Architects are the true artists of today. The warped, cubistic, titanium fantasy of Frank Gehry's Guggenheim Bilbao, the curvaceous wonder that is Norman Foster's Swiss Re building in London - where are the equivalents in contemporary art of these revolutionary buildings? The culture of the ready-made and the rise of video has all but exiled sculptural invention from the art gallery. Artists just don't think it is their job to imagine new shapes - or so you might think until you visit the Guggenheim Bilbao.
The shimmer of the building lifts you, but when you walk inside, architecture is trumped by the most extraordinary sculpture of our time. After you experience this work of art, Gehry's architecture - which looked so brilliant - starts to seem flimsy, no more than a series of clever moves. It lacks the authority of Richard Serra's sculpture, which seems more solid, heavier.
This diminution of its setting is not accidental. Serra is a very aggressive artist, as is all too obvious when you see early works by him currently showing in London's Gagosian gallery. They look as if they might easily topple and kill someone, and this is part of their meaning. In the 1980s, his public sculpture in New York, Tilted Arc, offended people so much it was demolished - not because of its content, he is an abstract artist, but because of its violent domination of space as it cut across a plaza. He is a friend of Gehry, yet his sculpture in Gehry's museum defies the famous building in scale, mass and style. This doesn't just mean it is big. It offers a series of spaces you can walk inside and through - can run, even, like the children who rush past you as you are contemplating death and memory in the depths of one of the passageways. Again, this might not be so special. Installation art is always creating rooms and interiors and corridors you can go inside. So what?
Serra's sculpture uses no theatrical lighting, no smoke and mirrors and is no way illusionistic. It consists simply of very large sheets of steel, twisted to form spirals, ellipses and long arcs. Walking among these dwarfing structures, you are constantly aware of being among things heavier and stronger than yourself. At the same time, you are aware that a human intelligence and human technology shaped this stuff. It is sculpture, just as surely as chiselled stone is sculpture. Serra once made a list of sculptural actions: "to roll, to crease, to fold, to bend, to shorten, to twist, to dapple, to crumple . . ." The applicable verb here is "to twist". In his recent sculpture he subjects tons of steel to the forces necessary to torque, that is, twist it laterally. It is something a jeweller might do, and seen from above, the cluster of his objects almost resembles jewellery. Then again, from closer up, its convex and concave curves, browned by oxidation, makes you think of African ceramics.
Most of all, and unmistakably, the sculptures reflect and remake the wavy contours of Gehry's building. The Guggenheim is all serpentine and bulbous organicism, paying homage to Gaudí, but more profoundly inspired by the baroque. If Gehry is, in his style and spectacle, today's version of the flamboyant 17th-century sculptor and architect Gian Lorenzo Bernini, Serra is the reincarnation of his rival, Francesco Borromini. This is not an idle comparison. Serra has said he was inspired to start making ellipses partly by a visit to Borromini's church, San Carlo alle Quattro Fontane in Rome, with its ovoid dome. Borromini is a disturbing architect, and the oval is an uncomfortable form that goes against intuitive beliefs about natural harmony. But Serra's standoff with Gehry also resembles the baroque titans in a more personal, dirty way.
Borromini hated Bernini, who in turn is said to have depicted a figure on his Four Rivers fountain in Rome looking up in terror at a Borromini church, implying it is unstable. Serra is on video in a little cinema in Gehry's museum, talking about how he loathes architects. But surely you must be grateful to Gehry, objects the interviewer. "Oh, yeah! I should be grateful!" says Serra. He goes on to assert that he draws better than Gehry - "and Frank would agree" - and to argue that architects are just plagiarists who cannibalise sculpture. Apart from being hilarious, it is totally at odds with the way most people speak of the arts today. A multimedia conception of the arts is almost universal now. This has even been labelled "the age of the post-medium condition". Yet here is an artist who speaks of himself as a sculptor, committed to a medium as ancient as humanity. A retrospective of his work at the Museum of Modern Art in New York this summer is simply called Richard Serra Sculpture: Forty Years. Not Richard Serra Artworks, though he has also made Warhol-influenced films and was one of the first artists to experiment with video.
At the Gagosian gallery near King's Cross, there is a chance to see how Serra evolved towards his current triumph in works going right back to the 1960s, including Sign Board, made in 1969. In that same year he made One Ton Prop (House of Cards), in which four plates of lead like the one at Gagosian were propped against each other to make a roughly cubic enclosure. What you see, looking around the Gagosian show, is how dangerous this new kind of sculpture was - and I'm not speaking metaphorically. Some of the pieces in this big, white space made me want to go under and behind them; I was particularly drawn to stand underneath Corner Prop No 8 (Orozco and Siqueiros), until I realised I might not get out alive. For there is nothing holding this immensely heavy steel sculpture together except an equilibrium of forces. The lower plate, facing outward, supports the upper one - nearly two metres square of steel - that is propped into the corner. It hit me as a physical shock. What's he doing? How can art be so arrogant?
The danger is real. A rigger was killed attempting to install a Serra sculpture in the early 1970s. For Serra, the toughness of steel is natural. He comes from a blue-collar background, one of his formative memories is a wartime visit to a shipyard where his father worked - and he himself worked in steel mills to pay his way through Yale. He delights in the heat and violence of steel production. But there is so much more than machismo to Serra. Above all, these sculptures make you think of architecture. That is what makes them utterly different from the other modern sculpture in the Gagosian show, by Giacometti, Fontana and Twombly. While their pieces are shown much like sculpture down the ages, even on plinths, his press menacingly against the walls of the gallery, like Samson pushing apart the temple.
Those big chunks of steel look unstable, an effect architecture generally tries to avoid. People want architecture to look good but won't accept a bridge that wobbles. Serra makes objects that resemble buildings - or rather, what is inside modern buildings: the steel frame - but these skeletal structures only loosely and tenuously cohere. Is Serra celebrating or attacking the arrogance of the designing mind? Is he for or against western reason? This is the ambiguity of his art at its most serious level.
At Bilbao, you see where the young man who propped sheets of metal together ended up after divorcing his first wife because she said One Ton Prop wasn't sculpture. From the harsh and threatening counter-architecture of the early works at Gagosian, he has come to an art that matches and mirrors the best in modern architecture. For whatever he says about Gehry or Foster, this is a creative, not destructive, contest. Curves have entered the best design of today with a vengeance, just as they subverted Renaissance classicism at the end of the 16th-century.
Serra claims to make you recognise the fluidity of time. He might equally say what Mark Rothko said of his cycle of abstract paintings the Seagram murals: "I have made a place." Then again, being inside this art in Bilbao, I found myself thinking of Jackson Pollock's voice talking about action painting: "When I'm inside my work ..." Being inside the work, making a place - these are the highest ambitions of the greatest American artists. Serra's achievement belongs beside their heroic efforts to make an embracing world of art, saturated in memory and poetry. This is as good as it gets. If you don't like this, you don't like modern art. If you do, you must revere Serra.
· Living, Looking, Making: Giacometti, Fontana, Twombly, Serra at Gagosian Gallery, London (020-7841 9960), until May 19
· Richard Serra's The Matter of Time is part of the Guggenheim Bilbao's permanent collection