In 2003, the Lower Manhattan Development Corporation held an international competition to design a memorial to the 3,000 men, women and children who were murdered in the terrorist attacks of 11 September 2001 - predominantly in the north and south towers of the World Trade Center but also in the Pentagon and the four commercial passenger planes which crashed that morning.
The competition received some 5,200 entries from 63 nations - a remarkable response by any measure - and was won by the Israeli-American architect Michael Arad, in partnership with Peter Walker, a landscape architect. The memorial they designed, which President Barack Obama will unveil on 11 September, consists of two gigantic, square voids, set within the footprints of the twin towers. Each is an acre in size and 30 feet deep, and contains a pool fed on all four sides by a continuous cascade of water that, in turn, disappears down a smaller, central void. Occupying about half of the original 16-acre site, the memorial extends beyond the two pools to calming groves of swamp white oak trees, through which the mourner or visitor approaches. While the voids are a stark reminder of the enormous scale of the destruction and human loss, the 438 deciduous trees evoke regeneration and life.
On bronze parapets surrounding the two pools are the names of all those killed in the atrocities. The names, cut out rather than engraved so that they appear to be written in light, are arranged in such a way that those belonging to one group - employees of the same company, members of the New York City fire department, passengers and crew of the same flight - are listed together. More than 1,200 surviving colleagues and family members requested the names of specific individuals next to whom they wanted their relatives or friends to be commemorated. Every request was met.
This collaborative element is carried over into the adjacent museum, which opens in September 2012. The Memorial Museum is appealing to the public to donate to its permanent collection artefacts such as photographs, voice messages, clothing, letters, diaries, emails and other effects "that help to illuminate people's experiences during and after 9/11". It has already received personal property, much of it poignantly intimate or domestic.
The museum has also entered into partnership with StoryCorps, an oral history project, "to record at least one remembrance for each of the victims . . . as well as the narratives from survivors, rescue workers, witnesses", and so on. Visitors will be able to record and listen to shorter witness statements, thus becoming part of the display - an inversion of the usual viewer-object relationship.
The first thing visitors to the collection will encounter is a corridor lined with portrait photographs of the dead, as if in an installation by the French artist Christian Boltanski. The museum building itself is as much an exhibit as the contents of the displays. The entrance is through a glass pavilion enclosing two of the steel tridents that supported the exterior of the twin towers. The centre of the building is dominated by a ramp that descends three storeys, past a section of the "survivors' stairs" used by hundreds as an escape route, to the bedrock on which the towers were built, their outlines clearly visible. The unidentified partial remains of some of the victims are buried behind a large, stone wall.
The museum's huge west chamber houses two powerful, iconic objects: a portion of the slurry wall, 60 feet high, which withstood the collapse of the towers and prevented the site from being flooded by the Hudson River; and the "last column", the final piece of structural steel to be retrieved from the wreckage in May 2002, covered with tributes to the dead - scrawled, spray-painted, photographic.
The museum's educational programme asks many questions: about the human capacity for extremes of good and evil, alien ideologies, civil liberties. It does not claim to know all of the answers, recognising "the unfinished nature of [the 9/11] story and the continuing ambiguities surrounding a full understanding of the significance of these relatively recent, historical events". This open-ended aspect, combined with the interactive character of the displays, reflects the more sceptical attitude to memorialisation of recent decades: memory as an evolving public debate, rather than something certain or fixed.
How we commemorate the victims of mass murder, especially innocent civilians who were obliterated (about 40 per cent of those killed in lower Manhattan left not so much as a DNA trace), has changed. The best artists today avoid the constricting iconography of a vertical, figurative monument cast in bronze or carved in stone, in favour of a more flexible and allusive sculptural language. An environmental dimension, requiring participation by the onlooker, is now common. Softer materials, such as trees and water, are increasingly used.
The presence on the 9/11 competition jury of James E Young, a US authority on Holocaust memorials, was an encouraging sign. Young coined the term "counter-memorial" to describe the new approach to commemoration, which rejects the monumental and heroic. The National September 11 Memorial is the latest in a succession of minimalist public memorials over the past 25 years, in which formal repetition and "negative" space, whether actual or implied, are employed to evoke absence and loss, but also to resolve and transcend these feelings in a form that gives meaning and hope.
The best-known examples include the Vietnam Veterans Memorial in Washington, DC, by Maya Lin (another member of the 9/11 competition jury); Micha Ullman's Library, commemorating the Nazi book-burnings of 1933 in Berlin; Rachel Whiteread's Judenplatz Holocaust Memorial in Vienna; and Peter Eisenman's Memorial to the Murdered Jews of Europe, also in Berlin. Of the above, Library is arguably the most minimal. A large, white, underground chamber lined with empty bookshelves, it is invisible to the casual pedestrian except through a glass pane set into the cobbles of Bebelplatz above. As Young has written: "A monument built into the ground . . . forces visitors to shoulder memory by themselves, to become, in essence, the monument."
The memorials by Lin, Whiteread and Eisenman were all controversial. Public memory has always been vulnerable to exploitation by political, religious or ethnic interests, and to appropriation by survivors and victims' families. Whether the National September 11 Memorial can avoid these pitfalls remains to be seen. By involving survivors and victims' next of kin at every stage, however, and in its preservation of as much as possible of the specific place where 2,752 lives were lost, the memorial designers and museum curators have done their best to garner widespread popular support. Nationals of 93 countries and people of many faiths, including Muslims, were among the dead. An estimated two billion people around the world watched the attacks live on television. 9/11 was a global atrocity: we all, in a sense, "own" it.
Richard Calvocoressi is director of the Henry Moore Foundation