The world seems to be putting itself together piece by piece on this damp gray morning along the coast of Maine. First the spruce and white pine trees that cover High Island materialize from the fog, then the rocky headland, and finally the sea, as if the mere act of watching has drawn them all into existence. And that may indeed be the case. While this misty genesis unfolds, the island's most eminent residentdiscusses notions that still perplex him after seven decades in physics, including his gut feeling that the very universe may be constantly emerging from a haze of possibility, that we inhabit a cosmos made real in part by our own observations.

John Wheeler, scientist and dreamer, colleague of Albert Einstein and Niels Bohr, mentor to many of today's leading physicists, and the man who chose the name "black hole" to describe the unimaginably dense, light-trapping objects now thought to be common throughout the universe, turned 90 last July. He is one of the last of the towering figures of 20th-century physics, a member of the generation that plumbed the mysteries of quantum mechanics and limned the utmost reaches of space and time. After a lifetime of fundamental contributions in fields ranging from atomic physics to cosmology, Wheeler has concerned himself in his later years with what he calls "ideas for ideas."

"I had a heart attack on January 9, 2001," he says, "I said, 'That's a signal. I only have a limited amount of time left, so I'll concentrate on one question: How come existence?'"

Why does the universe exist? Wheeler believes the quest for an answer to that question inevitably entails wrestling with the implications of one of the strangest aspects of modern physics: According to the rules of quantum mechanics, our observations influence the universe at the most fundamental levels. The boundary between an objective "world out there" and our own subjective consciousness that seemed so clearly defined in physics before the eerie discoveries of the 20th century blurs in quantum mechanics. When physicists look at the basic constituents of reality— atoms and their innards, or the particles of light called photons— what they see depends on how they have set up their experiment. A physicist's observations determine whether an atom, say, behaves like a fluid wave or a hard particle, or which path it follows in traveling from one point to another. From the quantum perspective the universe is an extremely interactive place. Wheeler takes the quantum view and runs with it.

As Wheeler voices his thoughts, he laces his fingers behind his large head, leans back onto a sofa, and gazes at the ceiling or perhaps far beyond it. He sits with his back to a wide window. Outside, the fog is beginning to lift on what promises to be a hot summer day. On an end table near the sofa rests a large oval rock, with one half polished black so that its surface resembles the Chinese yin-yang symbol. "That rock is about 200 million yearsold," says Wheeler. "One revolution of our galaxy."

Although Wheeler's face looks careworn and sober, it becomes almost boyish when he smiles, as he does when I extend a hand to help him from the couch and he says, "Ah, antigravity." Wheeler is short and sturdily built, with sparse white hair. He retains an impish fascination with fireworks— an enthusiasm that cost him part of a finger when he was young— and has on occasion lit Roman candles in the corridors of Princeton, where he became a faculty member in 1938 and where he still keeps an office. At one point a loud bang interrupts our interview. Wheeler's son, who lives on a cliff a few hundred yards away, has fired a small cannon, a gift from Wheeler.

Wheeler is gracious to a fault; one colleague describes him as "a gentleman hidden inside a gentleman." But that courtly demeanor also hides something else: one of the most adventurous minds in physics. Instead of shying away from questions about the meaning of it all, Wheeler relishes the profoundand the paradoxical. He was an early advocate of the anthropic principle, the idea that the universe and the laws of physics are fine-tuned to permit the existence of life. For the past two decades, though, he has pursued a far more provocative idea for an idea, something he calls genesis by observership. Our observations, he suggests, might actually contribute to the creation of physical reality. To Wheeler we are not simply bystanders on a cosmic stage; we are shapers and creators living in a participatory universe.

Wheeler's hunch is that the universe is built like an enormous feedback loop, a loop in which we contribute to the ongoing creation of not just the present and the future but the past as well. To illustrate his idea, he devised what he calls his "delayed-choice experiment," which adds a startling, cosmic variation to a cornerstone of quantum physics: the classic two-slit experiment.

That experiment is exceedingly strange in its own right, even without Wheeler's extra kink thrown in. It illustrates a key principle of quantum mechanics: Light has a dual nature. Sometimes light behaves like a compact particle, a photon; sometimes it seems to behave like a wave spread out in space, just like the ripples in a pond. In the experiment, light — a stream of photons — shines through two parallel slits and hits a strip of photographic film behind the slits. The experiment can be run two ways: with photon detectors right beside each slit that allow physicists to observe the photons as they pass, or with detectors removed, which allows the photons to travel unobserved. When physicists use the photon detectors, the result is unsurprising: Every photon is observed to pass through one slit or the other. The photons, in other words, act like particles.

But when the photon detectors are removed, something weird occurs. One would expect to see two distinct clusters of dots on the film, corresponding to where individual photons hit after randomly passing through one slit or theother. Instead, a pattern of alternating light and dark stripes appears. Such a pattern could be produced only if the photons are behaving like waves, with each individual photon spreading out and surging against both slits at once, like a breaker hitting a jetty. Alternating bright stripes in the pattern on the film show where crests from those waves overlap; dark stripes indicate that a crest and a trough have canceled each other.

