ahmed Almheiri.

Below are some of the random bits that I wrote while reading Polchinski's memoir.

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Polchinski's undergraduate school was Caltech, and Richard Feynman, having already pocketed his Nobel Prize, was a star professor there. Polchinski's recollections about Feynman, however, were mixed. He considered Feynman's famous The Feynman Lectures on Physics as lacking examples and calculations, and he sort of blamed Feynman's offhand remark that one doesn't need to know much math for his not having learned much math. I'm not surprised that Polchinski should have mixed recollections about Feynman, since they were two very different types of physicists ‒ almost contrary to each other in most aspects: Feynman's physics was very down-to-earth in the sense that it was deeply entangled with phenomena; Polchinski's physics, however, was highly abstract. In terms of math, Feynman's was classical and analytical, while Polchinski's was modern and more geometrical. But I don't know whether it's because they were different types of physicists, so Polchinski's feelings about Feynman were retrospectively mixed; or because he had mixed feelings about Feynman, so he became what he is rather than a "next Feynman", which some of his classmates once proclaimed him to be.

Polchinski's graduate study was at Berkeley, with Stanley Mandelstam being his doctoral advisor. In Polchinski's recollections, Mandelstam seems to be a thinker far deeper than a partial outsider such as myself used to know. For instance, a problem he assigned to Polchinski somewhat foresaw a research direction 25 years ahead of its time, and he talked about 11-dimensional supergravity in relation to QCD confinement long before string theory made it a popular topic. None of these were mentioned on Mandelstam's Wikipedia page, so I suppose it's not just me as a partial outsider who was ignorant. Another interesting recollection in relation to Mandelstam was: Edward Witten, the later "King of String Theory" but then a mere postdoc, once asked Polchinski about Mandelstam's work, and Polchinski shockingly noticed that Witten's understanding of Mandelstam exceeded his own, despite his years of study under Mandelstam. :-)

Polchinski's second postdoctoral position was at Harvard, with Sidney Coleman being his advisor. Reading this part of his memoir brought me a rare feeling of nostalgia, since it was almost a showcase of the so-called "small-world phenomenon" between Polchinski and none other than myself. The showcase has three layers: 1. my secondary doctoral advisor, Erick Weinberg, was a doctoral student of Coleman; 2. my primary doctoral advisor, Kimyeong Lee, was Erick's doctoral student (therefore a "grandson" of Coleman in academic genealogy); 3. I once worked (unfruitfully though) on a topic called "Q-ball" for which Coleman was a major pioneer, and Polchinski was the one who suggested the name "Q-ball" to Coleman.

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Steven Weinberg is a physicist whom I admire a lot. I enjoyed reading his books and I resonated, to various degrees, with almost all his scientific and social views. After he passed away, I collected many reminiscences that his colleagues wrote, and I began to pay more attention to writings about him. Weinberg belongs to a generation that preceded, and, perhaps because of that, was generally less enthusiastic about, the rise of string theory. But Weinberg himself was an exception, and was rather supportive of string theory. As such, and since support from a person of his caliber carried huge weight, it is only natural that Weinberg appeared in Polchinski's memoir. In fact, Polchinski was among the first group of young faculty colleagues Weinberg hired to form his research circle at the University of Texas at Austin (which happened to be Polchinski's first post-postdoctoral job).

Steven Weinberg, if judged by his publications, was a largely solitary author. But he was by no means a solitary person. In fact, he had, according to Polchinski, a strong bond with and was proud of his research group, which held a weekly gathering dubbed the "family meeting". He also enjoyed lunch conversations with the group on subjects which Polchinski jokingly summarized as English history, Israeli politics, and DOS versus Windows. Those conversations were surely very casual, and only Israeli politics, if my reading is sufficiently exhaustive, received rather limited coverage in Weinberg's essay collections. Among the subjects, DOS versus Windows was particularly amusing and yet perhaps reflected a profound characteristic of Weinberg's research style ‒ namely he was, in Polchinski's words, "a notably text-oriented thinker", and "used very few figures in his books and papers". I would add one thing to the last point that I myself vividly remember: Weinberg, in his most famous textbook Gravitation and Cosmology, boldly deviated from the traditional geometrical pedagogy, and with that, "used very few figures" was completely natural. :-)

Polchinski also portrayed Weinberg from the angle of personal interactions: "my first impression of him", he said, adding that he felt embarrassed to say, "was that he was a little slow" and "seemed to get stuck on things that seemed obvious". Though he continued by saying, a little bit insincerely I dare (and, to copy his words, "feel embarrassed to") say, that he soon realized that this was part of Weinberg's genius since he never took anything for granted. This style of Weinberg's reminded me of Richard Feynman's habit of trying to derive everything in his own way. Sure enough, that sometimes also made Feynman, though far outsmarting most people he conversed with, look slow. In his best-seller memoir Surely You're Joking, Mr. Feynman!, he acknowledged he was slow in understanding people and often asked dumb questions, until eventually, he not only understood but might surprise people by catching their mistakes.

