John Archibald Wheeler,
high priest of quantum mysteries,
suspects thatreality exists not
because of physical particles but
rather because of the actof
observing the universe. "Information
may not be just what we learn
about the world," he says. "It may
be what makes the world."

The world seems to be putting itself together piece by piece on thisdamp gray morning along the coast of Maine. First the spruce and whitepine trees that cover High Island materialize from the fog, then therocky headland, and finally the sea, as if the mere act of watching hasdrawn them all into existence. And that may indeed be the case. Whilethis misty genesis unfolds, the island's most eminent residentdiscusses notions that still perplex him after seven decades inphysics, including his gut feeling that the very universe may beconstantly emerging from a haze of possibility, that we inhabit acosmos 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 thename "black hole" to describe the unimaginably dense, light-trappingobjects now thought to be common throughout the universe, turned 90last July. He is one of the last of the towering figures of20th-century physics, a member of the generation that plumbed themysteries of quantum mechanics and limned the utmost reaches of spaceand time. After a lifetime of fundamental contributions in fieldsranging from atomic physics to cosmology, Wheeler has concerned himselfin his later years with what he calls "ideas for ideas."

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

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

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

AlthoughWheeler's face looks careworn and sober, it becomes almost boyish whenhe smiles, as he does when I extend a hand to help him from the couchand he says, "Ah, antigravity." Wheeler is short and sturdily built,with sparse white hair. He retains an impish fascination withfireworks— an enthusiasm that cost him part of a finger when he wasyoung— and has on occasion lit Roman candles in the corridors ofPrinceton, where he became a faculty member in 1938 and where he stillkeeps 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 fireda small cannon, a gift from Wheeler.

Wheeler is gracious to afault; one colleague describes him as "a gentleman hidden inside agentleman." But that courtly demeanor also hides something else: one ofthe most adventurous minds in physics. Instead of shying away fromquestions about the meaning of it all, Wheeler relishes the profoundand the paradoxical. He was an early advocate of the anthropicprinciple, the idea that the universe and the laws of physics arefine-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, hesuggests, might actually contribute to the creation of physicalreality. To Wheeler we are not simply bystanders on a cosmic stage; weare shapers and creators living in a participatory universe.

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

Seeing Double (Click here to enlarge)
In his delayed-choice thought experiment,
Wheeler suggests that a single photon
emitted from a distant quasar (far right) can
simultaneously follow two pathsto Earth,
even if those paths are separated by many
light-years. Here onephoton travels past two
different galaxies, with both routes deflected
by thegravitational pull of the galaxies. Stranger
still, Wheeler theorizes, the observations
astronomers make on Earth today decide the
path the photon tookbillions of years ago.
Graphic by Matt Zang

That experiment is exceedingly strange in its own right, even withoutWheeler's extra kink thrown in. It illustrates a key principle ofquantum mechanics: Light has a dual nature. Sometimes light behaveslike a compact particle, a photon; sometimes it seems to behave like awave spread out in space, just like the ripples in a pond. In theexperiment, light— a stream of photons— shines through two parallelslits and hits a strip of photographic film behind the slits. Theexperiment can be run two ways: with photon detectors right beside eachslit that allow physicists to observe the photons as they pass, or withdetectors removed, which allows the photons to travel unobserved. Whenphysicists use the photon detectors, the result is unsurprising: Everyphoton is observed to pass through one slit or the other. The photons,in other words, act like particles.

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

The outcome of the experimentdepends on what the physicists try to measure: If they set up detectorsbeside the slits, the photons act like ordinary particles, alwaystraversing one route or the other, not both at the same time. In thatcase the striped pattern doesn't appear on the film. But if thephysicists remove the detectors, each photon seems to travel bothroutes simultaneously like a tiny wave, producing the striped pattern.

