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 the other. 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.

universe_3.jpgBirthday 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 to illustrate
that our universe may have been
paired with another when it was
born. Wheeler, with glass in
hand (bottom),chats with Ravi
Ravindra, a professor emeritus
of comparative religion at
Dalhousie University in Nova Scotia.
Photographs by Brian Finke universe_4.jpg


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.