Cloud of steel: Sou Fujimoto's temporary pavilion for London's Serpentine Gallery unveiled

How do you make a cloud out of steel? If you’re the Japanese designer Sou Fujimoto, one of architecture’s rising international stars, you take 28 kilometres of thin metal tubing, cut it up into 27,000 sections, join them together in 10,000 places, and put the structure on the grass next to London’s Serpentine Gallery.

And presto! A pavilion that is a beautiful mystery of light, space and geometry that will be used through the summer for music, screenings, lectures and, most of all, as somewhere to hang out in Hyde Park.

Everything about the pavilion is ambiguous. It looks finished and unfinished, delicate and substantial, hard-edged and softly indistinct. The charming 42-year-old Fujimoto has created a seductive maze of perspectives that lead your gaze into the structure and then, very teasingly, turns apparently strict structural order into impossible visual riddles.

We can blame that on Albert Einstein, the scientist whose theory of relativity changed the way the world thought about space and time. “In high school I was fascinated by the theories of Einstein,” says Fujimoto. “My concept of space came to me through Einstein, not my architectural training.”

His other inspiration was the Catalan architect, Antoni Gaudi, whose vividly sculpted buildings and public spaces remain hugely popular in Barcelona almost a century after his death. “The first time I found Gaudi was when I was 12, and I thought – wow! Then I visited Gaudi’s work in Barcelona and, at his Parc Guell, I felt the diversity of the area – people behaving differently, no sense of boundaries.”

So we can think of the Serpentine Pavilion as an architecture that may look definite and “structural”, but is actually a subtle design that treats space and light as visions that constantly shift as you approach the building, or move through it. There’s no specific sense of outside or inside – Fujimoto wants to erase those kinds of architectural boundaries.

He erases the roof, too, by using horizontal clusters of thin, clear polycarbonate discs that riffle very slightly in the breeze; from some angles, they’re almost invisible. The pavilion is the most ethereal since the 2009 structure designed by the SANAA architectural practice. That was an exquisite, super-thin amoeba of highly polished steel that seemed to float above the grass, inverting reflections of trees, people – and even falling rain.

Sou Fujimoto is the youngest pavilion architect to have been commissioned by the Serpentine Gallery’s directors, Julia Peyton-Jones and Hans Ulrich Obrist. The latter is fascinated by the micro-macro effect of the building, and the fact that its image keeps changing, depending on the light.

“We met Sou in Tokyo seven years ago, when he was writing a manifesto on primitive architecture,” he explains, “and we’ve followed his work since then.” In Japan, Fujimoto’s house designs are invariably opened out, or give the impression of being deconstructed in some way.

The Serpentine Pavilion – it’s the 13 in as many years – is both primitive and highly sophisticated. In the first century BC, the Roman engineer Vitruvius declared that architecture should have firmness, commodity, and delight. Sou Fujimoto’s pavilion hits the “delight” button, in particular, with great panache.