It's the sort of question that could provoke a fistfight in a music pub quiz: what is the most listened-to piece of music ever?

The answer would be have to be a three-and-a-quarter-second-long piece of music (although you can argue about how to phrase the question) composed in 1995 by Brian Eno and still used as the startup sound on Microsoft Windows. Eno was paid a flat fee of $35,000. You might think that's not bad for 3.25 seconds of work, but if Eno had negotiated a royalty rate, Bill Gates would be broke.

Eno, who is indisputably the most interesting man in popular music, is a quiet genius. His brief from the ad agency, as he remembers, was "a piece of music that was inspiring, universal, blah- blah-blah, optimistic, futuristic, sentimental, emotional - this whole list of adjectives. And then at the bottom it said 'and it must be 3 1/4 seconds long'." The work made Eno focus on musical composition as measured in nanoseconds. Perhaps this is one of the reasons why he remains, pound for pound, the best music producer going.

When Apple wanted to add on a super-duper music application to their iPhone, the first person on the list was Eno and his ambient partner in crime, Peter Chilvers. They came up with a fascinating and absorbing application called "Bloom", which looks set to become the Rubik's cube for the download generation.

Bloom gives you a series of multicoloured spots on your iPhone touch screen that pay different notes every time you hit them. A slider helps you control the notes, and there are optional bass and treble notes. You get a better idea from watching www. youtube.com/watch?v=rBOk-gbC3Uc. Or go to YouTube, type in Peter Chilvers and play the first return.

It's when you don't play Bloom, though, that things get interesting. The generative music player kicks in and Bloom begins to offer up its own self-created symphonies. It's insanely addictive, and anecdotal evidence suggests that people are using it for hours on end - and it does have that sort of soothing affect on you. The application is available at a steal price of €2.99 from the iTunes store.

"One of the inspirations for this was Mozart," says Chilvers. "He used to have a way of composing which was called 'dice music' - the music you heard was dependent on the roll of dice. So we sort of worked off that a bit and brought in the generative aspect because the technology is now out there to use it. You couldn't have Bloom on a CD."

Apple is being a bit coy about the sales figures for Bloom (it's only been out for a few weeks) and, according to Chilvers, it has been uncharacteristically low-key about the introduction of and publicity for the application.

"So far it's been entirely word of mouth. They released it at 10.30pm on a Tuesday night with no fanfare. It immediately exploded on the tech and music blogs. A lot of people are remarking on how it is an ignorable but interesting piece of music. It's more of a 'presence' than anything else. Myself and Brian had a huge sample library we have built up over the years to delve into, and from the beginning we had the idea of providing a basic musical drone and then having different coloured circles appearing on the screen, which if you touched them would change the pitch."

Chilvers says you can listen to Bloom for hours on end because "there's no rhythm there. If you have rhythm in music, your time allocation is immediately reduced. It's why most pop songs are three minutes long."

Bloom may seem like a state- of-the-art music application for groovy, with-it iPhone users, but Mozart isn't its only influence. There's also the 19th-century composer Eric Satie who, in 1893, composed Vexations, a short musical phrase that could be repeated indefinitely. Satie used the phrase "furniture music" to describe his work - as in the music should be unobtrusive or "ignorable". In this sense, Satie is the founder of what we now call ambient music.

At present, though, at €2.99 iPhone's Bloom is the best buy for an infinite set of albums.