The attempt to find out how a composer composes, to try to locate the precise connection between a creative impulse and its realisation in an orchestral piece, a chamber work, a song, or an opera is something that every writer about music has wanted to do for as long as there has been music that has inspired, moved and changed us. How do composers do it? What's their relationship with the world around them, with music history, with the people in their lives, and how does all that affect and shape the music they write?

I talked to Thomas Adès, one of today's most brilliant composers (and conductors, and pianists, and programmers) to get him to explain his musical universe. Did he explain? Only up to a point. But our conversations are sketches towards an understanding; their illumination, I hope, comes from how Adès answers, parries, and sometimes obfuscates the questions I ask him. Along the way, there are insights into the music he loves and loathes, especially the operas of Janacek and Wagner, aspects of Mahler's symphonies, and Brahms's and Britten's music. He also reveals his priorities as a composer and listener: the "magnetism" he hears in every note or combination of notes he plays on the piano or puts down on manuscript paper.

Adès talks about stability and instability; the challenges of writing opera (he has composed two so far – Powder Her Face and The Tempest, which he is conducting at the Metropolitan Opera in New York in a production by Robert Lepage next month – and is currently writing a new opera on Luis Buñuel's film The Exterminating Angel) and his problems with Wagner; about how he has dealt with the issue of endings in his music and what this tells us about the times in which we are living.

What is the principal difference between writing an opera and writing something else?

Thomas Adès: Well obviously it takes more time, it's longer. Aside from that there's the question of the scale of the gestures. There is a mysterious thing that happens when you set actions to music: a third shape that emerges when something non-visual like a musical score is acted out by people moving on a stage. To make this completely absurd thing watchable in any way, it is not that straightforward. There's no absolute way to do it. You just have to do it instinctively or not at all. Most of the time I sit and watch operas and think: this is all absurd. Really we shouldn't all be here!

Are you trying to get past that?

TA: No, that is the point, the more absurd, the more indefensible, the more it makes sense. Operas that are worthily about something, some idea or ideal, and that try to make a point, especially a political point, are just absurd in an off-putting way. Operas should instead be absurd in a way that is truer than reality. But that's just the most absurd form of something that is absurd from the start: music. Music should have no excuse, other than itself. Music is its own excuse.

So concerts are absurd too?

TA: Oh, completely. What are we doing here? What are all those musicians doing? More so in opera, because you have this further absurdity of the supposed psychology of the characters on stage. I really want to do something where their psychology is not the important point. Because you can't just believe that these characters have a psychology of their own unless it is genuinely, unequivocally encoded in the music. Psychological problems in themselves are not really a strong enough force for musical structure. This is the root of my problem with Wagner.

But it's the opposite of a composer like Janacek, whom I love. In Jenufa, for example, the wicked stepmother throws the baby in the lake. And when this is discovered there is a huge reaction from the chorus. That's absurd because there is no baby and there is no lake. But they have to have that reaction because there is a seismic harmonic event that creates the baby in the lake, and their reaction follows. The lighting and the direction have to follow the harmony too – to do what the music tells it, not the drama. Otherwise it's boring.

Can you expand on that? What about the power of Wagner's music to cause seismic events in something like Tristan und Isolde?

TA: Well, I find that much less interesting than Janacek's operas about fate – because I think music in an opera should be a sort of fate that the characters are going to be subjected to.

But isn't that exactly what Tristan is about?

TA: No! Because they're taking drugs, aren't they? It's artificial. They're not really that keen on each other. I can hear that in the music, it's inorganic.

But the music in Tristan – that's surely the fate that drives them, that they can't escape from? … The whole thing is about an unstable situation from bar one, which ends up in an image of stability, which the whole thing is striving for and reaches only at the end of the piece – it's a place that they're all trapped in until the very end.

TA: I don't know, I find it a bit too long.

I'm not defending it, necessarily, in those terms, but surely in Wagner the fate of the characters is in the music. Why doesn't Wagner do that for you?