The outcome of the experiment depends on what the physicists try to measure: If they set up detectors beside the slits, the photons act like ordinary particles, always traversing one route or the other, not both at the same time. In that case the striped pattern doesn't appear on the film. But if the physicists remove the detectors, each photon seems to travel both routes simultaneously like a tiny wave, producing the striped pattern.

Wheeler has come up with a cosmic-scale version of this experiment that has even weirder implications. Where the classic experiment demonstrates that physicists' observations determine the behavior of a photon in the present, Wheeler's version shows that our observations in the present can affect how a photon behaved in the past.

To demonstrate, he sketches a diagram on a scrap of paper. Imagine, he says, a quasar — a very luminous and very remote young galaxy. Now imagine that there are two other large galaxies between Earth and the quasar. The gravity from massive objects like galaxies can bend light, just as conventional glass lenses do. In Wheeler's experiment the two huge galaxies substitute for the pair of slits; the quasar is the light source. Just as in the two-slit experiment, light — photons — from the quasar can follow two different paths, past one galaxy or the other.

Suppose that on Earth, some astronomers decide to observe the quasars. In this case a telescope plays the role of the photon detector in the two-slit experiment. If the astronomers point a telescope in the direction of one of the two intervening galaxies, they will see photons from the quasar that were deflected by that galaxy; they would get the same result by looking at the other galaxy. But the astronomers could also mimic the second part of the two-slit experiment. By carefully arranging mirrors, they could make photons arriving from the routes around both galaxies strike a piece of photographic film simultaneously. Alternating light and dark bands would appear on the film, identical to the pattern found when photons passed through the two slits.

Here's the odd part. The quasar could be very distant from Earth, with light so faint that its photons hit the piece of film only one at a time. But the results of the experiment wouldn't change. The striped pattern would still show up, meaning that a lone photon not observed by the telescope traveled both paths toward Earth, even if those paths were separated by many light-years. And that's not all.

By the time the astronomers decide which measurement to make — whether to pin down the photon to one definite route or to have it follow both paths simultaneously — the photon could have already journeyed for billions of years, long before life appeared on Earth. The measurements made now, says Wheeler, determine the photon's past. In one case the astronomers create a past in which a photon took both possible routes from the quasar to Earth. Alternatively, they retroactively force the photon onto one straight trail toward their detector, even though the photon began its jaunt long before any detectors existed.

It would be tempting to dismiss Wheeler's thought experiment as a curious idea, except for one thing: It has been demonstrated in a laboratory. In 1984 physicists at the University of Maryland set up a tabletop version of the delayed-choice scenario. Using a light source and an arrangement of mirrors to provide a number of possible photon routes, the physicists were able to show that the paths the photons took were not fixed until the physicists made their measurements, even though those measurements were made after the photons had already left the light source and begun their circuit through the course of mirrors.

Wheeler conjectures we are part of a universe that is a work in progress; we are tiny patches of the universe looking at itself — and building itself. It's not only the future that is still undetermined but the past as well. And by peering back into time, even all the way back to the Big Bang, our present observations select one out of many possible quantum histories for the universe.

Does this mean humans are necessary to the existence of the universe? While conscious observers certainly partake in the creation of the participatory universe envisioned by Wheeler, they are not the only, or even primary, way by which quantum potentials become real. Ordinary matter and radiation play the dominant roles. Wheeler likes to use the example of a high-energy particle released by a radioactive element like radium in Earth's crust. The particle, as with the photons in the two-slit experiment, exists in many possible states at once, traveling in every possible direction, not quite real and solid until it interacts with something, say a piece of mica in Earth's crust. When that happens, one of those many different probable outcomes becomes real. In this case the mica, not a conscious being, is the object that transforms what might happen into what does happen. The trail of disrupted atoms left in the mica by the high-energy particle becomes part of the real world.

At every moment, in Wheeler's view, the entire universe is filled with such events, where the possible outcomes of countless interactions become real, where the infinite variety inherent in quantum mechanics manifests as a physical cosmos. And we see only a tiny portion of that cosmos. Wheeler suspects that most of the universe consists of huge clouds of uncertainty that have not yet interacted either with a conscious observer or even with some lump of inanimate matter. He sees the universe as a vast arena containing realms where the past is not yet fixed.

Wheeler is the first to admit that this is a mind-stretching idea. It's not even really a theory but more of an intuition about what a final theory of everything might be like. It's a tenuous lead, a clue that the mystery of creation may lie not in the distant past but in the living present. "This point of view is what gives me hope that the question — How come existence? — can be answered," he says.

William Wootters, one of Wheeler's many students and now a professor of physics at Williams College in Williamstown, Massachusetts, sees Wheeler as an almost oracular figure. "I think asking this question — How come existence? — is a good thing," Wootters says. "Why not see how far you can stretch? See where that takes you. It's got to generate at least some good ideas, even if the question doesn't get answered. John is interested in the significance of quantum measurement, how it creates an actuality of what was a mere potentiality. He has come to think of that as the essential building block of reality."

