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 on Friday 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.
Dr.
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, Dr. 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 email interview that Dr. Polchinski “ranks among the greatest
theorists of the last half-century.”
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, Steven 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 particle colliders’ capabilities, 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.
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 10500 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. 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
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.”
Correction: February 7, 2018
An earlier version of this obituary misstated the day Dr. Polchinski died and misspelled the given name of one of his sons. He died on Friday, not Saturday, and his son is Steven, not Stephen.
An earlier version of this obituary misstated the day Dr. Polchinski died and misspelled the given name of one of his sons. He died on Friday, not Saturday, and his son is Steven, not Stephen.
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