An eternal movement from zero hour

Proms; Sir Harrison Birtwistle's Exody is the theme tune for the end of the millennium . It doesn't so much develop as evolve. Especially in Daniel Barenboim's capable hands. CHICAGO SYMPHONY ORCHESTRA ROYAL ALBERT HALL LONDON

WITH EACH arrival comes a new departure, with each answer a new question, and with each ending a new beginning. Sir Harrison Birtwistle's tremendous orchestral essay Exody ("23: 59: 59") begins in the vast empty space between the highest and lowest C naturals his orchestra can access, from violin harmonics and glockenspiel almost beyond the reach of hearing to string basses and electric piano on the bottom of the world. But it never truly ends, or indeed "arrives", but rather pauses, suspended on a single E natural as the digital display of your imagination finally clicks over from "23:59:59" to "24:00". It's the last midnight of the old millennium.

Thirty minutes of real time have elapsed in that one second. But what is a second in eternity? What is eternity but a never-ending journey? And so Exody journeys, unconstrained by time - real or imagined - until it stops (or doesn't, as the case may be). You may sense a resolution of sorts, but in fact there is none, because Birtwistle decided long, long ago that it was better to journey than to arrive. Whether active or inactive (and the tension between the two is one of the work's key features), Exody is music in a perpetual state of exposition - meaning that it is forever revealing itself.

Many strands, many colours, many ideas - heard individually or as dense weaves of sound - give the score its imperative. There are haunting, melismatic solos for the winds of the orchestra, not least the "alien" saxophone (whose exotic, strangely insidious arabesques so characterise this composer); long-bowed lamentations in the strings are pitted against hyperactivity in the rest of the orchestra (itchy percussion suggesting the workings of some cosmic timepiece); sudden suspensions, like time-outs or cardiac arrests, freeze-frame the action. Because this is music that exists "in the moment", in the magnified split-second before nature's timepiece clicks over.

Birtwistle is very much a composer of our time, of our century, but in his lifelong rush to zero hour (whenever that may be) he drags a lot of history with him. His music is full of primitive and mythological resonance. It doesn't develop; it evolves. It isn't composed; it just is. At least, that's how it sounds. And it sounded very well in the capable hands of the orchestra and conductor that commissioned it: Daniel Barenboim and the Chicago Symphony.

The rest of their eagerly anticipated first Prom - namely Mahler's Fifth Symphony - was oddly dispiriting, as if our energy, and theirs, had already been well spent. The whole performance seemed to come and go (mainly go) in strict accordance with Barenboim's own level of engagement. Sometimes he was there, sometimes not at all. Some of the playing was beautiful, articulate; some flaccid. As witness the quite shockingly sloppy attack, or rather lack of, into the second movement, marked, incidentally "with the greatest vehemence". Vehemence (as in those fierce Mahlerian contrasts) was not in the vocabulary of this performance. It was far too comfortable (sonically and spiritually) and self-satisfied; pat. The opening trumpet summons augured well with a blast from the past; a phantom bugler whose wide vibrato was very much from the old world. But the ensuing funeral march, whilst shot-through with the appropriate klezmer band colours, was very much about appearances and not about feelings.

But this is a tale of two Barenboims and two orchestras. Their second Prom, on Friday, was another story. First there was Till Eulenspiegel - rogue, master prankster - ducking and diving through the orchestra, whistling Richard Strauss the while. Lightning reflexes, wry smiles, rude rubatos, and a scrawny E-flat clarinet grimly anticipating the noose tightening around his neck. The Mahler of the previous night was already a faded memory, notwithstanding, of course, the spectral waltzes and militaristic marches and fatalistic hammer blows, the Mahlerian refractions of Alban Berg's sensational Three Pieces for Orchestra. Barenboim sought and found the heart of it - a cello solo lasting no more than a couple of bars. But its reach was a whole symphony's worth.

And so Barenboim was engaged, his musical authority (or should that be autonomy) possessed now of a deeper expression. You could hear it, feel it, almost touch it with the arrival of the great second subject of Tchaikovsky's Sixth Symphony. Just the way in which the first phrase was breathlessly anticipated, the way it bloomed and resonated. Very beautiful, very old school. As was the entire performance.

One could argue that the first movement development was too measured truly to unleash the furies (though bass trombone and tuba thrillingly marked out our descent to the abyss), that the finale's soulful adagio lamentoso was initially too rosy, but the artistry at work here was considerable, the Chicago strings investing everything in the kind of meaningful sostenuto you rarely hear outside Vienna or Berlin. And unlike his Mahler, Barenboim's Tchaikovsky didn't just tell us about Barenboim.