r. Peter G. Bergmann, a physicist who worked with Albert Einstein and played a leading role in the advancement of Einstein's theories in the years after World War II, died on Saturday in Seattle. He was 87.

As a professor at Syracuse University from 1947 to 1982, Dr. Bergmann taught relativity to several generations of physicists and was a pioneer in efforts to reconcile Einstein's general theory of relativity, which explains gravity as the warping of space-time geometry, with the paradoxical quantum laws that rule atomic affairs. That quest is now at the center of modern physics.

Dr. Bergmann, born in Berlin, was only 21, with a fresh Ph.D. from the German University in Prague, when he joined Einstein at the Institute for Advanced Study in Princeton, N.J., in 1936 as a research assistant.

Peter Bergmann's association with Einstein began, without his knowledge, in 1933, when his mother, Dr. Emmy Bergmann, a pediatrician, sent a letter by courier to Einstein, who was then in Belgium hiding from the Nazis, extolling her son's virtues and asking if he could study with Einstein.

Einstein wrote back, again by courier, offering to bring her son to the Institute for Advanced Study, where Einstein had just accepted a position, once the young Bergmann had finished his work for a degree.

But Dr. Bergmann's mother never told her son of the correspondence. Two years later, Dr. Bergmann wrote of his own accord to Einstein, who in turn asked Dr. Bergmann's professor, Dr. Philipp Frank, for a character evaluation. Dr. Frank gave a glowing one.

In five years at the Institute for Advanced Study, Dr. Bergmann collaborated with Einstein on attempts to construct a so-called unified field theory to explain all the forces of nature. Among the attempts was a 1938 paper, building on a notion developed by the mathematicians Theodor Kaluza and Oskar Klein, that suggested that space-time was not four-dimensional, but had a fifth dimension that was not ordinarily perceived because it was very small.

Although Einstein and his collaborators subsequently turned to other ideas, the notion is now at the center of modern attempts to create a theory of everything.

"Bergmann and Einstein were the first to explain how the fifth dimension could be real and on a par with the others but just smaller," said Dr. Edward Witten of the Institute for Advanced Study. "It is a very modern idea."

While he was in Princeton, Dr. Bergmann also wrote the first textbook about general relativity, "Introduction to the Theory of Relativity." Einstein wrote the introduction.

"For a long time it was the book everyone read when they were studying general relativity," said Dr. Steven Weinberg, a physicist and Nobel laureate at the University of Texas in Austin.

After leaving Princeton, Dr. Bergmann taught at Black Mountain College in North Carolina and at Lehigh University in Pennsylvania as well as working for the Navy doing research on underwater sound at Columbia University and at the Woods Hole Oceanographic Institute in Massachusetts.

In 1947, when Dr. Bergmann joined the faculty at Syracuse, the study of general relativity was nearly dormant, said Dr. Engelbert Schucking, a physicist and longtime friend at New York University, because it was thought to be mathematically abstract and difficult to test experimentally. Over the years Dr. Bergmann created a center for the study of relativity, particularly its mathematical foundations, at Syracuse, guiding 32 students to their doctorates, and organizing visits of outside scholars.

"In those days, Syracuse was the place to be if you wanted to do general relativity," said Dr. Clifford M. Will of Washington University in St. Louis, "because no one was doing it anyplace else."

In the 1960's, a second center for relativity studies arose at Princeton under Dr. John Archibald Wheeler. This month, Dr. Bergmann and Dr. Wheeler were named winners of the American Physical Society's newly inaugurated Einstein Prize in Gravitational Physics.

Throughout his career at Syracuse, Dr. Bergmann commuted weekly from his apartment on Riverside Drive in Manhattan.

His wife, Margot Bergmann, who died three years ago, was a physical chemist at the Polytechnic University in Brooklyn.

After retiring from Syracuse in 1982, Dr. Bergmann took a post as a research professor at New York University. This year, he moved to Seattle to live with his son John, who survives him along with his son Ernest, of Bethlehem, Pa., and five grandchildren.