MICHELE ANGELO BESSO (1873–1955). Einstein’s closest friend. An engaging but unfocused engineer, he met Einstein in Zurich, then followed him to work at the Bern patent office. Served as a sounding board for the 1905 special relativity paper. Married Anna Winteler, sister of Einstein’s first girlfriend.

NIELS BOHR (1885–1962). Danish pioneer of quantum theory. At Solvay conferences and subsequent intellectual trysts, he parried Einstein’s enthusiastic challenges to his Copenhagen interpretation of quantum mechanics.

MAX BORN (1882–1970). German physicist and mathematician. Engaged in a brilliant, intimate correspondence with Einstein for forty years. Tried to convince Einstein to be comfortable with quantum mechanics; his wife, Hedwig, challenged Einstein on personal issues.

HELEN DUKAS (1896–1982). Einstein’s loyal secretary, Cerberus-like guard, and housemate from 1928 until his death, and after that protector of his legacy and papers.

ARTHUR STANLEY EDDINGTON (1882–1944). British astrophysicist and champion of relativity whose 1919 eclipse observations dramatically confirmed Einstein’s prediction of how much gravity bends light.

PAUL EHRENFEST (1880–1933). Austrian-born physicist, intense and insecure, who bonded with Einstein on a visit to Prague in 1912 and became a professor in Leiden, where he frequently hosted Einstein.

EDUARD EINSTEIN (1910–1965). Second son of Mileva Mari and Einstein. Smart and artistic, he obsessed about Freud and hoped to be a psychiatrist, but he succumbed to his own schizophrenic demons in his twenties and was institutionalized in Switzerland for much of the rest of his life.

ELSA EINSTEIN (1876–1936). Einstein’s first cousin, second wife. Mother of Margot and Ilse Einstein from her first marriage to textile merchant Max Löwenthal. She and her daughters reverted to her maiden name, Einstein, after her 1908 divorce. Married Einstein in 1919. Smarter than she pretended to be, she knew how to handle him.

HANS ALBERT EINSTEIN (1904–1973). First son of Mileva Mari and Einstein, a difficult role that he handled with grace. Studied engineering at Zurich Polytechnic. Married Frieda Knecht (1895–1958) in 1927. They had two sons, Bernard (1930–) and Klaus (1932–1938), and an adopted daughter, Evelyn (1941–). Moved to the United States in 1938 and eventually became a professor of hydraulic engineering at Berkeley. After Frieda’s death, married Elizabeth Roboz (1904–1995) in 1959. Bernard has five children, the only known great-grandchildren of Albert Einstein.

HERMANN EINSTEIN (1847–1902). Einstein’s father, from a Jewish family from rural Swabia. With his brother Jakob, he ran electrical companies in Munich and then Italy, but not very successfully.

ILSE EINSTEIN (1897–1934). Daughter of Elsa Einstein from her first marriage. Dallied with adventurous physician Georg Nicolai and in 1924 married literary journalist Rudolph Kayser, who later wrote a book on Einstein using the pseudonym Anton Reiser.

LIESERL EINSTEIN (1902–?). Premarital daughter of Einstein and Mileva Mari. Einstein probably never saw her. Likely left in her Serbian mother’s hometown of Novi Sad for adoption and may have died of scarlet fever in late 1903.

MARGOT EINSTEIN (1899–1986). Daughter of Elsa Einstein from her first marriage. A shy sculptor. Married Russian Dimitri Marianoff in 1930; no children. He later wrote a book on Einstein. She divorced him in 1937, moved in with Einstein at Princeton, and remained at 112 Mercer Street until her death.

MARIA “MAJA” EINSTEIN (1881–1951). Einstein’s only sibling, and among his closest confidantes. Married Paul Winteler, had no children, and in 1938 moved without him from Italy to Princeton to live with her brother.

PAULINE KOCH EINSTEIN (1858–1920). Einstein’s strong-willed and practical mother. Daughter of a prosperous Jewish grain dealer from Württemberg. Married Hermann Einstein in 1876.

ABRAHAM FLEXNER (1866–1959). American education reformer. Founded the Institute for Advanced Study in Princeton and recruited Einstein there.

PHILIPP FRANK (1884–1966). Austrian physicist. Succeeded his friend Einstein at German University of Prague and later wrote a book about him.

