The art installation "Gravity of Light," a high point of the innovative 2008 Pittsburgh International Festival of Firsts, is also free and available for perusal during user-friendly hours on a drop-in basis.

That means visitors may set individual agendas with this large work, controlling length and number of visits.

Early in the month, before New York-based identical twins Doug and Mike Starn arrived to oversee the set- up of their work in the Strip District's Pipe Building, the Post-Gazette published pictures of "Gravity" in its Stockholm incarnation.

The components are the same, but the difference between venues illustrates the importance of place to an installation's ultimate effect.

The Swedish space has a vast open center, brighter walls and a sweeping, level floor. As a working contemporary art site, it's also cleaner -- tidied and mostly devoid of reference to its past life as a paint factory.

In Pittsburgh, little was done beyond painting the building's high glass block windows to cut out daylight, leaving years of cumulative industrial emissions that have coated surfaces and filled cracks with a soft blackness.

All of this suits the Starns' aesthetic well.

Their leitmotiv, since they began exploring photography in their teens, has been light -- essential to the very existence of their medium, but also a recurring symbol of such integral human concerns as intellectual discovery, spiritual presence and biological sustainability. It can obliterate as much as reveal, frighten as much as calm. And they find its absence equally fascinating.

Mike Starn says that one of the things he and Doug appreciate about the Pittsburgh site is "how dark the space is. It really sucks up all of the light," which disappears into the shadowy coating on the floors, ceilings and walls.

That quality also makes the centerpiece 40,000-to-50,000-watt carbon arc lamp, the room's only source of illumination, more articulate.

It is the lamp's occasional sparking, it's variance from white heat to amber glow, that mediates the viewer's experience as it erratically illuminates the Starns' large photographic works, bathing those closest with light while the farthest linger in shadowy recess.

But the lamp also works on a physiological level, inducing subconscious emotional and physical responses as one lingers within its unmeasured rhythm.

The imagery -- moths, tree branches, skeletal leaves, the blind eighth-century Chinese monk Ganjin-- appear at first to be unrelated. But then the piece works its charm.

My first impression of "Gravity" was "distressed" -- not as in anxiety but indicating wear and passing: The bare trees to winter, the leaves to dust, the workplace to abandonment.

More significant, though, is the slow realization of how integral each element is to the whole: The tactile similarity between the moth body and the paper the image is printed on, the leaf's exposed structure to that of human circulatory and nervous systems. And more: of blind Ganjin's internal spiritual radiance and the arc lamp's blinding fire; of the sizzling spark, ignited when two carbon rods touch, to sexuality and reproduction; and all to cycles of mortality.

Such discovery is aided by the breathtakingly beautiful -- if almost sacrificial -- installation of the museum-quality images, which have been worked around architectural elements -- cut, bent, pierced.

By extension, the Starns remind of the intrinsic interrelatedness between ourselves and everything we move within: That we and the surrounding world are inseparable.

It seems an observation that might be keenly felt by artists who are identical twins.

The Starns produce exquisite exhibition catalogs that amount to artist books. One of five projected to accompany "Gravity" -- "To Find God Not the Devil's Insides" -- is for sale here ($75), as is "alleverythingthatisyou," which was inspired by Wilson Bentley's 19th-century snowflake photography, given the Starns' imprint ($60).

The Starns also have designed a "Carbon Arc Lamp Manual" in conjunction with the show's American tour, which may be purchased out of a machine for $10; so New York.

"Gravity" is at 3000 Liberty Ave., on the south side of the street. Free parking is available in a rough lot on the west side of the four-story brick building and directly across Liberty from where 30th Street enters. The installation is in the building's far end and open noon to 6:30 p.m. Tuesday through Sunday, extended until 8:30 p.m. Friday and Saturday.

Visitors are given ultraviolet-filtering glasses to wear in "Gravity" and must sign a "responsibility and liability disclaimer." The floors are very uneven, so allow time to adjust to the lighting before walking around. There is a short stairway to climb at entry.

People of all ages who love the arts and the magical impulses that birth them won't find a more inspiring story than the one documented in "A Man Named Pearl," at Pittsburgh Filmmakers' Regent Square Theater through Sunday (pghfilmmakers.org). To read Barry Paris' 3-star review visit www.post-gazette.com/pg/08290/920094-42.stm.