Joseph Polchinski, 63, Leading Theorist on Multiple Universes, Dies

Joseph Polchinski, 63, Leading Theorist on Multiple Universes, Dies

Joseph Gerard Polchinski Jr. was born in White Plains on May 16, 1954, the eldest of two children. His father was a financial consultant and manager; his mother, the former Joan Thornton, was an office worker and homemaker.

In a memoir, Dr. Polchinski said he had been a painfully shy child with an avid interest in science and science fiction. When he was in the sixth grade, his family moved to Tucson, where he developed an interest in telescope-making and chess.

He later enrolled at the California Institute of Technology, where his freshman adviser was Kip S. Thorne, a future Nobel laureate (he shared the prize in physics last year) who was already a renowned black-hole theorist.

After graduating from Caltech, he obtained a doctorate from the University of California, Berkeley, in 1980. It was there that he met Dorothy M. Chun, who was a graduate student in German and is now a professor of education at the Santa Barbara campus.

They were married in 1980. She survives him, along with two sons, Stephen and Daniel, and a sister, Cindy Reid.

After postgraduate stints at the Stanford Linear Accelerator Center and Harvard, Dr. Polchinski joined the faculty of the University of Texas in Austin. He left for Santa Barbara in 1992 and stayed there.

“My greatest failure as head of the Theory Group here in Austin was to lose Joe to Santa Barbara,” said Steven Weinberg, a 1979 Nobel laureate at the University of Texas.

Dr. Polchinski joined a revolution. By the time he entered the profession, Dr. Weinberg and others had completed the Standard Model, a set of equations that explained most of particle physics but left out gravity.

String theory, as developed in 1984, was a revolutionary triumph in that it included gravity in the scheme. But its effects could be seen only at energies far beyond anything particle colliders could ever achieve, and required nature to have 10 dimensions of space and time.

In 1995, Dr. Polchinski showed that the theory not only included strings but also described reality as built by extended objects with various numbers of dimensions, called “branes,” short for membranes. His work led to a burst of theorizing, often called “the second superstring revolution.”

In this new conception, the universe could be a hologram — a three-dimensional mirage like the images on bank cards — suspended in a vast extradimensional space like a leaf in a fish tank, perhaps colliding with other such island universes and setting off events like the Big Bang, with which our own universe began. And those fearsome black holes would be dense tangles of strings and branes crumpled together into a ball, like a wad of paper that is tossed into a wastebasket.

“Remarkably little theoretical physics is done today that doesn’t build on Polchinski’s work,” said Dr. Bousso, who collaborated with him on string theory calculations of the number of universes.

The work on counting universes arose from an effort to understand a fudge factor known as the cosmological constant, an antigravitational force associated with empty space that Einstein invented in 1917 to explain why the universe was stable. (It has also been called Einstein’s greatest blunder.)

Astronomers had long concluded that the value of the cosmological constant was zero, as the universe was expanding nicely, even though, according to the Standard Model, it should be ginormous. Theoretical calculations could not explain why.

Dr. Weinberg, who once referred to this discrepancy as “the bone in our throats,” suggested in a paper in 1987 that the value of the cosmological constant was random and could be high or low depending on where you were in the cosmos. He pointed out that human beings could live only where the constant was zero or very small — otherwise the universe would have blown itself apart before galaxies and stars had had time to coalesce out of the primordial mists.

In other words, the main parameters of the universe were determined by chance, not by some deep, elegant principle or theory. The universe had the features it did, like a minimal cosmological constant, because those were the conditions necessary for humanity’s own existence in it, a notion called the anthropic principle.

Photo

Dr. Polchinski at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, in 2017.

Credit
Matt Perko/U.C. Santa Barbara

That prospect was so dismaying to many physicists, who seek a deeper explanation for things, that Dr. Polchinski vowed to quit physics if a cosmological constant were ever found.

In 1998, astronomers did measure a very small cosmological constant in the form of a “dark energy,” which seems to be speeding up the expansion of the universe.

