[tt] the reality tests

[Eugen Leitl]

http://www.seedmagazine.com/news/2008/06/the_reality_tests_1.php?page=all&p=y

A team of physicists in Vienna has devised experiments that may answer one of
the enduring riddles of science: Do we create the world just by looking at
it?

by Joshua Roebke • Posted June 4, 2008 11:10 AM

Anton Zeilinger heads up the IQOQI lab in Vienna. Photograph by Mark Mahaney.

To enter the somewhat formidable Neo-Renaissance building at Boltzmanngasse 3
in Vienna, you must pass through a small door sawed from the original
cathedrallike entrance. When I first visited this past March, it was chilly
and overcast in the late afternoon. Atop several tall stories of scaffolding
there were two men who would hardly have been visible from the street were it
not for their sunrise-orange jumpsuits. As I was about to pass through the
nested entrance, I heard a sudden rush of wind and felt a mist of winter
drizzle. I glanced up. The veiled workers were power-washing away the
building's façade, down to the century-old brick underneath.

In 1908 Karl Kupelwieser, Ludwig Wittgenstein's uncle, donated the money to
construct this building and turn Austria- Hungary into the principal
destination for the study of radium. Above the doorway the edifice still
bears the name of this founding purpose. But since 2005 this has been home of
the Institut für Quantenoptik und Quanteninformation (IQOQI, pronounced
"ee-ko-kee"), a center devoted to the foundations of quantum mechanics. The
IQOQI, which includes a sister facility to the southwest in the valley town
of Innsbruck, was initially realized in 2003 at the behest of the Austrian
Academy of Sciences. However, the institute's conception several years
earlier was predominantly due to one man: Anton Zeilinger. This past January,
Zeilinger became the first ever recipient of the Isaac Newton Medal for his
pioneering contributions to physics as the head of one of the most successful
quantum optics groups in the world. Over the past two decades, he and his
colleagues have done as much as anyone else to test quantum mechanics. And
since its inception more than 80 years ago, quantum mechanics has possibly
weathered more scrutiny than any theory ever devised. Quantum mechanics
appears correct, and now Zeilinger and his group have started experimenting
with what the theory means.

Some physicists still find quantum mechanics unpalatable, if not
unbelievable, because of what it implies about the world beyond our senses.
The theory's mathematics is simple enough to be taught to undergraduates, but
the physical implications of that mathematics give rise to deep philosophical
questions that remain unresolved. Quantum mechanics fundamentally concerns
the way in which we observers connect to the universe we observe. The theory
implies that when we measure particles and atoms, at least one of two
long-held physical principles is untenable: Distant events do not affect one
other, and properties we wish to observe exist before our measurements. One
of these, locality or realism, must be fundamentally incorrect.

For more than 70 years, innumerable physicists have tried to disentangle the
meaning of quantum mechanics through debate. Now Zeilinger and his
collaborators have performed a series of experiments that, while neatly
agreeing with the theory's predictions, are reinvigorating these historical
dialogues. In Vienna experiments are testing whether quantum mechanics
permits a fundamental physical reality. A new way of understanding an already
powerful theory is beginning to take shape, one that could change the way we
understand the world around us. Do we create what we observe through the act
of our observations?

Most of us would agree that there exists a world outside our minds. At the
classical level of our perceptions, this belief is almost certainly correct.
If your couch is blue, you will observe it as such whether drunk, in high
spirits, or depressed; the color is surely independent of the majority of
your mental states. If you discovered your couch were suddenly red, you could
be sure there was a cause. The classical world is real, and not only in your
head. Solipsism hasn't really been a viable philosophical doctrine for
decades, if not centuries.

But none of us perceives the world as it exists fundamentally. We do not
observe the tiniest bits of matter, nor the forces that move them,
individually through our senses. We evolved to experience the world in bulk,
our faculties registering the net effect of trillions upon trillions of
particles or atoms moving in concert. We are crude measurers. So divorced are
we from the activity beneath our experience that physicists became relatively
assured of the existence of atoms only about a century ago.

Physicists attribute a fundamental reality to what they do not directly
perceive. Particles and atoms have observable effects that are well described
by theories like quantum mechanics. Single atoms have been "seen" in
measurements and presumably exist whether or not we observe them
individually. The properties that define particles—mass, spin, etc.—are also
thought to exist before we measure them. In physics this is how reality is
defined; particles and atoms have measurable properties that exist prior to
measurement. This is nothing stranger than your blue couch.

As a physical example, light consists of particles known as photons that each
have a property called polarization. Measuring polarization is usually
something like telling time; the property can be thought of like the
direction of a second hand on a clock. For unpolarized light, the second hand
can face any direction as with a normal clock; for polarized light the hand
will face in only one or a few directions, as if the clock were broken. That
photons can be polarized is, in fact, what allows some sunglasses to
eliminate glare—the glasses block certain polarizations and let others
through. In Vienna the polarization of light is also being used to test
reality.

