Tate Britain has some good news and some bad news to deliver. The good news is that Post-Modernism is dead - which must surely come as quite a relief to all those who were never sure in the first place precisely what Post-Modernism was. Now they no longer need even try to get their heads round the problem. The Post-Modern has been outmoded, apparently, like pre-decimal money. But by what?

That's where the bad news comes in, for according to Nicholas Bourriard, the French cultural theorist and the curator of the fourth Tate Triennial, the new era of the Altermodern has dawned. And if you come out of this great jumble of a show with a lucid overview of what this term embraces, then you can probably give yourself a pat on the back - the physical exertions this involves will be nothing compared with the mental contortions you have just undertaken.

Altermodern: Tate Triennial 2009 is a huge, heterogeneous exhibition of work including drawings, sculptures, videos, photographs, slide shows, installations, soundtracks, documentaries and performances - and usually a mixture of several of them at once - by some 28 artists. These hail from anywhere from America to the Far East, from communities as prominent as New York or as marginalised as Yaoundé. But more than half of them are British. The oldest contributor is an octogenarian; the youngest is only 26. But the overwhelming majority of these contemporaries are in their thirties and forties. You will recognise some names from past Turner Prize shortlists. Others will often be completely unknown. But now they are all assembled together to point us the way towards a fresh future.

So what will this new Altermodern era entail? Don't expect the catalogue to help you. Bourriard is a Frenchman. He has svelte Gallic looks and a Left Bank aroma of Gauloises. And he seems to have been brought up on Baudrillard and Foucault in the way that the rest of us were brought up on our ABC. His introductory essay, under such headings as “Rails and Networks: The ‘Viatorisation of Forms'”, offers sentences such as this: “Altermodernism can be defined as that moment when it became possible for us to produce something that made sense starting from an assumed heterochrony, that is, from a vision of human history as constituted of multiple temporalities, disdaining the nostalgia for the avant-garde and indeed for any kind of era - a positive vision of chaos and complexity.”

But as you make your way through the “chaos and complexity” of a show that can involve anything from sitting down at a desk to leafing through a copy of The Times through playing a giant accordion to lounging around on beanbags and watching soft porn, a few vague trends start to emerge.

The Altermodern, it would seem, is essentially about global culture. The starting point of Post-Modernism, curators suggest, is the question “where am I from?” But now, thanks to such innovations as the internet, we need no longer define ourselves within traditional boundaries. The artist is a wanderer, drifting about in space and time, drawing from a vast, fluid fund of collective ideas. And his or her work is far less about a single finished object than about continuing processes of development and connection in which one thing always seems to be leading to the next.

Bourriard offers the archipelago as a useful simile. He is presenting a gathering of individual talents which, though each is distinct, come together to form a pattern. Likewise, each artist's contribution tends to be an assemblage of related matter rather than one single piece.

Certainly, the territory that this show maps out is not a solid landmass. The spectator is never quite sure of the ground he is standing on. Nothing is ordered or stable. Even the furniture cannot be trusted. Simon Starling plays a complicated game of Chinese whispers with a desk that wasoriginally made by Francis Bacon for the author Patrick White and has been replicated by a succession of craftsmen who, taking their commission from a mobile phone image, create works that, displayed on packing-case plinths, reveal the mix and match processes of cultural mutation.

Marcus Coates flicks back and forth from ancient folklore to the political future in a performance that brings together a shamanic spirit and an Israeli mayor. Mike Nelson creates an installation that has you constantly questioning what is inside and what is outside, what is real and what is imagined, what is rational and what is completely paranoid. He certainly puts you in the right frame of mind to visit Joachim Koester's Hashish Club or Gustav Metzger's tranced-out Liquid Crystal Environment.

The non-existent and invisible become present and perceivable as Katie Paterson creates a glittering map of dead stars or Loris Gréaud, having recorded his brain activity while thinking about an artistic project, translates his mental wonderings into physical vibrations that the spectator can tap into as he steps into the room. Pascale Marthine Tayou digs up contemporary culture and displays it like trophies in a museum of the future. Rachel Harrison challenges our systems of classification and disrupts the orders of progression. And Charles Avery takes us on a journey to the imaginary island that for the past several years he has been elaborating in his head.

This is an iconoclastic show. Tear up traditional expectations, fling them in the air and watch them flutter down all about you, in any old order and size and shape and all mixed up and in a mess. So don't come hoping for pictures on the wall - except where Peter Coffin adapts the Tate's collection with his projected animations. Don't expect craftsman-like skills. Art is a process of production. You might find the occasional beautiful image - Darren Almond takes photographs of Chinese landscapes by the light of the full moon - but more often the works are mixed-media mish-mashes, assemblages of bits.

After Brit Art with its easy one-liners, this confusing junk room of images feels full of possibilities. Certainly, some of the more explicable contributions seem disappointingly trite. Ruth Ewan, for instance, presents the largest squeezebox in the world. Every day the craftsman who made it will play one of Ewan's growing collection of protest songs. It will show us how difficult and cumbersome the processes of social change can be. But I don't think we need a giant squeezebox to show us that.

The occasional work makes a visually pleasing and philosophically succinct point. I liked Subodh Gupta's vast mushroom-cloud explosion of saucepans and kitchen utensils as a metaphor of explosive encounter and its potentially prosperous fall-out. But mostly the spectator moves through this show, his mind flickering to what soon starts to feel like the omnipresent hum, flick and clack of a continuing slide show that drops image after image on to your retina. You might wait at first for a final resolution, but in the end you will realise that this is a never-ending chain. The entire show is a continuing and constantly adapting event. Bob and Roberta Smith, for instance, will be making a new piece each week in response to responses to the show.

It's less a question of “what's next”, Bourriard explains, than a question of “what if”. I can't help thinking that for many it will be a question of “what's that?” What is that garish heap of clutter, that bank of blurred screens, that pool of yellow ping -pong balls in a blue rock?

Most of the time, I never found out. This show is all about distraction. Without any one focus, the eye hops restlessly about. The thoughts shuffle about in your head. At their most engaging, they are making unexpected connections. You enjoy the sudden strike and leap of a spark. More often they drift off bored by conglomerations of clutter that, quite frankly, feel about as fascinating as a file of student research notes.

It's just as well that this triennial is supposed to be about travelling because I didn't linger long with anything much. The most optimistic message is that artists, fed up with the idea of the brand, the one-liner, the shock factor, appear to be searching for a fresh way forward. The most pessimistic is that they haven'tfound it yet.

Altermodern: Tate Triennial 2009, is at Tate Britain, SE1 (020-7887 8888; www.tate.org/britain), from today to April 26

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