String theorists lament the death of one of their field’s most esteemed and respected thinkers
SONIA FERNANDEZ, UCSB PUBLIC AFFAIRS & COMMUNICATIONS
Modesty is not a quality often found in abundance in physicists. Maybe that’s because Joe Polchinski had all of it.
Substantial ego is arguably a necessary qualification for anyone attempting to wrest nature’s deepest secrets from their mathematical lairs. And in many cases, ego seems proportional to the magnitude of a physicist’s accomplishments. But if you divided Polchinski’s accomplishments by his ego, the answer would be a lot closer to infinity than unity.
Polchinski died this month. He was 63. Physicists worldwide lamented the loss of one of their most creative theorists. And one of their most respected persons.
Polchinski was an early pioneer of string theory, the mathematical apparatus picturing the basic particles of matter and force as supertiny wriggling strands of energy known as superstrings. His contributions to the field were immense. As a young professor at the University of Texas at Austin in the 1980s, he developed a branch of superstring theory involving objects called supermembranes.
Superstrings are one-dimensional objects (like lines, hence “strings”) vibrating like rubber bands in multidimensional space. (String math presupposed more dimensions than the usual three.) Polchinski explored the possibility that those multiple dimensions could contain two-dimensional membranes, kind of like the film forming the surface of a soap bubble. He and his students derived the math describing such supermembranes living in 11 dimensions (10 of space, one of time).
Supermembranes did not catch on at first. “Most string theorists, myself included, assumed that they were an aberrant offshoot of the real theory,” Polchinski wrote in his memoirs. But a few theorists, especially Michael Duff (then at Texas A&M University) became vocal supermembrane advocates. When Duff visited Austin, Polchinksi said he had just been joking when he invented supermembranes. To which Duff replied, “Many a true word is spoken in jest.”
The truth of Polchinski’s jest became apparent a few years later, after he moved to the University of California, Santa Barbara. He found that a variant of his supermembranes, known as D-branes, played a key role in making sense of superstring theory. Some strands of string needed a surface for their end points to attach themselves to. D-branes provided such a surface. D-branes also helped explain other string theory mysteries. Suddenly strings and branes were no longer separate branches of inquiry, but integrated parts of a more powerful theory.
Polchinksi’s D-brane advance meshed well with a recent demonstration from physicist Edward Witten that various versions of string theory were just different disguises of an overarching mathematical edifice called M theory. Hopes rose that the ultimate theory of physics was at hand — a theory that would unify gravity with quantum physics, along with specifying all the varieties and properties of matter and force in the universe. Polchinski certainly hoped so. In particular, he wanted the theory to eliminate the possibility of a repulsive energy perfusing all of space. But then astronomers discovered that something (most likely just such repulsive energy) was indeed driving space to expand at an accelerating rate.
Maybe, string/brane/M theory would explain the amount of that mysterious “dark” energy in space and all would be well. But no. Working with physicist Raphael Bousso, Polchinski found that string theory did not specify how much energy the vacuum of space contained. Instead the theory predicted a virtually countless number of vacuum states, with nearly any amount of repulsive energy you could imagine. In other words, string theory described a giga-gaggle of different universes — a multiverse.
Polchinski’s modesty manifested itself in his reaction to this situation. He hated the idea of a multiverse, because it implied that some questions had no answers that physicists could calculate. No equation could specify the amount of dark energy; it would just be luck — determined by which universe had the right amount of dark energy to make it hospitable to life (an idea known as the anthropic principle). Polchinski once vowed to quit physics if dark energy existed and implied the need for anthropic reasoning. But unlike many physicists who adamantly defend their views against contrary evidence, Polchinski changed his mind.
At first, he was reluctant to admit it. And then a civilian asked him what physicists thought about the anthropic principle.
“I said nobody believes that, and when I said that, I knew I was lying,” he told me. “I knew that the evidence was mounting for the anthropic principle. And it was really that I didn’t want to lie anymore. That’s why I started talking about it.”
He was not only modest. He was polite, friendly and always enormously helpful to anyone needing help — such as journalists attempting to write about string theory. When I interviewed him in 2006, he explained the technicalities of his work with colloquial clarity. I then asked him about a prominent physicist’s complaint that string theorists were basically doing macrame. Polchinski responded that “at worst, you could say I’m doing mathematics.” And then he said, “What is macrame?”
When I asked about the multiverse and the anthropic principle, he pointed out that it’s part of science to find out what sorts of things science can predict. “We don’t know a priori how predictive science is,” he said. “We don’t know which things are absolutely predictable and which things are not, and we have to figure out the answer to this to the best of our ability. There are a number of trains of thought which point to things being less predictable, and if that’s the way it is, that’s the way it is. Arbitrary definitions of what science is shouldn’t affect one’s attempts to figure out the way things really are.”
In a talk prepared for a meeting in Munich in December 2015, Polchinski presented his closing arguments in favor of string theory and the multiverse. He did not get to deliver that talk — he was diagnosed with brain cancer just a few days before. Another physicist presented the talk for him, and the text is available online. Toward the end of that paper, Polchinski offered his prediction for the year 2031:
“It is that we will have figured out the theory of quantum gravity, and that it will be built upon string theory. What and how much will it be able to predict I cannot say; how much can be predicted is something for us to discover.” He expressed optimism that not only would physicists discover the theory of quantum gravity by 2031, but they would also know for sure that it was correct. “Of course I am working hard to reduce the interval,” Polchinski said. “I want to know the answer.”
He didn’t live to see that answer. But because of his work, it’s more likely that those still around in 2031 will.
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