Reach for the sky - and see the light

The stony lane meanders upwards through a landscape ectoplasmed by mist. Somewhere below, beyond the stands of sitka, spruce and magenta toupées of heather dissolving into the sky's soft grain, is the vast expanse of Kielder Water. A mile ahead, on a most inauspiciously grey and featureless day, lies a lesson about the delicacies of light.

The stony lane meanders upwards through a landscape ectoplasmed by mist. Somewhere below, beyond the stands of sitka, spruce and magenta toupées of heather dissolving into the sky's soft grain, is the vast expanse of Kielder Water. A mile ahead, on a most inauspiciously grey and featureless day, lies a lesson about the delicacies of light.

The structure is set into the side of a hillock, a dry-stone drum 25ft in diameter rising about 10 feet out of earth and rock. It has no notable features apart from a white-painted concrete lip and a grey tunnel cutting into it. There is nothing in its form that can possibly signal its purpose; in this scrag-end of landscape the architecture seems arbitrary.

The opening ceremony is over. A red ribbon trails in the mud and the invitees of Northern Arts are heading inquisitively into the tunnel. They are entering the newest building by by the American perceptualist James Turrell. They are entering a Plato's cave for the new millennium - its obverse, in fact.

It's a place where an elemental truth is stripped down to its essentials and re-rendered as a physical experience. The light and shadow here - and the architecture that delivers them - are man-made; all who enter become creators.

Initially, everything about the interior space of the drum is to do with erasure. The off-white painted walls and ceiling 20ft above, the continuous circular concrete seating in matt battleship grey, the central disc of dark stone-shards, the sand-stippled floor; all of it is part of a carefully orchestrated process of reduction, and it is as quickly achieved as blanking text on a child's magic slate.

To sit in Turrell's Skyspace, even tangled in the febrile chatter, flash-photography and papoosed babies, is to become inexorably isolated - prepared, in effect, for the real deal. The echoing voices become as ambiently vague as an Aphex Twin track. The eye is drawn upwards, rather than around the chamber, and the watcher begins to be reeled into Turrell's subtle sleight-of-hand.

In the centre of the high ceiling is a 13ft-wide aperture open to the sky. At first, there seems nothing to consider: the eye takes in the upper reaches of the space and the misty light poised within the aperture.

And then, gradually but distinctly, the sharp edge of the circle seems to darken slightly, accentuating the light within it, and the eye becomes attuned to something else. After a minute or two, light that seemed as drearily unremarkable as skimmed milk on the slow yomp up to the Skyspace, begins to reveal itself as something possessed of an otherness that is both natural and unnatural. It is, as Turrell says, a case of "dealing with the light itself, not as the bearer of revelation, but as revelation itself".

So it proves. The light framed in the circle becomes other things, which reveal unexpected qualities: for a while, it cannot possibly be the sky because it resembles a perfect evenly-lit designer fluorescent ceiling light, something flawless designed in Italy. But time passes and the rogue light fitting metamorphoses into other forms - a pale meniscus seeming to bulge slightly into the space. Then the details begin to come; gradations of light and shade in the mist become precisely obvious.

It is only light, but not as we normally know it. In an odd way, Turrell is doing with light what Tracey Emin has done with beds and beach huts. In both cases, something obvious is isolated and made in some way extraordinary. The difference is that Turrell encourages spiritual illumination with architecture that delivers a surrealness that is natural rather than absurd. Emin's detritus comes from occluded places, and it decidedly wants to stay there.

Turrell, a Quaker whose doctorate was in the psychology of perception, creates spaces that will "directly connect you to a thought that is wordless, a thought that doesn't have a story-line. It's an arena of thought that has a kind of loneliness, but also a great beauty". He is therefore a paid-up member of the TS Eliot Signals Corps: a man in search of Eliot's "word that speaks no word".

The architecture of the Skyspace reflects this quest. Its obvious form has a less obvious purpose; it's a cipher carrying a meaning that has no direct link to its configuration. Yet it may contain the germ of an idea that could illuminate other forms of architecture. Light, shade, volumes, colours and proportions are key issues in design. But natural light in buildings is usually thought of in terms of windows, high-tech steel and glass facades, reflections, atria. Turrell's Skyspace - the Arts Council-funded Kielder structure is his fifth worldwide and there are more on the way - suggests that there may sometimes be more meaningful ways to deliver light into buildings.

Turrell's use of internal lighting is equally significant. Optic fibre illumination, at the base of the seating and concealed behind the seating, is triggered and modulated by light conditions outside. This means that, in the chamber, the alternative realities of light and dark skies are at their most potent around dawn and dusk.

Alan Sykes, one of a group of 10 who sat in the Skyspace at dusk on the evening of the building's opening, was astonished by the moment's power. "It was an extraordinary and mystical experience," he says. "It was literally mind-altering, or light-altering. The darkening sky became solid and seemed to be coming into the space. The whole sky became a Quink blue-black - an unreal black - and seemed to be bulging inwards. And then I found I was seeing green light rotating phosphorescently on the rim of the circle. It's as if you're touching the heavens".

Sykes had not partaken liberally of mandrake root. His experience is rather typical of those who have experienced Turrell's sky-focusing structures in Japan, France and Holland. The charming, softly-spoken artificer from Flagstaff, Arizona gives good visionary experience, as befits one fond of quoting St Exupéry and enthusing that in Cornwall he had seen "spaces within the spaces in the sky".

The Cornish connection is apt because the Kielder Skyspace, a kind of temple on a hill, is not far removed in elemental spirit from the remains of the nine iron-age fogous that can still be found in that county. These mysterious, curved subterranean chambers are thought to have been used for rituals linked to the transit of the sun, and with tin and copper lode-lines. At the Carn Euny fogou in West Penwith there is still a chamber off the main tunnel - and it has a hole in the roof.

Turrell's fogou at Kielder is an altogether more efficient affair, of course: at its zenith, the mid-summer sun will fall precisely on the roundel of stone chips on the floor of the chamber; and at the winter solstice, it will beam along the tunnel.

The Skyspace will thus attract New Agers and tired walkers eating dank, fractured pasties; it will become a haven of contemplation for the pure of mind, and an observatory for those with chemically-challenged synapses; Aldous Huxley would surely have loved it enough to scribble a slim volume entitled The Veluxes of Perception; and maybe Tracey will pitch up, accompanied by William Blake's luminous shade.

And should architects trudge up the track and into the tunnel, they, too, will learn something about the strange physicalities of light in this rebirthing chamber for radiance, which belongs to a time when religious and artistic flashovers depended on dramatic, and preferably unexplainable, revelations. Perhaps a New Jerusalem awaits them, encoded in a glittering circlet of the Milky Way.