Prophecy is always news. We desperately want to be told what's going to happen, even if, quite probably, it isn't going to happen. This applies across all human activity, but especially it applies in art – because in art quite often prophecy actually works.
It's the way art comes down to us, in a sequence of historical packages, otherwise known as movements – all those famous "isms". Some of them were retrospective labels. But many of those "isms" were announced at the time, and took off, and proved to be self-fulfilling prophecies.
So surely someone can do it now. Survey the scene, spot a trend, give it a name, start a bandwagon, determine the future. People are always trying. Every year, there's a wave of international biennials, each one confidently setting up a signpost telling the world that art is now going that-a-way. This year, we also have our local version of these foretellings: a Tate Triennial.
There have been Tate Triennials since 2000, so this is the fourth. They're designed to celebrate current trends in British art. I don't think they have ever been considered particularly successful as exhibitions, but Tate sticks with them, and this time it's trying something a little different, something a little more directional.
There are 28 artists in the show that has just opened at Tate Britain, spilling all over the gallery, and their average age is late thirties. Nine of them, however, are in no way British, even by the normally vague criteria – neither born here, nor working here. It's a sign that this Triennial has a concept that overrides its usual national consistency. Its curious name is Altermodern.
The business world has its gurus. The art world has them too. The Malcolm Gladwell of contemporary art is Nicolas Bourriaud, a good-looking Frenchman in his early forties. Bourriaud is a critic and curator, currently employed by Tate, and part of his job is to invent new agendas for art, to point out where things are going and where they ought to be going. Business gurus just have to offer a secret of success; art gurus have to have their fingers on the pulse on history.
The position of art guru goes back about 50 years, to the post-war New York scene, to when the critic Clement Greenberg acted as advocate, adviser and impresario to the Abstract Expressionists. He had a theory that explained why this painting was the inevitable and essential step forward for art.
Ever since, a certain kind of critic has sought a similar leadership role. Bourriaud has played it very successfully. A few years ago, he devised something called Relational Aesthetics. The artwork now will be judged by the relationship it implies with its audience. Does it (good) involve us in an open democratic dialogue? Or does it (bad) impose itself on us in a closed, fascistic way?
But now he's come up with a bigger deal, a proper signpost: Altermodern. As the Tate visitors' caption-board explains, it means "other modern", and it tells us that the next big thing is upon us. Postmodernism has come to an end (hurrah). A new art form for the 21st century is emerging.
"If early Modernism is characterised as a broadly Western cultural phenomenon, and Postmodernism was shaped by multiculturalism, origins and identity, Altermodern is expressed in the language of global culture." An art for the age of globalisation – polyglot, cosmopolitan, nomadic – that's the way things are heading. So, on its first outing, it would obviously be wrong to confine it to the art of a single country.
Do I believe in Altermodern? Well, the name seems weirdly irrelevant, and the idea sounds like something fabricated on a drunken evening – or rather, on an insufficiently drunken evening. Globalisation is surely the first thing anyone would think of, if they had to make up a new art movement.
As for the end of Postmodernism, so far as I can remember what it was, there seem to be a lot of overlaps. Besides, I wouldn't have thought that cultural globalisation was necessarily so benign, any more than economic globalisation has been. And the way the world is going, all forms of globalisation may be going into reverse.
Never mind. These agendas make amusing talking points, handy topics for countless art conferences – and many people in the art world would much rather be discussing art tendencies than art itself. But if you mind about art, then a programme like Altermodern is probably harmless. It may be misleading, but it is only likely to mislead artists who aren't much good anyway. It can be happily ignored.
Inside the show, though, it can't quite be ignored. Some of the work fits the polyglot bill so perfectly – the art version of fusion cuisine. Rachel Harrison has a photo-gallery of miscellaneous heads, an ID parade of cosmopolitan cultural icons; Pascale Marthine Tayou does something similar, but the heads are very moulded in semi-3D and V C embedded in tablets. And Shezad Dawood's film Feature mixes up all kinds of clichés – cowboys and Indians and Krishna and Wagner – amid a general mayhem.