MARCEL GROSSMANN (1878–1936). Diligent classmate at Zurich Polytechnic who took math notes for Einstein and then helped him get a job in the patent office. As professor of descriptive geometry at the Polytechnic, guided Einstein to the math he needed for general relativity.

FRITZ HABER (1868–1934). German chemist and gas warfare pioneer who helped recruit Einstein to Berlin and mediated between him and Mari. A Jew who converted to Christianity in an attempt to be a good German, he preached to Einstein the virtues of assimilation, until the Nazis came to power.

CONRAD HABICHT (1876–1958). Mathematician and amateur inventor, member of the “Olympia Academy” discussion trio in Bern, and recipient of two famous 1905 letters from Einstein heralding forthcoming papers.

WERNER HEISENBERG (1901–1976). German physicist. A pioneer of quantum mechanics, he formulated the uncertainty principle that Einstein spent years resisting.

DAVID HILBERT (1862–1943). German mathematician who in 1915 raced Einstein to discover the mathematical equations for general relativity.

BANESH HOFFMANN (1906–1986). Mathematician and physicist who collaborated with Einstein in Princeton and later wrote a book about him.

PHILIPP LENARD (1862–1947). Hungarian-German physicist whose experimental observations on the photoelectric effect were explained by Einstein in his 1905 light quanta paper. Became an anti-Semite, Nazi, and Einstein hater.

HENDRIK ANTOON LORENTZ (1853–1928). Genial and wise Dutch physicist whose theories paved the way for special relativity. Became a father figure to Einstein.

MILEVA MARI (1875–1948). Serbian physics student at Zurich Polytechnic who became Einstein’s first wife. Mother of Hans Albert, Eduard, and Lieserl. Passionate and driven, but also brooding and increasingly gloomy, she triumphed over many, but not all, of the obstacles that then faced an aspiring female physicist. Separated from Einstein in 1914, divorced in 1919.

ROBERT ANDREWS MILLIKAN (1868–1953). American experimental physicist who confirmed Einstein’s law of the photoelectric effect and recruited him to be a visiting scholar at Caltech.

HERMANN MINKOWSKI (1864–1909). Taught Einstein math at the Zurich Polytechnic, referred to him as a “lazy dog,” and devised a mathematical formulation of special relativity in terms of four-dimensional spacetime.

GEORG FRIEDRICH NICOLAI, born Lewinstein (1874–1964). Physician, pacifist, charismatic adventurer, and seducer. A friend and doctor of Elsa Einstein and probable lover of her daughter Ilse, he wrote a pacifist tract with Einstein in 1915.

ABRAHAM PAIS (1918–2000). Dutch-born theoretical physicist who became a colleague of Einstein in Princeton and wrote a scientific biography of him.

MAX PLANCK (1858–1947). Prussian theoretical physicist who was an early patron of Einstein and helped recruit him to Berlin. His conservative instincts, both in life and in physics, made him a contrast to Einstein, but they remained warm and loyal colleagues until the Nazis took power.

ERWIN SCHRÖDINGER (1887–1961). Austrian theoretical physicist who was a pioneer of quantum mechanics but joined Einstein in expressing discomfort with the uncertainties and probabilities at its core.

MAURICE SOLOVINE (1875–1958). Romanian philosophy student in Bern who founded the “Olympia Academy” with Einstein and Habicht. Became Einstein’s French publisher and lifelong correspondent.

LEÓ SZILÁRD (1898–1964). Hungarian-born physicist, charming and eccentric, who met Einstein in Berlin and patented a refrigerator with him. Conceived the nuclear chain reaction and cowrote the 1939 letter Einstein sent to President Franklin Roosevelt urging attention to the possibility of an atomic bomb.

CHAIM WEIZMANN (1874–1952). Russian-born chemist who emigrated to England and became president of the World Zionist Organization. In 1921, he brought Einstein to America for the first time, using him as the draw for a fund-raising tour. Was first president of Israel, a post offered upon his death to Einstein.

THE WINTELER FAMILY. Einstein boarded with them while he was a student in Aarau, Switzerland. Jost Winteler was his history and Greek teacher; his wife, Rosa, became a surrogate mother. Of their seven children, Marie became Einstein’s first girlfriend; Anna married Einstein’s best friend, Michele Besso; and Paul married Einstein’s sister, Maja.

HEINRICH ZANGGER (1874–1957). Professor of physiology at the University of Zurich. Befriended Einstein and Mari and helped mediate their disputes and divorce.