An opera set in the World Trade Center on 9/11? How is that possible? How can opera attempt to dramatise the thousand-fold tragedies of the twin towers? I’ll be honest. My first reaction to hearing about Tansy Davies’s Between Worlds was one of incredulity. Isn’t the whole notion of an opera set high on the north tower, above the impact zone of the first plane, recreating the last minutes of lives that ended in some of the most terrifying circumstances it’s possible to imagine, an operatic risk too far, which could trivialise the tragedy and fail to honour its world-changing impact?

It’s the opera’s director Deborah Warner who starts to scotch my ideas, when I meet her along with Davies and the writer Nick Drake, who is making his first foray into opera with his libretto for Between Worlds. As Warner says, the issue isn’t that the events of 9/11 are a theatrical no-go zone or a terra incognita, but rather the opposite. “In fact, what’s remarkable is how touched this territory is. It’s quite careless that it’s all there, that people can go online and find a fragment of one of those lives that were lost. There’s very little that one can’t find. So in fact it’s important that this material is dealt with seriously and with love.”

Drake reminds me how often the events of that day in 2001 have already been remade by the media. “The subject of the twin towers has been used in documentaries, in strip cartoons, in novels, and in movies like United 93, and there’s Nicolas Cage’s heroic fireman in Oliver Stone’s World Trade Center. In a shocking way, the day offers itself as a dramatic shape. There were 102 minutes between the first plane going in and the second tower going down [which was the north tower, the first to be hit; the south tower had collapsed earlier]. So you have Greek circumstances to this story, unities of time and place. And there are ironies that are extraordinary: it was a famously beautiful day – all of the newscasts started out talking about sunny highs and blue skies – and out of that, there’s this utterly unexpected thing, the most changing drama of our times.”

ENO’s Between Worlds collaborators (left to right) Deborah Warner, Tansy Davies and Nick Drake. Photograph: Martin Godwin for the Guardian

For composer Davies, “it was quite a terrifying idea to step into this. But I thought, actually art needs to be reacting to these huge events: if you’re not, why not? It’s our responsibility. I felt I had to go with this instinct to keep going.”

That’s a compelling manifesto of artistic necessity, but the roots of Between Worlds lie in another project that brought Davies and Drake together. Their first idea was to turn Philippe Petit’s tightrope walk between the World Trade Center towers into an opera, a story familiar from the documentary Man on Wire, and which Drake had already turned into a play. But they discovered that Petit has now sold all of the rights to Disney. Later, it was Deborah Warner’s realisation that “it would be impossible to be in relation to the towers in a piece of music-theatre without being in relation to what happened to them”. Drake remembers the moment when the fate of the twin towers on 9/11 came into focus as their subject: “suddenly it was undeniable, and once we realised that could be the subject – daunting as it is – we could not say no.”

But it’s one thing to feel the artistic pull to embark on an opera, and another to conceive the drama, words, and music of how it all might work in the theatre. What audiences will see is an opera staged between multiple worlds: there are the characters who are trapped on the high floor of the north tower and a chorus of onlookers on the street, out of which the voices of the loved ones of the doomed protagonists emerge. But there is also a third layer of the drama, the Shaman character (sung by countertenor Andrew Watts), who occupies a cosmic space beyond life and death. Among the terrified denizens of the tower is the Janitor (sung by Eric Greene), whose own knowledge of personal sorrow and grief (in his back story, his wife has died) makes him a bridge between the worlds. The Janitor consoles the characters and gives them the chance to communicate with the most important people in their lives while they still have time, but he also confronts them with the devastating and inescapable reality of their situation.

These are all invented characters, but they spring from the reality of what we know happened in those appalling 102 minutes. As Nick Drake says, “there were half a million pager messages sent in that time, and they cover the whole gamut of communication – from personal messages to the armed forces to the police – and they start out in an ordinary way and become extraordinary. They are like an archaeology of the day. And within those messages, what’s amazing is the imperative that people felt to communicate: at first to find out what was happening, but when they knew, their absolute imperative was to make contact with their loved ones. You get messages that start out saying: what do you want for dinner? And within an hour, they’re saying, you make my life, please go on living, please go on loving. It’s a human drama that’s absolutely elemental.”

Watch a trailer for Between Worlds – video

It’s a unique challenge for a composer to turn into music. Davies describes her score as a “dark cradle” for the drama, voicing the “cosmic perspective that music gives us”. She’s found a new kind of harmonic universe for this piece, and each of the 11 scenes of the opera is based on a series of 11 chords, which stand both metaphorically and musically for the twin towers: the visual shape of the number “11”, but also, in the score, the way the chords “stand there, pretty solid, and yet there are these waves of energy that just keep breaking against them, so there’s kind of a subversive element even within a structure that’s quite rigid”.

The places that Davis and Drake have gone to in their imaginations to create Between Worlds – and the places they invite their audiences to inhabit – seem to confront a horror and darkness that could be irredeemable. Yet that’s not where the drama of Between Worlds will end up. As Drake says, “we don’t want to leave people in darkness – although that has to be acknowledged. What the people in the towers really needed to say to each other in the end was: love. It sounds simplistic to say, but it wasn’t simplistic to those people. They were the last words they were going to say, the last things they wanted to convey. That says something very important about human beings, which is at the heart of what we’re trying to do”. Davies agrees: “The words and the music go to some very dark places, but if you go deep enough, it’s universal, and there is life there. And the music is full of that.”

• Between Worlds is at the Barbican, London, 11-25 April. Box office: 020-7638 8891.