In his concern for the nature of quantum measurements, Wheeler is addressing one of the most confounding aspects of modern physics: the relationship between the observations and the outcomes of experiments on quantum systems. The problem goes back to the earliest days of quantum mechanics and was formulated most famously by the Austrian physicist Erwin Schrödinger, who imagined a Rube Goldberg-type of quantum experiment with a cat.

Put a cat in a closed box, along with a vial of poison gas, a piece of uranium, and a Geiger counter hooked up to a hammer suspended above the gas vial. During the course of the experiment, the radioactive uranium may or may not emit a particle. If the particle is released, the Geigercounter will detect it and send a signal to a mechanism controlling the hammer, which will strike the vial and release the gas, killing the cat. If the particle is not released, the cat will live. Schrödinger asked, What could be known about the cat before opening the box?

If there were no such thing as quantum mechanics, the answer would be simple: The cat is either alive or dead, depending on whether a particle hit the Geiger counter. But in the quantum world, things are not so straightforward. The particle and the cat now form a quantum system consisting of all possible outcomes of the experiment. One outcome includes a dead cat; another, a live one. Neither becomes real until someone opens the box and looks inside. With that observation, an entire consistent sequence of events— the particle jettisoned from the uranium, the release of the poison gas, the cat's death— at once becomes real, giving the appearance of something that has taken weeks to transpire. Stanford University physicist Andrei Linde believes this quantum paradox gets to the heart of Wheeler's idea about the nature of the universe: The principles of quantum mechanics dictate severe limits on the certainty of our knowledge.

"You may ask whether the universe really existed before you start looking at it," he says. "That's the same Schrödinger cat question. And my answer would be that the universe looks as if it existed before I started looking at it. When you open the cat's box after a week, you're going to find either a live cat or a smelly piece of meat. You can say that the cat looks as if it were dead or as if it were alive during the whole week. Likewise, when we look at the universe, the best we can say is that it looks as if it were there 10 billion years ago."

Linde believes that Wheeler's intuition of the participatory nature of reality is probably right. But he differs with Wheeler on one crucial point. Linde believes that conscious observers are an essential component of the universe and cannot be replaced by inanimate objects.

"The universe and the observer exist as a pair," Linde says. "You can say that the universe is there only when there is an observer who can say, Yes, I see the universe there. These small words — it looks like it was here— for practical purposes it may not matter much, but for me as a human being, I do not know any sense in which I could claim that the universe is here in the absence of observers. We are together, the universe and us. The moment you say that the universe exists without any observers, I cannot make any sense out of that. I cannot imagine a consistent theory of everything that ignores consciousness. A recording device cannot play the role of an observer, because who will read what is written on this recording device? In order for us to see that something happens, and say to one another that something happens, you need to have a universe, you need to have a recording device, and you need to have us. It's not enough for the information to be stored somewhere, completely inaccessible to anybody. It's necessary for somebody to look at it. You need an observer who looks at the universe. In the absence of observers, our universe is dead.

"Will Wheeler's question— How come existence?— ever be answered? Wootters is skeptical. "I don't know if human intelligence is capable of answering that question," he says. "We don't expect dogs or ants to be able to figure out everything about the universe. And in the sweep of evolution, I doubt that we're the last word in intelligence. There might be higher levels later. So why should we think we're at the point where we can understand everything? At the same time I think it's great to ask the question and see how far you can go before you bump into awall."

Linde is more optimistic.

"You know, if you say that we're smart enough to figure everything out, that is a very arrogant thought. If you say that we're not smart enough, that is a very humiliating thought. I come from Russia, where there is a fairy tale about two frogs in a can of sour cream. The frogs were drowning in the cream. There was nothing solid there; they could not jump from the can. One of the frogs understood there was no hope, and he stopped beating the sour cream with his legs. He just died. He drowned in sour cream. The other one did not want to give up. There was absolutely no way it could change anything, but it just kept kicking and kicking and kicking. And then all of a sudden, the sour cream was churned into butter. Then the frog stood on the butter and jumped out of the can. So you look at the sour cream and you think, 'There is no way I can do anything with that.' But sometimes, unexpected things happen.

"I'm happy that some people who previously thought this question — How come existence? — was meaningless did not stop us from asking it. We all learned from people like John Wheeler, who asks strange questions and gives strange answers. You may agree or disagree with his answers. But the very fact that he asks these questions, and suggests some plausible — and implausible — answers, it has shaken these boundaries of what is possible and what is impossible to ask."

And what does the oracle of High Island himself think? Will we ever understand why the universe came into being?

"Or at least how," he says. "Why is a trickier thing." Wheeler points to the example of Charles Darwin in the 19th century and how he provided a simple explanation — evolution through natural selection — for what seemed an utterly intractable problem: how to explain the origin and diversity of life on Earth. Does Wheeler think that physicists might one day have a similarly clear understanding of the origin of the universe?

"Absolutely," he says. "Absolutely."