Specimen Days, by Michael Cunningham

A tired opposition is sometimes pressed between traditional and experimental novels. But each great novel is in some sense an experiment that came off - think of Mansfield Park or The Brothers Karamazov. So perhaps fictions we persist in calling experimental are either those where the jury is permanently out, or else books that failed.

Michael Cunningham's Pulitzer-winning The Hours, with its three interlocking stories about depressed lesbians - one of them Virginia Woolf, and each imprisoned in her different decade - was an experiment that worked. First, as a book; later, as a film with Nicole Kidman in a false nose, writing Mrs Dalloway.

Although Specimen Days has three narratives too, Cunningham is not repeating a formula. True, where Woolf dominated The Hours, here Walt Whitman's Leaves of Grass echoes throughout. In the first section ("In the Machine"), set in Victorian, industrial Manhattan, a magical changeling child called Luke quotes Whitman ceaselessly and inexplicably, and meets the great man on the streets.

Luke puts his hand into the nameless machine that devoured his brother Simon, in order, it turns out, to distract Catherine, who grieves the loss. He asks to see Catherine because her workplace, like the Twin Towers, will be destroyed by apocalyptic fire: absence saves her life.

Fresh incarnations of Luke keep recurring. He is a gifted sensitive out of both Salinger and American Beauty, acquainted with the terrible beauty of existence. Part two - "The Children's Crusade" - sees Luke as a terrorist-child in (roughly) contemporary New York, trained by a fanatical mother, like his two brothers before, to attach himself to an adult then detonate them both. His chosen victim is the second Catherine, a black civil servant with a new but still handsome Simon-lover. This second Luke, like the first, has been indoctrinated to recite Whitman. He and "Cat" drift off into an ambiguous future.

Part Three ("Like Beauty") is set in a futuristic New York beset by friendly aliens from another troubled planet. Here, Simon recurs as an android longing to be human, a wish granted only after he has suffered the death of a new and extraterrestial lizard-Catherine, and Luke's escape towards a Whitmanesque third planet. This final Simon has been programmed to declaim from Leaves of Grass: "The smallest sprout shows there is really no death... All goes onward and outward, nothing collapses,/ And to die is different from what anyone supposes, and luckier". Beautiful, invigorating, visionary lines, that make you want to read Whitman properly.

But Cunningham never decides what he wants to do with this rich meditation on the terrors of machinery, the mysteries of loss, impermanence and continuity, and the ambiguities of religion. He evidently wants his fiction to surprise its author as much as its readers, and thus Specimen Days is always unexpected and brave.

Its recurrent triangle - boy, man, woman - seeking repeatedly to build a nest in successive unstable centuries recalls the experimental kind of family he explored in his lyrical novel, A Home at the End of the World. Good writers must of course continue experimenting, even when - as in this whimsical confection - they cannot convince.

Peter J Conradi's life of Iris Murdoch is published by HarperCollins