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Polchinski mentioned in the memoir that he, at least in the early days of his research career when he was a lecturer at the University of Texas at Austin (i.e., in the late 1980s), was shy of speaking about his work, because "I rarely felt that my work was important enough". That was a very familiar feeling that also haunted me, when I was a PhD student, and I couldn't agree more with the reason for such shyness. Though of course, my feeling that my work was not important enough was far more justified than his. :-)

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To most outsiders among insiders, or insiders among outsiders, by which I mean physicists not specialized in string theory, Polchinski's textbook String Theory might be his most widely known legacy. In the memoir, he attributed his writing of it to three factors. First (and most decisive) was the "I rarely felt that my work was important enough" feeling I quoted above. Polchinski even mentioned the famous example of Murray Slotnick's six-month calculation (or PhD dissertation, as Polchinski called it) being reproduced (in fact vastly generalized) by Feynman in a mere night. The fierce competition, similar to what Slotnick encountered, with talents much higher on the intellectual ladder made Polchinski wonder, "why am I needed?", and writing a textbook became a sanctuary for him. Second was that he had just taught a one-year string theory course, and wanted to write a textbook that can match Weinberg's Gravitation and Cosmology (far more ambitious than his appetite in research!). Third (and most amusing) was that his nanny happened to be a woman who had traveled around the world and planned to write her own autobiography, and Polchinski figured "if my nanny could write a book, then so could I." That was in 1988, and it took him nine years, 30% of his time each year, to finish.

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The so-called "second superstring revolution" occurred when I was in graduate school, so though I never dived into the battleground, I had nevertheless sat through quite a few talks and learned some bits here and there. According to what I still remember, there were five string theories ‒ type I, type IIA, type IIB, heterotic E₈✕E₈ and heterotic SO(32) ‒ prior to the "revolution", and they, as well as the M-theory that emerged during the "revolution", are connected, or unified, as string theorists might prefer to say, through various dualities. Polchinski's major contribution, the D-branes, of which he was a co-discoverer, was the trigger of the "revolution", and that made him a renowned figure.

At the beginning, however, Polchinski was not very confident in his discovery, and, to quote his own words in the memoir, "gave zero talks" on his paper, whose title he coined as "Fun with Duality", which was vetoed by his collaborator and changed into a more serious one: "New Connections between String Theories". Among the various string dualities, Polchinski's direct contribution was, to again quote his own words, "that the type II theories were T-dual, and ... that the type I theory was dual to type II, but with the ground state of type I dual to an excited state of type II, with D-branes" (the italic highlights are mine).

An add-on story about Polchinski's D-branes: As quoted above, Polchinski "gave zero talks" on D-branes during the early days (BTW, I forgot to mention before that Polchinski's co-discovery of D-branes was in 1989, and that was when "early days" started). It was not until 1995 when Witten talked about dualities at the Strings ’95 conference, and Polchinski had a discussion with Witten, that he finally realized the importance of D-branes. Witten urged Polchinski to write a new paper. Witten was by then practically the King of String Theory. When the King urged someone to do something, that person would do exactly and exclusively that; and when people heard the King had asked someone to do something, they began to follow even before the paper was out. That was how D-branes, after a 6-year "hibernation", became almost instantly popular, and became part of the collective beginning of the "second superstring revolution".

In the memoir, Polchinski viewed the "second superstring revolution" as consisting of five successive "waves": 1. Witten's talk on dualities and the introduction of M-theory (1995); 2. Polchinski's D-branes (co-proposed in 1989, but largely hibernated until 1995); 3. Andrew Strominger and Cumrun Vafa's blackhole microstate counting via D-branes that is consistent with the Bekenstein-Hawking entropy (1996); 4. Tom Banks, Willy Fischler, Stephen Shenker, and Leonard Susskind's BFSS matrix model for M-theory (1997); 5. Juan Maldacena's AdS/CFT correspondence (1998).

BTW, whoever wants an easy-to-catch description of D-branes might appreciate these two lines from Polchinski's Memoir: "D-branes behave as higher-dimensional analogues of charged particles. They are the source of a higher-dimensional version of the electromagnetic field known as the Ramond-Ramond gauge field."

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An amusing story about Polchinski and the anthropic principle: Steven Weinberg once predicted, based on the anthropic principle, that the cosmological constant should have a nonzero value comparable to that of the average matter density in the universe. That was in 1987, when Polchinski was in Weinberg's group. Polchinski, however, was an anthropic principle denier, and he boldly (or recklessly) claimed that if such a cosmological constant were found, he would give up physics. He also made other comments such as that he would yield his office to a colleague, which he later conveniently forgot, but was immediately reminded of by that colleague in 1998, when a nonzero cosmological constant roughly as Weinberg predicted was confirmed by observation. Awkward though it was, Polchinski's love for physics didn't allow him to quit (and that colleague with too good a memory also didn't get his office). In fact, he even did some follow-up work in the area of the cosmological constant, trying to explain it from string theory. The last piece of the story is: Polchinski tried to avoid mentioning the anthropic principle in the paper, but a collaborator to whom Polchinski's institute planned to offer a position threatened to reject the offer should Polchinski reject the mention of the anthropic principle! In the end, the anthropic principle was mentioned twice in the paper: once in the introduction, once in the last paragraph :-)

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Polchinski’s memoir, as indicated at the very beginning of this article, has a sad ending. Polchinski passed away on the morning of February 2, 2018, at the age of 63, due to brain cancer that was diagnosed in late 2015 – which was pretty much where his memoir ended.

Polchinski’s family (his wife and two sons) added an afterword to the memoir that sketched his battle with brain cancer: Polchinski tried to maintain research activities for as long as possible, but reached roadblocks before long (he told his wife that he could start a calculation but could no longer figure out how to bring it to a successful end). He then switched gears and began to write his memoir. Meanwhile, his deteriorating condition gradually consumed his capability to read (though he could still understand spoken language). The only luck he had during this tragic period was his memory, which might have been his most remarkable gift only next to his talent in physics, remaining intact, and this memoir he wrote is the most detailed memoir I have ever read in terms of research activities.

Theoretical physicist Eugene Wigner once wrote in his obituary for John von Neumann that von Neumann was in deep despair when he realized he was dying, because it was impossible for him to imagine that he would stop thinking. This, stop thinking, is perhaps the saddest part of the death of any great intellectuals.

completed on August 16, 2025
posted on August 16, 2025
https://www.changhai.org/