EVENT The Incident ICA, London

"Every system of thought has a hard swallow," warns psychedelic guru Terence McKenna, as the backdrop of trippy computer visuals slides to a halt, and he prepares to explain why the world is going to end on 21 December 2012.

"There's always a preposterous initial idea. With Christianity, it is the resurrection. In science, it's the Big Bang theory, which, for me, is the ultimate test of credulity," he drawls, to appreciative murmurs from the tribe of techno kids and bead-and-feathered crusties who squat wide-eyed at his feet.

"Pay attention now," he wags, leaning forward in his chair like an avuncular Val Doonican, "and I'll explain the rules of the game."

The game is Timewave Zero; the rules, a computer model based on fractal dynamics, plotting a graph of every peak and trough in our planet's 4,500 million-year history. "Look, here are the first oxygen-producing plants, here the building of the Pyramids," burbles McKenna plausibly, whipping us along the rollercoaster of world events with the aid of a giant computer screen. What the necromancers of yesteryear proved with mirrors and fistfuls of gunpowder, today's visionaries unfold with technological wizardry and the incontrovertible evidence of mathematics.

Sixteen years and our time is up. Maybe we'll all be hit by a meteorite, but McKenna reckons it's more likely that we're just about to discover the secret of time travel.

Not that McKenna wishes to claim all the credit for his discovery. Loosened up with a nip of DMT (the visionary's psychotropic drug of choice), the theory, he maintains "came entire to me from megaspace. I simply pass it on."

And megaspace, it seems, is where all the best ideas come from; or so was the consensus, at "The Incident" this week, a five-day ideas-fest of talks, installation and performance at London's Institute of Contemporary Arts.

Californian UFOlogists, Swiss anthropologists and Colombian ethno-botanists rubbed auras with Glastonbury parapsychologists, archaeologists, cyberneticists and virtual realists, all sharing their thoughts on the paranormal and the nature of perception. A load of tosh? Possibly, but gripping stuff none the less.

Did earlier civilisations have access to other knowledge systems communicated to them by aliens? Is there a universal consciousness that we can all tap into? Are aliens visions of our future selves, glimpsed through gaps in the spiral of time? Or are they merely the projections of our fears that humans are evolving too fast? Are the snake-filled visions of Amazonian shaman a direct link to the twirling strands of DNA? We slipped between the parallel universes of art, spirituality and speculative science, as deftly as flying saucers through a worm-hole.

It was a prolonged encounter with a UFO, while he was piloting a plane in the Sixties, which first sent American artist James Turrell on his lifetime's trajectory of experimentation with ideas about limitless space and intelligent light. Unavoidably detained inside his current project, the Roden Crater, Turrell did not attend in the flesh but sent his installation Gasworks as emissary.

Participants experience Gasworks alone. Climb into a narrow tray, lie down and prepare to be rolled headfirst into a glowing white sphere. Staring up into the gleaming dome, you are enveloped in an intense white light. A single dazzling lightning flash suddenly seers the retina, inducing a giddying kaleidoscope of dancing geometrics. For 15 minutes a dizzying succession of Seventies kitchen lino patterns zoom around your field of vision. The frenzy of hexagons is inescapable, whether the eyes are open or closed. Eventually the patterns recede into gleaming purity and the participant slides blinking out of the capsule.

As abduction experiences go, Gasworks participants get off lightly, more likely to leave with a headache than a tummyful of gestating alien progeny.

A nest of Mayan rattlesnakes hiss and writhe a greeting to people entering Kathleen Rogers' installation Viper Science. Human movements in the room are detected by ultrasonic beams, unleashing a battery of fizzling rattler recordings and animating a twisting flickery video serpent. The Mayans used the plumed serpent as a sort of mystic abacus, counting its scales to compute their sacred calendar; and since the Mayan calendar also ends in the year 2012, I presume Rogers' snakes were communicating another vital message from megaspace.

Anne Bean's performance An Angel Called Gravity was sparked off by another brush with a UFO, this time floating past her studio in Canary Wharf. An evocative narrative recalling the experience drifted out of the semi- darkness while Bean filled the stage with bobbing helium-filled balloons, each moored to the floor by a shiny green apple. A further wave of fluorescent balloons gently deflated, exhaling into miniature harmonicas to breathe an eerie music of the spheres, while Bean led the audience out into St James's Park.

Here, two dozen glowing balloons, each bearing a teeny electronic birthday card, were released into the starlight; "Fur Elise" tinkling sweetly over the cabinet war rooms and up, up, up, to mega space.