For a few months in 2006, Simon Gröblacher, who had started his PhD not long
before, spent his Saturdays testing realism. Time in the labs at the IQOQI is
precious, and during the week other experiments with priority were already
underway. Zeilinger and the rest of their collaborators weren't too worried
that this kind of experiment would get scooped. They were content to let
Gröblacher test reality in the lab's spare time.

It was after 2 pm when I first met Gröblacher, and he had just woken up; they
are installing an elevator in his lab and so he works nights. He had told me
to come to the top floor of the IQOQI building to find him. I made my way up
the broad granite steps, and on the final landing I heard shouts from a
half-open door. There was a raucous game of foosball in the lounge. When
Gröblacher saw me, someone else grabbed the handles.

The lab where Gröblacher performed the first experiment on realism is on the
second floor of the Universität Wien physics department, which connects to
the IQOQI through a third-floor bridge. The original experiment has given way
to another, but, Gröblacher tells me, the setup looks roughly the same.

In the middle of the cramped space is a floating metal surface, about the
size of a banquet table, latticed with drill holes. A forest of black optical
equipment, like monocles atop tiny poles, seems to grow out of the table.
Beam splitters resemble exact, glass die. In the center is an encased crystal
that is not visible, and on the ends sit idle lasers.

Gröblacher walked me through the tabletop obstacle course: The laser light
passes through a series of polarizers and filters, hits the crystal, and
splits into two beams of single-file photons. Detectors in both beams measure
the polarization of each photon, which are related to one another. The data
is tested against two theories: one that preserved realism but allowed
strange effects from anywhere out there in the universe, and quantum
mechanics.

The whole experiment would fit snugly in a child's bedroom, and as I looked
at the table, I refrained from asking my first instinctual questions. "This
is it? This is where you tested realism?" I already knew how unfair these
questions were. It had taken a few months of tests, and almost two
yequantum's other leading progenitors, Niels Bohr and Albert Einstein, heard
about Heisenberg's completion of the work they began, their reactions were
almost immediate; Bohr was impressed, Einstein was not. Heisenberg's theory
emphasized the discrete, particle-like nature of matter, and Einstein, who
tended to think in images, could not picture it in his head.

In Switzerland, Erwin Schrödinger had also been "repelled" by Heisenberg's
theory. In the fall of 1925, Schrödinger was 38 years old and rife with
self-doubt, but when Einstein sent him an article describing a possible
duality between particles and waves, Schrödinger had an idea. Over a period
of six months, he published five papers outlining a wave theory of the atom.
Though it proved difficult to physically interpret what his wave was, the
theory felt familiar to Schrödinger. Heisenberg, who had moved to Copenhagen
to become Bohr's assistant, thought the theory "disgusting."

Schrödinger and Heisenberg independently uncovered dual descriptions of
particles and atoms. Later, the theories proved equivalent. Then in 1926
Heisenberg's previous advisor, Max Born, discovered why no one had found a
physical interpretation for Schrödinger's wave function. They are not
physical waves at all; rather the wave function includes all the possible
states of a system. Before a measurement those states exist in superposition,
wherein every possible outcome is described at the same time. Superposition
is one of the defining qualities of quantum mechanics and implies that
individual events cannot be predicted; only the probability of an
experimental outcome can be derived.

The following year, in 1927, Heisenberg discovered the uncertainty principle,
which placed a fundamental limit on certain measurements. Pairs of specific
quantities are incompatible observables; momentum and position, energy and
time, and other measurable pairs cannot be known together with absolute
accuracy. Measuring one restricts knowledge of the other. With this quantum
mechanics had become a full theaffects what it is used to observe. The
quantum world is discrete and so there can never be absolute precision during
a measurement. To know about quantum mechanics, we rely on classical devices.
To Bohr this implied that the hierarchy between observer and observed had no
meaning; they were nonseparable. Concepts once thought to be mutually
exclusive, such as waves and particles, were also complements. The difference
was only language.

By contrast Einstein was a realist who believed in a world independent of the
way it is measured. During a set of conferences at the Hotel Metropole in
Brussels, he and Bohr argued famously over the validity of quantum mechanics
and Einstein presented a number of thought experiments intended to show the
theory incorrect. But when Bohr used Einstein's own theory of relativity to
evade one of these thought experiments, Einstein was so stung he never tried
to disprove quantum mechanics again, though he continued to criticize it.

In 1935, from an idyllic corner of New Jersey, Einstein and two young
collaborators began a different assault on quantum mechanics. Einstein,
Podolsky, and Rosen (EPR) did not question the theory's correctness, but
rather its completeness. More than the notion that god might play dice, what
most bothered Einstein were quantum mechanics' implications for reality. As
Einstein prosaically inquired once of a walking companion, "Do you really
believe that the moon exists only when you look at it?"

