On the balcony of Tate Modern's Turbine Hall, a group of schoolchildren is staring down at the latest in the gallery's "Unilever Series" of large-scale installations. They would be forgiven for being disappointed after all the fun of the pre vious commissions: the child-friendly helter-skelters constructed for Carsten Höller's Test Site, and Olafur Eliasson's spellbinding sun projection, The Weather Project.
This work isn't nearly so entertaining. Shibboleth, by the Colombian artist Doris Salcedo, has been affectionately nicknamed "Doris's crack". It is simply a long gash in the floor which zigzags 167 metres from the glass entrance door, growing wider and splitting into smaller lightning strikes as it goes, before disappearing beneath the back wall. The schoolchildren gaze at Shibboleth with modest interest, but it is their teacher, a mild-mannered woman in a tweed skirt, who looks most bemused. "I just don't understand why they have done it," she says, almost to herself. "Ruined their lovely floor!"
It is an understandable response, but not exactly what Salcedo had in mind. Launching the installation, she told reporters that Shibboleth represents "borders, the experience of immigrants, the experience of segregation, the experience of racial hatred. It is the experience of a third world person coming into the heart of Europe." Similarly, the information the Tate gives out to visitors informs them: "Salcedo is addressing a long legacy of racism and colonialism that underlies the modern world. A shibboleth is a custom, phrase or use of language that acts as a test of belonging to a particular social group or class. By definition, it is used to exclude those deemed unsuitable to join this group."
Does all this explanation help a viewer to appreciate the work? By the time I entered the Turbine Hall my mind was so cluttered with ready-made "meanings" that it was difficult to admire Shibboleth on its own terms. It is a genuinely powerful spectacle. If it were allowed to speak for itself, it would play subtly into many dif ferent fears and associations. A gaping fissure is an uneasy presence, particularly on such a large scale. It creates a sense of instability and im permanence. The sight of it slashing down the middle of the Turbine Hall is shocking, an apparent violation of space that is as close to hallowed as you get in 21st-century London.
Shibboleth does not necessarily point towards "the experience of racial hatred" or illegal immigration, although perhaps some viewers would - perfectly validly - have understood it in that way. It might just as easily provoke thoughts about other personal, social, economic or cultural divisions: family breakdown, environmental destruction or economic inequality. Conceptual art, at least when it's as good as Shibboleth, is powerful because it works on many different conscious and unconscious levels. Its ambiguity, its openness to interpretation, is also what makes looking at it more enjoyable than, say, reading a polemical essay on race relations.
It is a commonly held view in the art world that the public resists conceptual work because it is difficult to understand. Its reputation for being pretentious and "not as good as painting" would improve, they argue, if it were properly explained. In many exhibitions - the pavilions at the Venice Biennale this year being one example - reams of text, and even books, are handed out to visitors to help them interpret what they see. The curator of Shibboleth, Achim Borchardt-Hume, argues that the Tate has a responsibility as a public institution to offer visitors a "way in". "It is a very difficult balance to strike," he says. "First and foremost, it should be the work that speaks - it is not defined by any one reading. However, with conceptual art, we have found that if people don't understand, their bafflement can lead to alienation."
In fact, the tendency to overexplain is the route of most of conceptual art's public relations difficulties. First, it is patronising, and implies that only experts in the field are capable of comprehending a work's "true" meaning. Second, it places the onus on the public to "understand" art, rather than on artists to produce work that provokes ideas or emotions. If I am not moved by the sight of a glass of water mounted on a gallery wall, or a small animal fashioned out of carpet fluff, is that because I have failed to understand it? Or could it be that the artist has failed to communicate with me?
Most importantly, however, it encourages artists, curators and galleries to come up with pretentious mission statements that are often actively misleading. Shibboleth is not "the experience of a third world person coming to the heart of Europe". It is a crack in the floor. Schoolchildren given that explanation would be quite right to feel confused by it and alienated from the work as a result. If, however, they were simply encouraged to explore how the crack made them feel, they might begin to appreciate the real value of Shibboleth and to understand the unique, mysterious power of an image. In that respect, the teacher's instinctive response was the best her students could have hoped for.
"Shibboleth" is at Tate Modern, London SE1, until 6 April 2008. More details: www.tate.org.uk