Wheelerhas come up with a cosmic-scale version of this experiment that haseven weirder implications. Where the classic experiment demonstratesthat physicists' observations determine the behavior of a photon in thepresent, Wheeler's version shows that our observations in the presentcan 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 thereare two other large galaxies between Earth and the quasar. The gravityfrom massive objects like galaxies can bend light, just as conventionalglass lenses do. In Wheeler's experiment the two huge galaxiessubstitute for the pair of slits; the quasar is the light source. Justas in the two-slit experiment, light— photons— from the quasar canfollow two different paths, past one galaxy or the other.

Supposethat on Earth, some astronomers decide to observe the quasars. In thiscase a telescope plays the role of the photon detector in the two-slitexperiment. If the astronomers point a telescope in the direction ofone of the two intervening galaxies, they will see photons from thequasar that were deflected by that galaxy; they would get the sameresult by looking at the other galaxy. But the astronomers could alsomimic the second part of the two-slit experiment. By carefullyarranging mirrors, they could make photons arriving from the routesaround both galaxies strike a piece of photographic filmsimultaneously. Alternating light and dark bands would appear on thefilm, identical to the pattern found when photons passed through thetwo slits.

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

By the time the astronomers decide which measurement tomake— whether to pin down the photon to one definite route or to haveit follow both paths simultaneously— the photon could have alreadyjourneyed for billions of years, long before life appeared on Earth.The measurements made now, says Wheeler, determine the photon'spast. In one case the astronomers create a past in which a photon tookboth possible routes from the quasar to Earth. Alternatively, theyretroactively force the photon onto one straight trail toward theirdetector, even though the photon began its jaunt long before anydetectors existed.

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

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

Birthday Bash
Andrei Linde, top, one of the
principal architects of inflationary
theory,helps celebrate John
Wheeler's pre-91st birthday at a
gathering at Princeton University.
Linde is using his hands toillustrate
that our universe may have been
paired with another when it was
born. Wheeler, with glass in
hand (bottom),chats with Ravi
Ravindra, aprofessor emeritus
of comparative religion at
Dalhousie University in Nova Scotia.
Photographs by Brian Finke

Does this mean humans are necessary to the existence of the universe?While conscious observers certainly partake in the creation of theparticipatory universe envisioned by Wheeler, they are not the only, oreven primary, way by which quantum potentials become real. Ordinarymatter and radiation play the dominant roles. Wheeler likes to use theexample of a high-energy particle released by a radioactive elementlike radium in Earth's crust. The particle, as with the photons in thetwo-slit experiment, exists in many possible states at once, travelingin every possible direction, not quite real and solid until itinteracts with something, say a piece of mica in Earth's crust. Whenthat happens, one of those many different probable outcomes becomesreal. In this case the mica, not a conscious being, is the object thattransforms what might happen into what does happen. The trail ofdisrupted atoms left in the mica by the high-energy particle becomespart of the real world.

At every moment, in Wheeler's view,the entire universe is filled with such events, where the possibleoutcomes of countless interactions become real, where the infinitevariety inherent in quantum mechanics manifests as a physical cosmos.And we see only a tiny portion of that cosmos. Wheeler suspects thatmost of the universe consists of huge clouds of uncertainty that havenot yet interacted either with a conscious observer or even with somelump of inanimate matter. He sees the universe as a vast arenacontaining realms where the past is not yet fixed.

Wheeler isthe first to admit that this is a mind-stretching idea. It's not evenreally a theory but more of an intuition about what a final theory ofeverything might be like. It's a tenuous lead, a clue that the mysteryof 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 comeexistence?— can be answered," he says.

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William Wootters, oneof Wheeler's many students and now a professor of physics at WilliamsCollege in Williamstown, Massachusetts, sees Wheeler as an almostoracular figure. "I think asking this question— How come existence?— isa good thing," Wootters says. "Why not see how far you can stretch? Seewhere 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 thesignificance of quantum measurement, how it creates an actuality ofwhat was a mere potentiality. He has come to think of that as theessential building block of reality."

