Yet the space isn’t empty. On the rotunda’s ground floor, a man and woman entwine in a changing, slow-motion amorous embrace. On the ramps above, people walk and talk in pairs or clusters at a leisurely pace, with new participants periodically joining conversations as others drop away.

Mr. Sehgal’s art is made up almost entirely of such balletic tableaus and social encounters. His work has features of theater and dance — he trained as a dancer — but is made for museums, galleries and art fairs, places that depend for their existence on a proliferation of valuable things.

Things are a problem for Mr. Sehgal, who lives in Berlin and studied political economy before he studied dance. He thinks the world has too many of them, that production is ceaseless and technology destructive. His art is a response to these perceived realities as they play out microcosmically in the context of the art industry. His goal is to create a counter-model: to make something (a situation) from virtually nothing (actions, words) and then let that something disappear, leaving no potentially marketable physical trace.

Arranging for disappearance isn’t easy in an age of omnipresent recording devices, which explains why the first thing you see at the Guggenheim is a sign forbidding the taking of photographs. This is standard Sehgal practice. But unless you know why, the prohibition comes across as calculatedly tantalizing.

His rule may be unenforceable in this day and age. Still, it turns the museum into a zone of sort-of secrecy. It piques both the voyeur and the skeptic in us. It amps up the star-power mystique surrounding artists, which in Mr. Sehgal’s case is considerable. Now 34, he has been having solo shows since his mid-20s and has become a fixture on the international biennial circuit.

When you arrive, though, tensions and doubts tend to dissipate. For one thing, there’s practically nothing to see, much less to catch on film. The sensuous pas de deux, titled “Kiss,” is in progress. As choreography it will hold no surprises for anyone familiar with contemporary dance. Taken as living sculpture, it has amusing moments: every so often, the performers strike erotic poses derived from Courbet, Rodin, Brancusi and Jeff Koons.

Far more interesting is the element of duration.

Through the run of the show, “Kiss” will be performed every day in the same spot during regular museum hours, from the time the doors open in the morning till they close at night. Each pair of performers — professional dancers rehearsed by Mr. Sehgal — will appear in roughly three-hour shifts, then be seamlessly replaced by another pair. Like a static sculpture, the piece is continually visible, but also constantly moving and changing. When the show ends, it will evaporate.

Mr. Sehgal created “Kiss” and other sculptural pieces like it in the early 2000s. He then moved on to work that makes viewers part of the action. The second of the two Guggenheim pieces, “This Progress,” which originated in 2006, is one of these. It is visually far less concentrated than “Kiss” — it is even in some sense invisible — but more embracing and filling.

It begins when you walk a short way up the rotunda ramp. A child comes over to greet you. My greeter, a girl of 9 or 10, introduced herself as Giuliana and stated matter-of-factly, “This is a piece by Tino Sehgal.” She invited me to follow her and asked if she could ask me a question. “What is progress?” I gave a broad answer, then at her request, a clarifying example. We went further up the ramp.

Soon we were joined by a young man, a teenager, who said his name was Will. Giuliana carefully and accurately paraphrased for him my response to her question and slipped away. I walked on with Will, who commented on my comments on progress, which prompted me to try to refine my initial thoughts.

About halfway up the rotunda, Will was replaced by Tom, whom I took to be in his mid-30s and who introduced a new topic.

He had read a scientific report that morning saying that dinosaurs, long envisioned as drab-gray and green, might have been brightly colored, even gaudily striped. We had both, we said, been fascinated by dinosaurs as kids, as was his young son today. And now everyone would have to reimagine them, though artists already had done that. So Maurice Sendak’s “Where the Wild Things Are” turns out to be natural history. Art beats science to the punch.

As we neared the last stretch of the ramp, Tom handed me over to Bob, who was, like me, in late middle age and who broached another topic. He had just returned from Bulgaria where he had talked with a range of people over 20 about their feelings about the state of their country and lives. He found, he said, a pervasive nostalgia for life under Communism, a yearning for a society that promised to take care of everyone.