When Pauline Einstein learned that her beloved son Albert was consorting with a fellow physics student--one who was older, of another faith and from the backwaters of the Balkans--she was devastated. "If she gets a child, you'll be in a pretty mess," his mother warned him. But the 22-year-old Albert, as roguishly independent in his personal life as he would be in his science, brushed off Mutti's agitated words and continued the romance. On Jan. 27, 1902, nine months after an idyllic interlude at Lake Como, Albert's classmate--and future wife--Mileva Maric secretly gave birth to a girl at her parents' home back in Serbia. Neither Mileva nor Albert ever talked about her, even to close friends. Like some brief, fiery meteor, the baby named Lieserl (diminutive for Elisabeth) soon vanished into the Balkan night.

The illegitimate child in Einstein's past did not come to light until more than 30 years after his death, when the first volume of his collected papers finally appeared, in 1987. Still, a mystery remains. What happened to Lieserl? And after they married, why didn't the couple bring her back to Switzerland and legitimize her birth? Was she given up for adoption, as many scholars believe, because she might have endangered Einstein's new career as a patent-office examiner in Calvinist Bern? And might she even still be alive somewhere in Serbia, a wizened relic of the great relativist's youthful indiscretion?

Even after publication of seven more volumes of Einstein papers--and many more embarrassing revelations about his private life (his flirtations, his stormy divorce from Mileva, his possible dalliance with the daughter of the woman who would become his second wife, his estrangement from his two sons, one of whom was schizophrenic)--Lieserl's fate shadows the Einstein legend like some unsolved equation.

Now an amateur scholar is convinced that she has sleuthed some answers--ones that are not only surprising but also sure to touch off still more controversy among fractious Einstein historians. In a new book titled Einstein's Daughter: The Search for Lieserl (Riverhead Books; $25.95), Michele Zackheim, 58, a Greenwich Village painter turned writer, argues that the toddler was severely retarded and probably had Down syndrome. A simpleton child, in the language of the time, she would have been considered uneducable. Zackheim contends that Mileva, unable to place the little girl for adoption or send her to an orphanage, left her with her parents at their home in Serbia's rural Vojvodina region on the fertile Danube plain.

And there Lieserl's life was poignantly short. According to Zackheim, the little girl died at 21 months after a bout of scarlet fever. Zackheim even gives the date of her death--Sept. 15, 1903, when Vojvodina was darkened by a solar eclipse, the sort of celestial ballet between sun and moon that would later provide the world with the first proof of the correctness of Einstein's radical new ideas about time and space.

Zackheim's case is intriguing if not entirely convincing. A feminist activist in the 1960s and early '70s, she says she decided to pursue the book when she discovered that Einstein, a great icon of her youth in Compton, Calif., had had a child he might have forsaken. "It fascinated me from a psychological point of view," she says. "How did his daughter feel about being abandoned, especially by somebody who was so important to the culture?"

Helped by small grants and loans, Zackheim set off on her five-year quest for Lieserl, crisscrossing Switzerland, Germany, England, Hungary and especially Serbia. Even while bombs burst, she visited Mileva's ancestral villages, seeking her kin or anyone close to her family, including Serbian Orthodox priests and nuns, and holding many hours of coffee-table conversation, to say nothing of rummaging through countless baptismal records and archives for key documents. Many of them turned out to have been lost in the endless Balkan wars; others relating directly to little Lieserl may have been destroyed by Mileva's protective father.

The result is a colorful glimpse of rural Serbian culture, with its patrimonial society, strong family loyalties, female subservience, slow, leisurely discourse. Zackheim does manage to eliminate a number of women as possible Lieserls, including a melodramatic Berlin actress who claimed in the 1930s to be Einstein's daughter. Zackheim's final conclusions, however--based on little more than inferences from a cryptic 1903 letter from Einstein to Mileva ("I am very sorry about what has happened to Lieserl. Scarlet fever often leaves some lasting trace behind") and vague comments about idiocy in the family by an elderly Maric descendant in the Serbian town of Kac--remain conjectural at best.

The book has produced strong reactions, both positive and negative, in the academic community. "It sounds reasonable," says the University of Louisiana's Lewis Pyenson, author of The Young Einstein (1985), of Zackheim's theory. "I'd like to see what evidence has been dug up to support it." But Boston University historian Robert Schulmann, director of the Einstein Papers Project, is much less impressed. He concedes that Zackheim's conclusions about Lieserl's fate are "as good as anything I could come up with, or anyone else. But," he emphasizes, "it's speculation." Harvard physicist and Einstein historian Gerald Holton is highly critical. "She worked very hard traipsing through all those Serbian cemeteries," he says of Zackheim's prodigious research effort, "and came up with nothing."