Dr. Polchinski did not quit. In 1999, he and Dr. Bousso set out to see if string theory could supply enough different possible universes to ensure a reasonable chance of one having a cosmological constant as small as what had been measured.

When all the ways that branes and the fields and forces threading through them were taken into account, there were more solutions than anybody needed: some 10^500 possible universes, which cosmologists refer to as the “landscape.”

“If the landscape proves correct, it is a revolution,” Dr. Bousso wrote. It would be the ultimate wrenching Copernican shift, from humans’ being at the center of creation to their inhabiting a universe that is less significant than a dust mote in the desert, and whose most important properties were attributed to chance.

The price of solving the cosmological constant problem would be to give up the Einsteinian hope of explaining the universe.

That idea was so discouraging that Dr. Polchinski did not want to mention the anthropic principle in the paper he and Dr. Bousso wrote. But Dr. Bousso, coming from a cosmology background, did, and he had a trump card.

“We had just offered him a senior postdoctoral position at I.T.P.,” Dr. Polchinski wrote, referring to what would become the Kavli Institute for Theoretical Physics, “and he said that he would accept only if I agreed to be on the paper.”

As it happened, the string theory landscape fit perfectly with a theory of the Big Bang called inflation, which seems to predict that the universe could go on spouting new branches forever.

To date, there is no evidence that either string theory, inflation or the landscape is correct or incorrect. Nor is there any better explanation for Einstein’s fudge factor.

Some scientists and other thinkers have argued that the idea of the landscape is unscientific, since it cannot be tested directly. Dr. Weinberg disagrees.

“To say that speculating about a multiverse is unscientific because you can’t observe its other parts is like saying that it is unscientific to suppose that there are galaxies farther away than 100 billion light years, because in an expanding universe they can never be observed,” he wrote in an email. “We believe that such galaxies exist because our cosmological theories, which have been verified in other ways, tell us that they do.”

Dr. Polchinski wrote a widely used two-volume textbook on string theory, and for his work on branes he was awarded the Dirac Medal, which has often been a precursor of the Nobel Prize in Physics, in 2008. He shared a $3 million Breakthrough Prize in Fundamental Physics with Andrew Strominger and Cumrun Vafa, both of Harvard, in 2017.

But his work went deeper than string theory. His research on black holes reframed a 40-year-old argument about whether black holes would erase the information about what falls into them, a violation of the rules of quantum mechanics that govern subatomic reality. After first claiming that they would, the famed British cosmologist and black-hole guru Stephen Hawking relented and conceded a bet about this in 2004.

In 2012, however, Dr. Polchinski concluded that Dr. Hawking had given up too soon. When he and his Santa Barbara colleagues Ahmed Almheiri, Donald Marolf and James Sully set out to explain how information gets out of a black hole, they ran into a contradiction.

According to general relativity, you would not notice anything untoward — “no drama,” in the parlance — as you fell past the edge of a black hole toward doom. But according to quantum theory, you would be flash-fried by a firewall of energy right inside the boundary. The contradiction meant that either Einstein or quantum theory had to be wrong.

“It points to something missing in our understanding of gravity,” Dr. Polchinski said in an interview in 2014.

Their work shocked many physicists, who first denied it and then leapt into a continuing frenzy of theorizing and speculation about space-time and quantum weirdness.

“It was fun to have once again kicked over the hive and watched the bees swarm,” Dr. Polchinski wrote in his memoir.

On Nov. 30, 2015, he gave a talk on string theory in Berlin to celebrate the 100th anniversary of Einstein’s general theory of relativity. Three days later he suffered a seizure, which sent him to the hospital, where his brain cancer was discovered.

After months of treatment, Dr. Polchinski put his energy into writing his memoir, which he posted on the internet.

“I have not achieved my early science-fiction goals, nor explained why there is something rather than nothing,” he wrote in an epilogue, “but I have had an impact on the most fundamental questions of science.”

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