THE great Turbine Hall at London’s Tate Modern, a former power station, is a notoriously difficult space for an artist to fill with authority. Its immensity can dwarf the imaginations of all but a select tribe of modern artists who understand the mysteries of scale, of how to say something interesting when you also have to say something really big. Louise Bourgeois’s giant spider once stood menacingly in this hall; Anish Kapoor’s “Marsyas,” a huge, hollow trumpet-like shape made of a stretched substance that hinted at flayed skin, triumphed over it majestically.

Last October the Chinese artist Ai Weiwei covered the floor with his “Sunflower Seeds”: 100 million tiny porcelain objects, each handmade by a master craftsman, no two identical. The installation was a carpet of life, multitudinous, inexplicable and, in the best Surrealist sense, strange. The seeds were intended to be walked on, but further strangeness followed. It was discovered that when trampled they gave off a fine dust that could damage the lungs. These symbolic representations of life could, it appeared, be dangerous to the living. The exhibition was cordoned off and visitors had to walk carefully around the perimeter.

Art can be dangerous. Very often artistic fame has proved dangerous to artists themselves. Mr. Ai’s work is not polemical — it tends towards the mysterious. But his immense prominence as an artist (he was a design consultant on the “bird’s nest” stadium for the Beijing Olympics and was recently ranked No. 13 in Art Review magazine’s list of the 100 most powerful figures in art) has allowed him to take up human rights cases and to draw attention to China’s often inadequate responses to disasters (like the plight of the child victims of the 2008 earthquake in Sichuan Province or those afflicted by deadly apartment fires in Shanghai last November). The authorities have embarrassed and harassed him before, but now they have gone on a dangerous new offensive.