The EPR paper begins by asserting that there's a real world outside theories.
"Any serious consideration of a physical theory must take into account the
distinction between the objective reality, which is independent of any
theory, and the physical concepts with which the theory operates." If quantum
mechanics is complete, then "every element of physical reality must have a
counterpart in the physical theory." EPR argued that objects must have
preexisting values for measurable quantities and that this implied that
certain elements of reality could not be determined by quantum mechanics.

Einstein and his colleagues imagined two electrons that collide and fly
apart. After the collision the electrons exist in a state of supe value of
the other's is instantly known and the superpositions collapse. Once the
momentum is known for a particle, we cannot measure its position. This
element of reality is denied us by the uncertainty principle. Even stranger
is that this occurs even when the electrons fly vast distances apart before
measurement. Quantum mechanics still describes the electrons as a single
system across space. Einstein could never stomach that an experiment at one
electron would instantaneously affect the other.

In Copenhagen Bohr began an immediate response. It didn't matter if particles
might affect one another over vast distances, or that particles had no
observable properties before they are observed. As Bohr later said, "There is
no quantum world. There is only an abstract quantum physical description."
Physicists' discourse on reality began just as the world slid inexorably
toward war. During WWII physicists once interested in philosophy worried
about other issues. David Bohm, however, did worry. After the war Bohm was a
professor at Princeton, where he wrote a famous textbook on quantum
mechanics. Einstein thought it was the best presentation of quantum mechanics
he had read, and when Bohm began to challenge the theory, Einstein said, "If
anyone can do it, then it will be Bohm."

In 1952, during the Red Scare, Bohm moved to Brazil. There he discovered a
theory in which a particle's position was determined by a "hidden variable"
even when its momentum was absolutely known. To Bohm reality was important,
and so to preserve it, he was willing to abandon locality and accept that
entangled particles influenced one another over vast distances. However,
Bohm's hidden variables theory made the same predictions as quantum
mechanics, which already worked.

In America Bohm's theory was ignored. But when the Irishman John Bell read
Bohm's idea, he said, "I saw the impossible done." Bell thought hidden
variables might show quantum mechanics incomplete. Starting from Bohm's work,
Bell derived another kind of hidden variables theory that could make
predictions different from those of quantum mechanics. The theories could be
tested against one another in an EPR-type experiment. But Bell made two
assumptions that quantum mechanics does not; the world is local (no distant
influences) and real (preexisting properties). If quantum mechanics were
correct, one or both of these assumptions were false, though Bell's theorem
could not determine which.

Bell's work on local hidden variables theory stirred little interest until
the 1970s, when groups lead by John Clauser, Abner Shimony, and others
devised experimental schemes in which the idea could be tested with light's
polarizations instead of electrons' momentum. Then in 1982 a young Frenchman
named Alain Aspect performed a rigorous test of Bell's theory on which most
physicists finally agreed. Quantum mechanics was correct, and either locality
or realism was fundamentally wrong.

During the 1980s and 1990s, the foundations of quantum mechanics slowly
returned to vogue. The theory had been shown, with high certainty, to be
true, though loopholes in experiments still left some small hope for
disbelievers. However, even to believers, nagging questions remained: Was the
problem with quantum mechanics locality, realism, or both? Could the two be
tested?

in may of 2004 Markus Aspelmeyer met Anthony Leggett during a conference at
the Outing Lodge in Minnesota. Leggett, who had won the Nobel Prize the year
before, approached Aspelmeyer, who had recently become a research assistant
to Zeilinger, about testing an idea he first had almost 30 years before.

In 1976 Leggett left Sussex on teaching exchange to the University of Science
and Technology in Kumasi, the second largest city in Ghana. For the first
time in many years, he had free time to really think, but the university's
library was woefully out of date. Leggett decided to work on an idea that
didn't require literature because few had thought about it since David Bohm:
nonlocal hidden variables theories. He found a result, filed the paper in a
drawer, and didn't think about it again until the early 2000s.

Leggett doesn't believe quantum mechanics is correct, and there are few
places for a person of such disbelief to now turn. But Leggett decided to
find out what believing in quantum mechanics might require. He worked out
what would happen if one took the idea of nonlocality in quantum mechanics
seriously, by allowing for just about any possible outside influences on a
detector set to register polarizations of light. Any unknown event might
change what is measured. The only assumption Leggett made was that a natural
form of realism hold true; photons should have measurable polarizations that
exist before they are measured. With this he laboriously derived a new set of
hidden variables theorems and inequalities as Bell once had. But whereas
Bell's work could not distinguish between realism and locality, Leggett's
did. The two could be tested.

