The other five shortlisted designs are equally adventurous, but are reassuringly fixed in attitude and motion. They include designs by Frank Gehry and Richard Serra and by Sir Anthony Caro and Sir Norman Foster, working together. Combining the disciplines of architecture and structural engineering and the questionable (and sometimes non-existent) discipline of the artist, each of the six supposedly anonymous designs (anonymity is a part of the rules of this open competition) offers a fresh way of crossing the Thames.
The Millennium Bridge competition is not to be confused with the Living Bridges exhibition currently showing at the Royal Academy of Arts. The RA's show is focused on inhabitable bridges (like old London Bridge or the Rialto in Venice) and the competition attached to it is for the design of a brand new inhabited bridge to be built, developers' money willing, across the Thames between Waterloo and Blackfriars bridges.
Living Bridges, in turn, is not to be confused with the ICA's ambitious and likeable plan to have the architect Will Alsop build it a new gallery spanning the Thames. Alsop's project is not related to the London Dockland Development Corporation's love of building small, yet perfectly formed new pedestrian bridges across the old dockland basins of London east of Tower Bridge.
Building bridges, in London at least, has become something of a craze in recent months. Architects, engineers, planners and politicians have turned their beady gaze on the River Thames because here they see a feature of the capital city that everyone can agree to love. Bridges themselves are not just means by which rivers are crossed, but metaphors for "building bridges" between riven or sparring communities. At a time when London has no central governing authority, elected or otherwise, and has not done so for 10 years, there is a real need to build bridges between local authorities and communities as well as across old docks and the Thames itself. Given that London is increasingly a world city, a sort of state within a state with its tentacles wrapped around the globe, it has been building bridges of other sorts - by satellite, computer and other forms of telecommunications - between its ever expanding perimeter and the capital cities of the world. Bridges can be seen as a kind of three-dimensional representation of the Internet, linking us together in readily accessible ways. The more bridges across the Thames (or the Tees or the Tyne), the more we might feel a part of a larger community.
Up to a point. It may well be that politicians like John Gummer are getting a little too excited about the caring-sharing qualities of new bridges. While there is a clearly identifiable use for a pedestrian bridge spanning the Thames between St Paul's and the new Tate, there is no obvious or good reason to build an inhabited bridge across the Thames between Blackfriars and Waterloo Bridges. This week, Harry Handelsmann, the sophisticated developer behind Manhattan Lofts, announced that he is interested in making a go of Zaha Hadid's competition-winning design for that inhabited bridge. Mr Handelsmann could undoubtedly get Hadid's bravura design built, but whether or not London wants a megastructure on the water that will block the peerless views from Waterloo Bridge - of Wren's St Paul's, looking east, and Barry and Pugin's Westminster Palace, looking west - is a question that should not be ducked. If Handelsmann, Hadid and Gummer were to consider building such a bridge further down river - in Docklands, perhaps - they would create something new and exciting without destroying existing beauty.
The pedestrian bridge between St Paul's and the Tate promises to be a thing of beauty, not least because it can afford to be pencil-thin and, lit up at night, will be a band, or arc, of colour across the river. It will be the first bridge across the Thames that pedestrians will have to themselves; to date, they have had to share river crossings with cars, buses, trams, trains, bicycles and, in the case of old London Bridge, traitors' heads.
Aside from being potentially very beautiful, the bridge encourage some of the 3 million or so visitors to St Paul's each year to cross to Southwark and thus visit the new Tate, the Globe Theatre and other forthcoming attractions on this rapidly redeveloping stretch of the south bank. The bridge has caused the Corporation of London and the Borough of Southwark to work closely with one another for the first time in many years, and this is probably a good thing. Finance should not be too much of a problem: a pedestrian bridge is, by nature, a light bridge and should be reasonably cheap to build and maintain. Millennium money and, somehow, some way, the rate-capped coffers of the City and Southwark should be almost enough, with a little help from developers such as Mr Handelsmann and local businessmen, who include David Sainsbury, the billionaire grocer.
Perhaps the reason that bridges have become such a cause celebre is that they are much liked and, compared with new buildings, relatively uncontroversial. It is interesting that when there is a controversy over a new bridge, it is either because the design is not modern enough, or because the costs involved upset local people in some way. The former case is well illustrated by the sad story of the new motorway bridge over the estuarine reaches of the Thames, between Kent and Essex. A plan had been hatched, led by the developer Stuart Lipton, to get Santiago Calatrava, the brilliant Spanish engineer, to design the new bridge. Calatrava is, is many ways (yes, I know journalists like to say these things), the Brunel of our day. We rejected him and a very beautiful bridge in favour of an elongated slab of dull concrete that does the job required of it in much the same way as a VW Golf does much the same job as a Mk2 Jag.
The latter case is illustrated by the ugly new road bridge that crosses over the sea to Skye in the Scottish Highlands. The design is banal in the extreme (and this in an area of stunning natural beauty, and with Over Arup & Partner's superb A82 road bridge only a few miles away); and yet the only issue local people appear to get their sporrans in a twist over is the heady fee that the road authorities charge for crossing the bridge. No one questioned the need for either Thames or Skye bridges. No one seems to have questioned the need for a second Severn road bridge, and certainly no one (perhaps there are one or two insular souls out there) has questioned the LDDC's policy of spanning old London docks and basins with filigree pedestrian bridges.
We should, however, question the design of bridges a little more closely than we do. They are truly thrilling structures and, because each is a special occasion, none must be as crass as the Skye Bridge or the Thames Crossing. None must steal stunning views of city centres (go east Mr Gummer, Mr Handelsmann, Ms Hadid); none should be built without the need to connect at that particular point, otherwise we end up with rivers crossed willy- nilly by bridges. Or bridges that, like the lovely Charles Bridge in Prague have become tacky tourist bazaars rather than a glorious way to cross from river bank to river bank. If bridges should ever become too popular amongst politicians and developers, it will be time to dig out that old David Lean video and to recall the fate of the bridge on the River Kwai n
Living Bridges continues at the Royal Academy, Piccadilly, London W1 until 18 December. An exhibition of the 220 entries to the Millennium Bridge competition is planned.