Inspirational … Gérard Grisey. Photograph: Salvatore Sciarrino/Casa Ricordi

The 1999 premiere of French composer Gérard Grisey's Quatre Chants pour Franchir le Seuil – Four Songs for Crossing the Threshold – was one of those concerts that went straight into the history books of contemporary music. The performance, by the London Sinfonietta, conducted by Grisey's friend George Benjamin, was never supposed to be a memorial for Grisey, but his sudden death at the age of 52 the previous November meant that these death-haunted songs would be his last completed work, music that imaginatively explores the existential inter-zone between life and death. There are apocalyptic visions (listen to the start of the fourth song, The Death of Humanity, to hear the world implode in a black hole of percussion writing), reflective resignation and spectral stasis in these songs. But more than anything, you're left with a sense of benign acceptance. That's true above all in the ethereal intertwining of the flute melody and the soprano voice in the final Berceuse movement, music which realises the post-cataclysmic serenity of a passage from the Epic of Gilgamesh, with its open-ended final words, "I looked at the sea's horizon, the world …"

Terrible coincidence as it was, the sense of expressive catharsis in that performance of Quatre Chants was only partly to do with Grisey's own death; much more, it was down to the astonishing musical space that this piece conjured in its unflinching exploration of existence. For Grisey, every single sound was a living, breathing entity; it was only logical that he should want to explore what happens at the end of the sonic life-cycle as well as the start. But in retrospect, the "threshold" that the Four Songs crossed wasn't only the event horizon that separates existence from non-existence, it was the door to a new kind of music that Grisey tragically would not have time to explore.

Yet the composer had already given the world visionary, inspirational and complete music. His achievement has often been reduced to yet another of new music's fetishistic labels, "spectralism" – a category that Grisey had rejected by the end of his life. The principles of spectralism are easy to describe, and like all good musical cliches, there's more than a grain of truth in the term. The essential idea is the creation of a new way of structuring the parameters of music by exploring the harmonic series, the overtones that are part of every musical note. If you analyse the complexity of the harmonic series of a single note played on a particular instrument – say a low E on a trombone – you find a teeming world of musical possibility. (That E, by the way, is exactly the note that Grisey took as the starting point for his ensemble work Partiels, the third piece of his epic, six-part cycle Les Espaces Acoustiques.) For Grisey, the possibilities of this approach were microscopic yet infinite. By atomising sounds in this way, he could structure large pieces of music and spans of time, such as Partiels, that were based on an intense process of listening to an individual sound, exploding the smallest of sonic phenomena, a single note, on to the largest possible scale. The harmonic implications of the overtone series also allowed Grisey to create a hierarchy within his micro-tonally enriched musical world, which gives his music a monumental dynamism.

So much for a decidedly spectral sketch of the theory, but let's get stuck into what Grisey wanted us to get stuck into, which is the stuff of sound, the sounds his music makes. Whether you use the "spectral" label or not, Grisey's music is about sound as material, as physical element, as living phenomena. As he said, "we are musicians and our model is sound not literature, sound not mathematics, sound not theatre, visual arts, quantum physics, geology, astrology or acupuncture". (He doesn't say "sound not metaphor", incidentally, because his music is not some abstract sonic science lesson, but about how sound affects our ears, about how we hear, about how it makes our brains and bodies vibrate, and what it can make us feel. Listen to the incantatory power of the love-obsessed voices and electronics of Les Chants de l'Amour for real sonic proof of what I'm on about.)

So here's the opening of Transitoires, the fifth part, for large orchestra, of the ever-increasing musical forces required for Les Espaces Acoustiques (the cycle starts with a solo viola and ends with a huge orchestra and four solo horns in the final part, Epilogue). A glittering, shimmering, light-filled chord begins Transitoires; the music then seems to stop time with its pregnant pauses, and with echoes of sounds – a guttural double-bass growl, a low gong, a mysterious middle-distance drone – that are sustained, seemingly into the infinite. As with so much of Les Espaces Acoustiques, Transitoires makes sounds that are simultaneously ancient and modern: there's something primeval, even ritualistic about those microtonal chords which chime towards the end of the piece, the way the gong's music tolls underneath the whole texture, and how the viola emerges with its solo song right at the end. That music is a bridge to the Epilogue of the whole cycle, with its whooping chords for four solo horns, the sounds of a universe of sound rejoicing in itself.

But it's part of the paradox of Grisey's music that just as he can slow down time so that you feel you're inside, say, a stretched-out gong stroke for 20 minutes, he can also speed it up with surreal velocity. (Grisey himself spoke of the difference between the sort of super-slow time experienced by whales as opposed to the frenetic time-scale of insects.) Listen to the opening of Vortex Temporum, and the way in which flourishes of notes in the ensemble spin out of the orbit of the piano music (based on a flute solo from Ravel's Daphnis et Chloé). Or, for another kind of mobile time, hear how Grisey makes a solo contrabass clarinet swing, slide and stride with mythic abandon in his evocation of Anubis-Nout, a piece written for the Canadian composer Claude Vivier, who was murdered in 1983. Or, for another surreal manipulation of musical time, listen to the funereal bass drums of Stèle, a piece for percussion that manages to be almost anti-rhythmic with its slides, scrapes and tollings.

Grisey's mastery of a gigantic spectrum of sound, texture and feeling makes his music some of the most special of the late 20th century. Le Temps et l'Écume, scored for chamber orchestra, percussionists and synthesisers, is a transcendence of the Grisey archetypes of scintillating harmony and orchestral colour, and it gives you that disturbing-but-seductive sensation of going through a mirror, the way it seems to connect the small scale of its details and surfaces – a skirl of woodwind sound, a trumpet solo – with an all-encompassing dimension of cosmic, coruscating chords and orchestral textures. It's not just those Quatre Chants: Grisey's music is always crossing thresholds of sound and space, of slowness and speed, of time at its grandest and most fleeting. To hear Grisey's music is to have adventures in the stuff of sound that will change your ears for ever. Enjoy.

Five key links

Quatre Chants pour Franchir le Seuil

Partiels

Transitoires

Le Temps et l'Écume

Vortex Temporum