By Dennis Overbye

New York Times News Service

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 Saturday at his home in Santa Barbara, California. 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.

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.

In recent years, Polchinski’s investigation of black holes, 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 email that Polchinski “ranks among the greatest theorists of the last half-century.”

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

In a memoir, 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 he developed an interest in telescope-making and chess.

He later enrolled at the California Institute of Technology, where his freshman adviser was Kip Thorne, a future Nobel laureate 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, Polchinski joined the faculty of the University of Texas in Austin. He left for Santa Barbara in 1992 and stayed there.

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

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, 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.”

“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.

For his work on branes, he was awarded the Dirac Medal, in 2008. He shared a $3 million Breakthrough Prize in Fundamental Physics 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, Polchinski concluded that Hawking had given up too soon. When he and his Santa Barbara colleagues 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 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.

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,” Polchinski wrote in his memoir.