IT WAS the Fifties and Britain was never having it so good. Post- war austerity was bottoming out; men were settling into homes fit for heroes; women with vacuum cleaners were dedicating themselves to dust; and composers were re-establishing themselves in the traditional genres like opera, where much British ground had been lost during the previous 200 years.

But on mainland Europe, where the scars of war cut deeper, it was a different story. Sure, they had vacuum cleaners; but they also had electronic music studios, peopled by a fierce, committed avant-garde concerned to make its connections with the future rather than the past. Mending the torn fabric of pre-war creativity was not on the agenda. New work required new languages: and so began the pluralist riot we know now as contemporary music - speaking in such a diversity of tongues that you'd think it impossible to summarise in a handful of works what the music of the Fifties amounted to.

But this is the task Simon Rattle sets himself in the latest instalment of his Millennium series: a 10-year project working its way decade by decade through the 20th century. He has just reached The Fifties, with a series of CBSO concerts that travel between Birmingham, London and Vienna; and very conspicuously they don't include a single British piece. There had been plans for a concert performance of Tippett's Midsummer Marriage, but they were scotched by competition from Covent Garden, and Rattle clearly didn't feel that any other British score quite caught the spirit of the age.

So, this Fifties snapshot focuses on a starkly black and white contrast between the separatism of the European avant-garde and the inclusiveness of American popularism. Leonard Bernstein represents the latter; in the opening concerts last weekend the representatives of the former were Stravinsky (b 1882 and in the home run of his creative life), Messiaen (b 1908 and in mid-stride), and Stockhausen (b 1928 and, along with Boulez and Cage, the dazzling new kid on the block). Three generations, united by the fact that they were living not just in the fall-out of Hiroshima but in the fall-out of serialism: the Big Issue that no European composer in the Fifties could duck.

As an example of how the grand old man of modern music was dealing with it, the CBSO began their opening concerts with Stravinsky's Agon: the ballet score whose unfolding movements summarise their author's journey from tonality to serial- ism. The title refers to an abstract idea of contest in the dancing. But equally it suggests a contest of language; and although serialism wins the battle, it loses the argument in that the earlier, non-serial music in the piece is far the richer and more engaging.

For richness of sound in new methods there followed Messiaen's Chronochromie, a prime contender for seminal status among Fifties scores which explores a posited relationship between time and colour and grows into a great, non-developmental, circular machine of sound - engineered here by the CBSO with a finesse worthy of the London Sinfonietta. The performance was exemplary of what this orchestra has become in recent years under Rattle: an instrument of stunning versatility that can one week be a period band in Beethoven, the next a high-precision advocate of modernism, and convincing to the nth degree of idiom and style in both.

But the piece de resistance was still to come in Stockhausen's Gruppen: a true epic of its time and rarely heard now - partly because Stockhausen's credibility isn't what it was (his claim to be an alien from outer space may be why) and partly because the demands of the score (three orchestras, three conductors, and a great deal of rehearsal) are impractical for a piece that lasts 25 minutes and fills less than half a programme.

The reason for the three orchestras is that Gruppen is an experiment in layered pulse and pace. Like so many others at the time, Stockhausen was obsessed with the idea that all the elements of music could be incorporated into one single system of organisation: a kind of musical totalitarianism that required him to demonstrate how apparent disparates like rhythm and pitch were actually the same matter and answerable to the same laws. Having, as he thought, done this - by showing how a regular pulse, speeded up, becomes a pitch - his next proposition was that rhythm and pace could be vertically massed into kinetic texture just as notes are massed into harmonic texture. But to attempt such music with one conductor giving one beat to one band was asking a lot. Hence the triplication. And by arranging the orchestras on three sides of a square - as was the case at the 1958 premiere - with the audience in the middle, he could also utilise spatial sound, rocketing signals back and forth between the bands.

All this, though, produced a piece that doesn't easily fit into a concert hall. In Birmingham, Rattle used the convention centre that adjoins Symphony Hall. In London he was stuck with the RFH, and it was a debilitating compromise. The platform layout didn't do enough to envelop the audience, and the effect of travelling signals was limited. But even so it had enormous aural impact as the textures massed and thinned, and strands of sound like complicated railway tracks converged and separated. For all the theory behind Gruppen, it strikes the ear less as a demonstration piece than as a coda to the lush excesses of German Romanticism. The ear wallows. And about the only response the intellect can make to such an overwhelming mass of sonic data is to register the fact that Gruppen is, and must be, an Event. The 1958 premiere certainly was. The conductors then were Pierre Boulez, Bruno Maderna and Stockhausen himself, and there exists a memorable photograph of them together in rehearsal, striking quintessentially characteristic poses: Maderna punchily aggressive, Boulez inscrutable, and Stockhausen staring into the distance like a small child waiting for a vision.

For the CBSO performances the triumvirate was remarkable, too, in that it spelt out a line of conductorial descent. In the middle was Rattle, to the right his teacher John Carewe, and to the left his brilliant young protege, Daniel Harding - who at 21 is clearly following in Rattle's footsteps, not to say his baton manner. Together they staged a feat of co-ordination as dazzling to see as it was to hear. And we were dazzled twice because, to make it worthwhile, the whole thing was repeated - with a five-minute break in which Rattle encouraged the audience to swap seats and get a different sound perspective second time around. I'm told that at this point, in Birmingham, there was mayhem. In London it was pure politeness - "After you. No, after you" - but perhaps that's because we were punch-drunk by the power and spectacle of what had gone before. This may not be what Harold Macmillan meant by having it good in the Fifties, but it would have got my vote.