NEW YORK — Joseph Polchinski, one of the most creative physicists of his generation, whose work helped lay the mathematical foundation for the controversial proposition that our universe is only one in an almost endless assemblage that cosmologists call the “multiverse,” died Feb. 2 at his home in Santa Barbara, Calif. He was 63.
His death was announced by the University of California Santa Barbara, where he was a longtime professor and a permanent member of the Kavli Institute for Theoretical Physics. He had been treated for brain cancer since late 2015.
Mr. Polchinski was a giant force in the development of string theory, the ambitious attempt to achieve a “theory of everything,” which envisions the fundamental particles of nature as tiny wriggling strings. The theory has brought forth ideas and calculations that have opened new fields of study and new visions of a universe that is weirder and richer than astronomers had dreamed.
In recent years, Mr. Polchinski’s investigation of black holes, the Einsteinian monsters that gobble light and everything else, uncovered a mystery that calls into question what scientists thought they understood about how these objects work. The Firewall Paradox, as he and his collaborators called it, led to a firestorm of speculation about the nature of gravity and space-time.
Raphael Bousso of the University of California Berkeley, who worked with him, said in an e-mail interview that Mr. Polchinski “ranks among the greatest theorists of the last half-century.”
Joseph Gerard Polchinski Jr. was born in White Plains, N.Y., 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, Mr. 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. In addition to his wife, Mr. Polchinski leaves two sons, Stephen and Daniel, and a sister, Cindy Reid.
After postgraduate stints at the Stanford Linear Accelerator Center and Harvard, Mr. 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.
Mr. Polchinski joined a revolution. By the time he entered the profession, 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 particle colliders’ capabilities, and required nature to have 10 dimensions of space and time.
In 1995, Mr. 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 Bousso, who collaborated with him on string theory calculations of the number of universes.
Mr. 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, Mr. Polchinski concluded that 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,” Mr. Polchinski said in an interview in 2014.
Their work shocked many physicists, who first denied it and then leapt into a 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,” Mr. Polchinski wrote in his memoir.