Rachel Whiteread's Monument is the most refreshing piece of public art currently on display in London. Having been commissioned to come up with something to put on top of Trafalgar Square's unoccupied fourth plinth, the artist has (not altogether surprisingly) made a clear resin cast of the plinth itself. This glassy, inverted simulacrum towers above the tourists and pigeons like a giant aquarium or see-through sarcophagus. It's a simple trick, but an effective one, and the associations it conjures - heaviness and lightness, earth and heaven, death and life - are thought-provoking and manifold.
The answer to the "what to do with the vacant plinth?" question is perfectly simple. It should be left vacant. People, especially municipal councillors, have a problem with empty spaces and get itchy fingers every time they spot one. But this pleasingly pointless edifice, with its resolute silence, is as integral a part of Trafalgar Square as are the occupied plinths and their half-forgotten generals. Think of it, if you like, as a monument to the unknown soldier. And Whiteread's Monument, as light and gleaming as the plinth is dark and squat, is the only one of the four commissioned pieces to allude directly to the plinth's defining emptiness. She sees it not as a space to be filled, but as an absence to be acknowledged, and she does it well.
Judging Monument as a piece of sculpture, however, you might be entitled to a few misgivings. It is, for obvious reasons, entirely site-specific, and would be meaningless if moved elsewhere. But this is what constitutes its strength. What some might find more disturbing is the way it seems to disavow the very possibility of sculpture. To top a plinth with its reflected image is to create a closed world from which human endeavour is absent. The bare place in which art takes form has been wholly, immaculately cancelled. Might this be, in fact, a depiction of artistic stasis or blockage?
Similar doubts attend Whiteread's new show at the Serpentine Gallery, her first solo exhibition in a public art gallery in London. It seems unsure if it is a retrospective or a showcase for recent work; wanting to be both, it ends up being neither. The largest of the two new pieces is Untitled (Upstairs), a cast of the interior of a staircase from an old synagogue in Bethnal Green, east London, which the artist is converting. Is this, then, art as an adjunct to interior design, sculpture for the Changing Rooms generation? Not exactly. Reaching high into the Serpentine's domed central gallery, Untitled (Upstairs) has the same air of exalted melancholy that characterises all Whiteread's large-scale works. You struggle to identify these vaguely temple-like blocks connected by stairs that lead to a blank wall. Things are further complicated by the cast not being upright, the staircase's fossilised doorway lying flat on the gallery floor.
It has a strange and dreamlike presence, but you can't quite banish the thought that we've been here before. With Ghost, Whiteread's much earlier cast of a suburban sitting room, the feeling of estrangement and the subsequent shock of recognition were disturbing and profound. Here was the all too physical ghost of a room, the "missing piece" that spells silence and extinction. Now there's a niggling suspicion that elderly, if not entirely dead, horses are being flogged. An initial brilliant idea is being honed and refined, and, for that reason, is losing its force.
Untitled (Cast Iron Floor) is the only other new work on display, a piece strongly reminiscent of the floor sculptures that Carl Andre produced in the late Sixties. But whereas Andre used uncompromisingly industrial slabs and exotic metals such as magnesium, Whiteread's piece alludes to the materials and techniques of traditional sculpture (an earlier version was cast in bronze). Here we have a British artist doing what British artists have always been accused of: making watered-down copies of the pioneering work of their European or American counterparts. Whiteread has excused the resemblance by claiming that, unlike Andre's, the patina on her tiles will wear down as people walk over them. That's all very well but, as the justification for a piece of art, it seems singularly unambitious.
The rest of the exhibition consists of a wide-ranging, if somewhat threadbare, selection of earlier work. It suffers from a surfeit of the bed casts that resemble actual mattresses far too closely to have the enigmatic charge of Whiteread's best (and strangest) work. One of her huge bath pieces is on display, tar-black and tomblike, as is the pretty-coloured plaster negative of shelves of paperback books. Elsewhere, there are translucent, purplish casts of the spaces beneath a table and chair, miniaturised temples sealed to everything but light. Some of these works are very beautiful - all of them an absolute gift to academics who like to write wordy theses with titles like "The Dialectics of Presence and Absence in the Sculpture of Rachel Whiteread". Still, the suspicion remains that, once you've got the visual trick (that these cast spaces resemble tombs or temples) and its attendant idea (that, whatever efforts we make to banish them, death and the sacred are inescapably present in our domestic lives), you're not left with very much.
"Rachel Whiteread" is at the Serpentine Gallery, Kensington Gardens, London W2 (020 7298 1515) until 5 August