Meanwhile, some works are so thumpingly dull (step forward Simon Starling, doing replications of replications of replications) that they can only have been chosen because somehow they ticked the right boxes. That's the occupational hazard of a programmatic show.
But this exhibition is not just a showcase for a new-coined movement. It performs a more simple function. It gives a prominent public platform to a new generation of artists. It may feature some experienced hands, like Tacita Dean and Mike Nelson. It includes that most ancient avant-gardist, Gustav Metzger, born in 1926, and a veteran of "auto-destructive art". But most of the artists here are "emerging" – the children of the late 1960s and the 1970s, and even the 1980s.
And if you are seeking trends among them, what's most noticeable is not a cosmopolitan theme but a particular aesthetic. It's an aesthetic of loose, disparate combinations. That old label "mixed media" now really means business, with this and that and the other co-existing and no attempt to make them fuse. A single installation could consist of, say, a laptop with a film playing, a diagram, a piece of model-making – and the broad hope that these bits are going to add up, or at least relate in a sparky way.
Conversely, you can be in a room, and not be sure whether it has works by one, two or three artists, and actually things play equally well each way. Take Spartacus Chetwynd's bank of monitors showing some wacky porno-detective show, and next to it David Noonan's sinister cut-out figures: the connections you can make between them (remote as they are) are just as plausible as those between the various diverse architectural elements of Matthew Darbyshire's Palace.
Or another tendency: undigested information. There are several works that are basically presentations of interesting facts, cultural references, research projects. This is the age not only of globalisation, but of Google and Wiki and YouTube. Olivia Plender is interested in a breakaway Boy Scout movement from the 1920s. Ruth Ewan looks at the history of the accordion. In his projection installation, Giantbum, Nathaniel Mellors draws on Pasolini, Rabelais, cannibalism.
There's a subject lurking somewhere behind or around the work, but it doesn't matter what form it takes. The artist's ability to give a lively interview is as important as what appears in the gallery. Read this Triennial's catalogue and you'll find that names are dropped like Saturday night litter.
Diffuse forms and diffuse subjects – these are the emerging trademarks. Some people may think it looks like freedom. To me, it looks like mere free-association, a fatal floppiness. The works exist in a potential state, half-made, not minding too much what it amounts to. This makes them into amenable curator-fodder. They put up little resistance to being mixed and matched in a display. But they don't cohere in a way that holds the eye or the mind.
You pass through this show in a state of low attention drift – interrupted occasionally by a crass spectacular like Subodh Gupta's mushroom cloud of metalware, or Franz Ackermann's room of flash-bang sci-fi/abstract paintings.
Still, there are a few focused and arresting moments, where imagination grips. Tacita Dean's The Russian Ending is a sequence of old photos of catastrophes – sombre, monochrome, enlarged, and marked up in white ink, in her beautiful handwriting, giving hints of latent story. (The title refers to the historic Danish cinema, making two versions of a film, happy and sad, for American and Russian distribution.) Mike Nelson's The Projection Room is a perfect reconstruction of some obsessive-conspiratorial character's den, a mad head-space to peer into.
And, from the more emerging end, there's Charles Avery's invented world of exploration, with its mysteriously witty fictional globe map-cum-mental map – with an ocean called The Sea of Clarity, an archipelago called The Procession of the Numbers, a headland called The Phenomenon of Sense.
Or there's Lindsay Seers's Extramission, the point of maximum intensity in this entire show, I thought. It's a film supposedly telling the artist's true story – how she didn't speak until she was aged eight, and then how, through her later life, she at first identified with the camera and then with the projection. It's a story told with tension, as documentary and as fantasy, in a sequence of consistently traumatic images. There are some lengthy film pieces to look at in this show, and the temptation is always to move along. This is a piece to stay through.
