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