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 resident discusses 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 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
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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 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. 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 Geiger counter
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 a
wall." 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."
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