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

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

Ifthere were no such thing as quantum mechanics, the answer would besimple: The cat is either alive or dead, depending on whether aparticle hit the Geiger counter. But in the quantum world, things arenot so straightforward. The particle and the cat now form a quantumsystem consisting of all possible outcomes of the experiment. Oneoutcome includes a dead cat; another, a live one. Neither becomes realuntil someone opens the box and looks inside. With that observation, anentire consistent sequence of events— the particle jettisoned from theuranium, the release of the poison gas, the cat's death— at oncebecomes real, giving the appearance of something that has taken weeksto transpire. Stanford University physicist Andrei Linde believes thisquantum paradox gets to the heart of Wheeler's idea about the nature ofthe universe: The principles of quantum mechanics dictate severe limitson the certainty of our knowledge.

"You may ask whether theuniverse really existed before you start looking at it," he says."That's the same Schrödinger cat question. And my answer would be thatthe universe looks as if it existed before I started looking atit. When you open the cat's box after a week, you're going to findeither a live cat or a smelly piece of meat. You can say that the catlooks 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 itlooks as if it were there 10 billion years ago."

Lindebelieves that Wheeler's intuition of the participatory nature ofreality is probably right. But he differs with Wheeler on one crucialpoint. Linde believes that conscious observers are an essentialcomponent of the universe and cannot be replaced by inanimate objects.

"Theuniverse and the observer exist as a pair," Linde says. "You can saythat 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 humanbeing, I do not know any sense in which I could claim that the universeis here in the absence of observers. We are together, the universe andus. The moment you say that the universe exists without any observers,I cannot make any sense out of that. I cannot imagine a consistenttheory of everything that ignores consciousness. A recording devicecannot play the role of an observer, because who will read what iswritten on this recording device? In order for us to see that somethinghappens, and say to one another that something happens, you need tohave a universe, you need to have a recording device, and you need tohave us. It's not enough for the information to be stored somewhere,completely inaccessible to anybody. It's necessary for somebody to lookat it. You need an observer who looks at the universe. In the absenceof observers, our universe is dead."

Schrödinger's Cat
Erwin Schrödinger, a founding
father of quantum mechanics,
asked what wouldhappen to a
cat locked in a box with a
radioactive element that may or
may nottrigger the release of
poison gas during the experiment.
The short answer: Thecat's fate
is undecided until the moment
someone observes the experiment.

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

Linde is more optimistic.

"You know, if you saythat we're smart enough to figure everything out, that is a veryarrogant thought. If you say that we're not smart enough, that is avery humiliating thought. I come from Russia, where there is a fairytale about two frogs in a can of sour cream. The frogs were drowning inthe cream. There was nothing solid there; they could not jump from thecan. One of the frogs understood there was no hope, and he stoppedbeating the sour cream with his legs. He just died. He drowned in sourcream. The other one did not want to give up. There was absolutely noway it could change anything, but it just kept kicking and kicking andkicking. And then all of a sudden, the sour cream was churned intobutter. Then the frog stood on the butter and jumped out of the can. Soyou look at the sour cream and you think, 'There is no way I can doanything with that.' But sometimes, unexpected things happen.
"I'mhappy that some people who previously thought this question— How comeexistence?— was meaningless did not stop us from asking it. We alllearned from people like John Wheeler, who asks strange questions andgives strange answers. You may agree or disagree with his answers. Butthe very fact that he asks these questions, and suggests someplausible— and implausible— answers, it has shaken these boundaries ofwhat 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?

"Orat least how," he says. "Why is a trickier thing." Wheeler points tothe example of Charles Darwin in the 19th century and how he provided asimple explanation— evolution through natural selection— for whatseemed an utterly intractable problem: how to explain the origin anddiversity of life on Earth. Does Wheeler think that physicists mightone day have a similarly clear understanding of the origin of theuniverse?

"Absolutely," he says. "Absolutely."





Geons, Black Holes & Quantum Foam: A Life in Physicsby John Archibald Wheeler with Kenneth Ford. New York: W. W. Norton& Company, 1998. Also check out Andrei Linde's Web site: http://physics.stanford.edu/linde.