It is hard to believe we're a whole decade away from The Joshua Tree — U2's very own Born in the U.S.A., their Purple Rain, their defining moment of megastardom. Seems like only yesterday that the band was gazing out from the wide-screen desertscape sleeve of the 15 million-selling album: four Dublin boys against the world, about to conquer it.

Then again, so much has happened since U2 packed the stadiums of America with soul-stirring anthems like "Where the Streets Have No Name" and "I Still Haven't Found What I'm Looking For." Like all of rock's most astute operators, the band has striven to reinvent itself at every turn, to stay at least one step ahead of the game. Most boldly of all, after Rattle and Hum's muddled flirtation with America's roots music, U2 pulled up stakes for dark, kinky Berlin and turned themselves into the mischievous, neo-glam rockers of Achtung Baby and Zooropa. It didn't matter that the Zoo TV Tour was post modern posing of the worst kind (who could forget Bono's cringe-producing telephone calls from the stage?), for U2 had succeeded in changing the way we looked at them. Even if you took Bono's demonic Mister MacPhisto, his Last Rock Star alter ego, with a large pinch of salt, you still had to credit the guy with a canny awareness of pop's cultural bankruptcy in the late 20th century.

Advance word on Pop, the new U2 album, suggested that it would edge still further away from rock & roll heroics that the band was even experimenting with the spooky, flim-noirish soundscapes of trip-hop. The album's very title seemed to indicate a conscious rejection of "rock," a shrewd move at a time when America is tiring of alternative guitar sludge and even Billy Corgan is talking of using "loops" on his next record. (R.E.M., U2's greatest rival in the Biggest Rock Band in the World stakes, may have called their last album New Adventures in Hi-fi, but the adventures in question sounded suspiciously old.)

As it turns out, you won't find much evidence of trip-hop on Pop, although sections of "Miami" and "If God Will Send His Angels" come close to that mutant strain of the genre. What you will find is a whole arsenal of sound effects, tape manipulations, distortions and treatments designed to mask the fact that U2 are still essentially a four-piece male rock band. Unlike R.E.M., U2 know that technology is ineluctably altering the sonic surface — and, perhaps, even the very meaning — of rock & roll. In that sense, their competition now is not so much R.E.M. as it is Orbital or Prodigy.

What we can say immediately is that Pop sounds absolutely magnificent. Working with Flood, who engineered Achtung Baby and co-produced Zooropa, the group has pieced together a record whose rhythms, textures and visceral guitar mayhem make for a thrilling roller-coaster ride, one whose sheer inventiveness is plainly bolstered by the heavy involvement of techno/trip-hop wizard Howie B(familiar from his work on Passengers' Original Soundtracks I).

Having messed with conventional rock sound ever since hiring Brian Eno to produce The Unforgettable Fire, on Pop, U2 stray considerably deeper into the world of loops and samples — of remix culture in general — than they did on Achtung Baby. There's a Byrds riff here, a snatch of Le Mystere des Voix Bulgares there. There are endless fascinating bleeps, squawks, drones and juddering — and a good deal less rattle and hum. (U2 aren't interested in "roots" anymore, or at least no longer treat them as articles of faith.) Even in the realm of the once-trusty electric guitar, the distortion of sound is so radical that you barely recognize the instrument. Indeed, the Edge has a veritable field day on Pop, one minute out-Neil Younging Neil Young, the next taking the psychedelic funk of "The Fly" and "Mysterious Ways" to new extremes. Those searing, sheared harmonics are still there, but they're compressed and warped and mangled into crazy new shapes.

And yet what makes U2 so fiendishly clever is the way they reinvent themselves without sacrificing the driving riffs and rhythms that have always powered their greatest songs. Coming after the superficial thrill of the glib opener, "Discotheque" — no more than good INXS, which isn't good enough — "Do You Feel Loved" arrives as a triumphant reaffirmation of U2's strengths, built on an Adam Clayton bass line as dominant as the one on "In God's Country" and boasting an instantly unforgettable chorus. And for anyone who finds the more experimental stuff on Pop too twisted, there's a trio of, well, rockers ("Staring at the Sun," "Last Night on Earth" and "Gone") that are going to sound just dandy in the coliseums this summer.

Interestingly, there's also a marked throwback on Pop to Bono's soul-searching of yore. References to God and Jesus abound, far more so than on Achtung or Zooropa. "Jesus, Jesus, help me/I'm alone in this world/And a fucked-up world it is, too," he sings with a groan on the closer, "Wake Up, Dead Man." "So where is the hope and where is the faith...and the love?" he asks on the tremendously pretty "If God Will Send His Angels." U2 may have given themselves permission to guzzle Dom Perignon and cavort with supermodels, but Bono badly wants us to know that he's still deeply perturbed by the ruin and spiritual decay of the world: "Intransigence is all around....Military still in town/Armor-plated suits and ties... Daddy just won't say goodbye" ("Staring at the Sun"). Is this man having a crisis of faith or what? Maybe it's just a crisis of conscience: On "Gone," which can be read as a desire to shake off fame ("this suit of lights"), Bono confesses that "you get to feel so guilty, got so much for so little."

Along with the wracked soul-searching comes a return to the singer's old preoccupation with Bad America. "The Playboy Mansion" is all about the sick faith in America's dream of redemption through glamour, decked with references to O.J. Simpson and Michael Jackson (and talk shows and Big Macs). On "Miami," a kind of reprise of "Bullet the Blue Sky," the city becomes a sinister pinky-blue dystopia, with "surgery in the air." (There's a witty touch, incidentally, when Larry Mullen Jr. kicks in, playing the sampled-to-death drum pattern from Led Zeppelin's "When the Levee Breaks.") While Bono the lyricist can still be unbearably gauche ("It's the blind leading the blonde/It's the stuff of country songs") or just pompously vague ("The more you take, the less you feel/The less you know, the more you feel/The less know, the more you believe"), at his most focused he can touch raw nerves like precious few other pop megastars. And all this without even mentioning two highlights that also happen to function as the poles between which Pop operates: the nightmarish industrial maelstrom of "MoFo," complete with shards of slashing guitar and a deranged lyric about incest, and the exquisite "If You Wear that Velvet Dress," a shimmering ode to a moonlight siren who beckons Bono away from the healthy, honest sunshine.

Pop may turn out to be a make-or-break album for U2. Alone among the giants of the '80s, they have a chance to carry their musical vision into the 21st century while still selling a ton of records. Are people still listening, or has rock & roll splintered into too many different tribes for a single band to shoulder the weight of our faith in its dream? Well, if people have stopped caring, it won't be U2's fault. With Pop, they've defied the odds and made some of the greatest music of their lives. Pretty heroic stuff, come to think of it.