Quite a handful ... Ai Weiwei with his sunflower seeds in Tate Modern's Turbine Hall. Photograph: Fiona Hanson/PA  At first sight Ai Weiwei's installation Sunflower Seeds presents us with an undifferentiated field of grey, filling the space between the bridge and the end wall of Tate Modern's Turbine Hall. It is almost disappointing. The late Felix Gonzalez-Torres's piles of cellophane-wrapped sweets, which he showed in the 1980s, were prettier, and you were free to eat them (the American artist liked the idea that people could leave his shows with a nice taste lingering in their mouths). But the sweets were also metaphors for the Aids crisis, and much besides. Nothing in art is what it seems. And you can't eat a single one of Ai Weiwei's sunflower seeds, any more than you could Marcel Duchamp's marble sugar cubes. They'd break your teeth.  But you can trudge over them, walk or skip or dance on these seeds, all of them Made in China. Or scoop up handfuls and let them run through your fingers, in the knowledge that someone, an old lady or a small-town teenager in Jingdezhen, has delicately picked up each one and anointed it with a small brush. Every seed is painted by hand. The town that once made porcelain for the imperial court has been saved from bankruptcy by making sunflower seeds. It is absurd.  I love this work. It is a world in a hundred million objects. It is also a singular statement, in a familiar, minimal form – like Wolfgang Laib's floor-bound rectangles of yellow pollen, Richard Long's stones or Antony Gormley's fields of thousands of little humanoids. Sunflower Seeds, however, is better. It is audacious, subtle, unexpected but inevitable. It is a work of great simplicity and complexity. Sunflower Seeds refers to everyday life, to hunger (the seeds were a reliable staple during the Cultural Revolution), to collective work, and to an enduring Chinese industry. But it is also symbolic. It joins several previous Turbine Hall commissions – most recently Doris Salcedo's 2008 Shibboleth and Miroslaw Balka's How It Is – in a dialogue about the social and cultural place of art.  The meanings are as multiple and singular as its form. Ai Weiwei has taken the lesson of Duchamp's readymade and Warhol's multiples and turned them into a lesson in Chinese history and western modernisation, and the price individuals in China pay for that. Every unique seed is homogenised into a sifting mass. Most contemporary Chinese art is a product made for western consumption, just as willow-pattern plates or porcelain vases were shipped out in huge quantities for the western market.  Ai is the best artist to have appeared since the Cultural Revolution in China. He has smashed ancient vases, taken a thousand Chinese citizens to a small town in Germany – his contribution to the five-yearly Documenta contemporary art show in Kassel in 2007 – made works about the Chinese government's response to the Sichuan earthquake in 2008, and the social crackdowns during the year of the Olympics. Ai takes on the world through an attitude, rather than a style. With his blogs and tweets, he is a constant communicator. However absurd his works might appear to be, he understands the place of the artist, recognising that his work exists in a global world of social, cultural and economic relations. He has a sort of social engagement Duchamp lacked, or couldn't have, and that Warhol dissembled.  Ai's field of sunflower seeds is both contemplative and barbed. Generous in spirit, everyone can get it. It will no doubt have a huge audience at Tate Modern, one that might see it as no more than an entertaining spectacle and treat it like a day at the beach. Yet Sunflower Seeds is contingent, oddly moving and beautiful. It is like quicksand.
Adrian Searle: Courtesy of Chinese artist Ai Weiwei, the Turbine Hall is now carpeted with a million hand-painted seeds – an image of globalisation both politically powerful and hauntingly beautiful