TA: It's too psychological. I'm thinking of The Ring more than Tristan, there's an awful lot of psychology in it which I find tedious. And naive, in a sort of superficial way. I mean, so much of Parsifal is dramatically absurd, which would be fine if the music was aware of the absurdity, but it is as if the whole piece is drugged and we all have to pretend that it's not entirely ridiculous. And it seems to me that a country that can take a character as funny as Kundry seriously, this woman who sleeps for aeons and is only woken up by this horrible chord, a country that can seriously believe in anything like Parsifal without laughing, was bound to get into serious trouble.

You're obviously not convinced by the music?

TA: I don't find Wagner's an organic, necessary art. Wagner's music is fungal. I think Wagner is a fungus. It's a sort of unnatural growth. It's parasitic in a sense – on its models, on its material. His material doesn't grow symphonically – it doesn't grow through a musical logic – it grows parasitically. It has a laboratory atmosphere.

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TA: The music we listen to is the residue of an endless search for stability. I think you can make a sort of illusion of stability in a piece; you can fix it in a certain way. In a musical work, you permanently fix something that in life would be appreciable only for a moment. If I put a note under the microscope I feel I can see millions, trillions of things. In Polaris, my recent orchestral piece, a "voyage for orchestra", I was looking and looking at a particular C sharp, and as I put it under the microscope I saw or heard a writhing that turned into the piece.

You talk about "magnetism" in your programme note for Polaris: is that a term for the pull between what's stable and what's unstable in your music?

TA: That's really what one is dealing with all the time, magnetism: understanding the magnetic pull of the notes put in a given disposition, their shifting relative weights. I don't believe at all in the official distinction between tonal and atonal music. I think the only way to understand these things is that they are the result of magnetic forces within the notes, which create a magnetic tension, an attraction or repulsion. The two notes in an interval, or any number of chords, have a magnetic relationship of attraction or repulsion, which creates movement in one direction or another. A composer, whether of a symphony or a pop song, is arranging these magnetic objects in a certain disposition. That means that sometimes, in order to understand the weight of one note and the next note to it, you might have to transfer meaning from one to another. In Polaris, I had to transfer meaning from the C sharp to the A in order to do that. And it was difficult in some ways, because to really discover what the notes want to do, you might have to go against what they at first appear to want to do, and then they start to resist and you have to use other magnets to see what they are really feeling.

Is that a pre-compositional idea in Polaris, that decision to look at that C sharp, to work out what that move from C sharp to A meant?

TA: There's no such thing as "pre-composition": as soon as you start you're really composing. You're dealing with something that is chronically volatile. It's like lava, except my material doesn't actually exist in physical reality. They are evanescent sounds. These notes are not objects that are in front of you – although in another sense it helps to treat them like that; maybe they are, in fact, a sort of invisible object. But that very invisibility is frustrating, because one's brain can't necessarily define them clearly at first.

In your own music, where have you faced the issue of endings most keenly?

TA: One way I tried of ending things – I did this as early as in my Chamber Symphony (1990) – is suddenly to have an aerial view of the whole thing. In Seven Days also pulls the camera out at the end. Tevot does that, Asyla does that. In the Violin Concerto I worried very much about the end, which is different. When I was about 10, I used to play a game of guessing, with classical pieces, whether the end would be loud or soft. I thought those were the two kinds. I was usually wrong. I remember being very dismissive about The Rite of Spring, because I got it right.

Isn't the ending of the Violin Concerto more like a release than a cosmic zooming-out?

TA: Well, there's an alternative version in my head, which is quiet, where the caravan moves on and fans out, endlessly. That can often happen, an alternative, opposite ending, and it stays there like a ghost in your head for a while. For some reason that would have also been a true way to end the piece. I tried it, but it didn't stick. I'm glad it seems like a release because the first movement is already an aerial view.

You said that at the beginning of Mahler's Ninth Symphony, everything had already collapsed in terms of his vision of music, and possibly of the world too. So I wonder: where are we now? Are we in a time of stability or instability?

TA: We're in a time of total freefall.