Oh, Altermodern, schmaltermodern. Trends, tendencies, themes, agendas – they can always be discovered and promoted, and sometimes they foretell and sometimes they sink. But in the end they count for nothing. Weak artists look for rallying cries and disappear into the gang. Rare strong ones make the going. The only call the critic can really give to the artist is what Diaghilev said to Jean Cocteau: étonnez-moi! Amaze me! And there are one or two glimpses of amazement to be grabbed here.
Altermodern: Tate Triennial 2009, Tate Britain, London SW1, to 26 April; admission £7.80, with concessions (020-7887 8888; www.tate.org.uk/britain)
Protest inspires 28-year-old Ruth Ewan: previous works include a gallery full of parrots squawking slogans chanted by demonstrators at the G8 summit in Gleneagles; a colouring book filled with pictures of anti-war marches; and a jukebox playing left-wing protest songs. The Scottish-born artist has developed the theme for Altermodern, creating the world's largest squeeze-box on which protest songs are played daily.
A name you're unlikely to forget. Born Lali Chetwynd in 1973, the eccentric artist adopted the name of the Roman rebel in 2006. Since graduating from the RCA in 2003, she has roped her band of followers into various bizarre happenings in the name of art. These have included a remake of Born Free, An Evening with Jabba the Hut and a snail race. At Tate Britain, she showcases her humorous, lo-fi, sex'n'violence spoof soap video.
Seers, 42, apparently uses her extraordinary life-story as the inspiration for her art. Watched from inside a replica of Thomas Edison's first film studio, which was called Black Maria, her semi-autobiographical and dreamlike film Extramission – which includes talking heads from people purporting to be her mother and her psychologist – has her describing her childhood inability to speak and her adolescent desire to become a camera and a projector. Super strange, but certainly one of the must-see pieces in the exhibition.
Almond had a busy year last year, with two solo shows at London's White Cube and Parasol Unit galleries. A Turner Prize nominee in 2005, the 38-year-old has lately turned his hand to the little-practised discipline of night-time photography. His brooding shots of nocturnal landscapes, bathed in the milky, tranquil light of the moon, are technically stunning and undeniably picturesque.
Nick Hackworth started his trendy London gallery, Paradise Row, in Shezad Dawood's attic while Dawood, 34, toiled downstairs on his multicultural mash-ups. He has previously re-imagined movie posters in different settings – setting Taxi Driver in Karachi, for example. Feature, the 55-minute film he is showing at Tate Britain, is a cowboys and Indians shoot-'em-up with a difference – that difference being that the piece also features zombies, Krishna and the Cambridgeshire countryside.
In December, the 27-year-old Glaswegian was voted the winner of The Independent's first Creative 30 award for her recordings of both the internal gurgles of glaciers in Iceland and Beethoven's "Moonlight" Sonata having been transmitted to the Moon and back. Her other works, which all capture ephemeral natural phenomena – tides, glaciers, stars, waves – include a Moon bulb, a chart of all the dead stars ever recorded (27,000 or so), and a mobile phone that connects to a microphone inside a melting Icelandic ice-cap.
Avery was thrown out of Central St Martins after just six months, and the 35-year-old has spent the ensuing decade creating an imaginary island. Some have praised The Islanders, with its made-up maps, sketches of imaginary creatures and explorer's "notes", as a return to storytelling in art and a witty dissection of utopian tropes, while others dismiss his life's work as twee whimsy or sci-fi geekiness. Either way, it's hard to fault the dedication of the one-time Isle of Mull resident.
The youngest artist on show at the Altermodern show, aged just 26, is Vonna-Michell, who combines his installations with live performance. He tells stories, often speaking in an incomprehensible, rapid-fire manner, with baffling swerves of logic and in rhythms that recall the spoken-word work of Allen Ginsberg and William S Burroughs. Vonna-Michell was born in Southend, and his art is often concerned with his identity and heritage. In one tale, he tells of the day he asked his father: "Why was I born in such a place?" – only to receive the cryptic answer: "Ask Henri Chopin – all you need to know is that he loves quail eggs, lives in Paris and is 82 years old."