THE harpist Emily Hoile was 19 the first time we met. She had never done an interview before. She was just through secondary school in Edinburgh, newly a college student at Julliard in New York, still getting to grips with life outside the UK. She told me about the dismal calibre of tea drinking she encountered in the United States, and the lifeline that was her mother’s regular care package of chocolate bars. She was utterly self-effacing about having just been booked for a major five-concert residency at the Lammermuir Festival.

Seven years later, Hoile’s voice comes down the phone with her native Newcastle vowels now rounded by stints in New York, Munich and Berlin. Much has happened since we last spoke. She completed her studies at Julliard and immediately won a place on the world’s most prestigious orchestral apprenticeship scheme – Berlin’s Karajan Academy, in which select young players work side-by-side with members of the Berlin Philharmonic. Which is to say, a month after finishing her undergraduate, Hoile found herself touring with the most august orchestra on the planet. But even that didn’t last long – because a year into the scheme, Hoile was poached by another top German band. At the age of 23, she became principal harp of the Bavarian Radio Symphony Orchestra.

Now she is back in Berlin and, for the first time since she was a toddler, she isn’t tied down by any kind of training scheme or orchestral contract. She says she’s taking stock, trying out new things. She recently recorded an album of chamber music with members of the Deutsche Kammerphilharmonie Bremen. And she’s returning to Lammermuir Festival this month to perform one of the most challenging and outlandish works ever written for harp.

Karlheinz Stockhausen. The cult iconoclast of Germany’s postwar avant-garde who pioneered electronic music, coined ultra complex structural theories – and evangelised music as a kind of cosmic ceremony. He designed his own house with no right angles. He believed that God gave birth to him, and that he came not from Earth but from a planet orbiting the star Sirius. For many years he dressed in orange jumpers, then all in white. His late music, especially, is a weird amalgam of all of that: mathematical process meets alpha-male bravura meets far-out spirituality, literally. Call it astral eroticism, call it spiritual kitsch in the age of gurus. Miles Davis, Bjork and Brian Eno were among umpteen celebrity devotees. The composer/conductor Esa-Pekka Salonen described Stockhausen as "the rock star of my youth”.

As well as groundbreaking works for electronics, vocal ensembles and orchestras, Stockhausen created vast mystic theatre pieces. For 30 years he assembled a seven-opera, 29-hour behemoth cycle called Licht, which involved an opera for every day of the week and a star turn for string quartet and four helicopters. And then there was Klang, dating from his very last years. The name means simply ‘sound’, and the plan was for a cycle of 24 hour-long pieces, each with its own designated colour, one for every hour of the day. Some of the “hours” can be performed in 20 minutes, while the third, Natural Durations, sprawls across two hours. Maybe time is more elastic up there on Sirius.

Stockhausen never finished his day. The last three hours of Klang remain silent, with no sketches found after his death. In its entirety -– 21 pieces – the cycle is epic, and yet much of the music feels extremely intimate. It’s also extremely stylised. Stars loom large in Stockhausen’s mythology, and performers in Klang often stand and turn like little planets rotating on axes of their own. It’s music of obsessive ritual, strict ceremony loaded with galactic tat. But there was no room for irony in Stockhausen’s ethos. One thing that characterised all the work he made was absolute conviction. The man was not lacking in self-belief – and that, according to Hoile, is what any performer of his work must also channel. “To do this music justice,” she says, “you have to commit to his whole world.”

And what a world Klang is. The 13th hour, Cosmic Pulses, is a trippy, resplendent magic carpet ride – Stockhausen’s last purely electronic work. Other hours are scored for solo instruments or various ensembles. A wind trio features in the eighth hour, Bliss; flute and electronics in the 21st hour, Paradise; septet and light sculpture in the tenth hour, Brilliance. The fourth hour, Heaven’s Door, is a theatre piece for air raid sirens, a little girl and a percussionist beating on a door. Apparently that image came to Stockhausen in a dream in which he had to play his way into heaven.

The second hour is called Freude, meaning Joy. The allocated colour is a medium blue. Two harp players chant in Latin – a Pentecost hymn called Veni Creator Spiritus – and do all kinds of things to the strings. According to Stockhausen, the piece consists of “plucking, picking, caressing, stroking, pinching, rubbing, striping, striking, pinking, jubilating”. He explained the structure: “in accordance with the 24 lines of this hymn, I have composed 24 musical moments like the 24 hours of the day, so that the Second Hour of KLANG is a full day within one hour of the day.”

I know how it often feels to listen to this music: for me, a mixture of bemusement, irritation and genuine, involuntary awe. How does feel it feel to play it? “At the end of a performance: tired!” says Hoile. “Like I’ve come out of some sort of trance. And strangely energised. To be honest, it depends how mentally prepared I am. There are very specific emotions that we have to get across, and they demand full attention. The actual elements of the piece are pretty intense: a lot of loud chords and a lot of waiting. There are clear instructions in the score to wait for ten seconds here, wait for seven seconds there. It’s very brutal but there is also a lot of space. And at some point we have start singing…” she groans. Neither she nor her duo partner, the French harpist Marion Ravot, is a trained singer. Does it matter? “Well, it was pretty excruciating to start. But we played it for Stockhausen’s daughter in Berlin, and she was very encouraging. Which was enough of a green light for us!”

Emily Hoile and Marion Ravot play Stockhausen’s Freude at 10pm on 22 September at St Mary’s Parish Church, Haddington. The Lammermuir Festival is 14-22 September at various locations around East Lothian