When Aspelmeyer returned to Vienna, he grabbed the nearest theorist he could
find, Tomasz Paterek, whom everyone calls "Tomek." Tomek was at the IQOQI on
fellowship from his native Poland and together, they enlisted Simon
Gröblacher, Aspelmeyer's student. With Leggett's assistance, the three spent
six months painfully checking his calculations. They even found a small
error. Then they set about recasting the idea, with a few of the other
resident theorists, into a form they could test. When they were done, they
went to visit Anton Zeilinger. The experiment wouldn't be too difficult, but
understanding it would. It took them months to reach their tentative
conclusion: If quantum mechanics described the data, then the lights'
polarizations didn't exist before being measured. Realism in quantum
mechanics would be untenable.

Anton Zeilinger stands in front of the door to his office. To his left is a
glass cabinet that holds the numerous medals he has won for tests of quantum
mechanics. Photograph by Mark Mahaney.

On my final morning in vienna, snow was tumbling like dryer sheets as I
stared out the window of the IQOQI waiting to speak again with Zeilinger.
Suddenly, there was a great flash of lightning and a long roll of thunder as
snow continued to fall. I turned around to no one and Zeilinger's assistant
appeared. He now had time to talk.

Though less robust and more intimidating, Zeilinger bears a slight
resemblance to the American Kris Kringle. Born in 1945, he is tall and stout
with a beard and white mane of hair. He wears tailored jackets, though
insists he is a hands-on kind of guy.

As a student in Vienna in the 1960s, Zeilinger never attended a single course
in quantum mechanics, which may help to explain the way he has investigated
it since—with the zeal of a late convert. In the past decade or so, Zeilinger
and his many collaborators were the first to teleport light, use quantum
cryptography for a bank transaction (with optical fibers in the sewers of
Vienna), realize a one-way quantum computer, and achieve entanglement over
large distances through the air, first across the Danube River and then
between two of the Canary Islands. Zeilinger's work had also previously shown
the greatest distinction between quantum mechanics and local realism.

Zeilinger's office is large and sparsely decorated. A few books lean on a
lengthy, glass-fronted bookshelf. As he spoke, Zeilinger reclined in a black
chair, and I leaned forward on a red couch. "Quantum mechanics is very
fundamental, probably even more fundamental than we appreciate," he said,
"But to give up on realism altogether is certainly wrong. Going back to
Einstein, to give up realism about the moon, that's ridiculous. But on the
quantum level we do have to give up realism."

With eerie precision, the results of Gröblacher's weekend experiments had
followed the curve predicted by quantum mechanics. The data defied the
predictions of Leggett's model by three orders of magnitude. Though they
could never observe it, the polarizations truly did not exist before being
measured. For so fundamental a result, Zeilinger and his group needed to test
quantum mechanics again. In a room atop the IQOQI building, another PhD
student, Alessandro Fedrizzi, recreated the experiment using a laser found in
a Blu-ray disk player.

Leggett's theory was more powerful than Bell's because it required that
light's polarization be measured not just like the second hand on a clock
face, but over an entire sphere. In essence, there were an infinite number of
clock faces on which the second hand could point. For the experimenters this
meant that they had to account for an infinite number of possible measurement
settings. So Zeilinger's group rederived Leggett's theory for a finite number
of measurements. There were certain directions the polarization would more
likely face in quantum mechanics. This test was more stringent. In mid-2007
Fedrizzi found that the new realism model was violated by 80 orders of
magnitude; the group was even more assured that quantum mechanics was
correct.

Leggett agrees with Zeilinger that realism is wrong in quantum mechanics, but
when I asked him whether he now believes in the theory, he answered only "no"
before demurring, "I'm in a small minority with that point of view and I
wouldn't stake my life on it." For Leggett there are still enough loopholes
to disbelieve. I asked him what could finally change his mind about quantum
mechanics. Without hesitation, he said sending humans into space as detectors
to test the theory. In space there is enough distance to exclude
communication between the detectors (humans), and the lack of other particles
should allow most entangled photons to reach the detectors unimpeded. Plus,
each person can decide independently which photon polarizations to measure.
If Leggett's model were contradicted in space, he might believe. When I
mentioned this to Prof. Zeilinger he saidquat box of Chinese plum wine on his
desk facing Markus's. When I asked about the wine, thinking it the theorists'
complementary tradition, he laughed and said he just needed a counterbalance.
Brukner has an easy manner and has been with Zeilinger's group almost
continuously since arriving in Austria in 1991 after leaving then Yugoslavia.

Last year Brukner and his student Johannes Kofler decided to figure out why
we do not perceive the quantum phenomena around us. If quantum mechanics
holds universally for atoms, why do we not see directly its effects in bulk?

Most physicists believe that quantum effects get washed out when there are a
large number of particles around. The particles are in constant interaction
and their environment serves to "decohere" the quantum world—eliminate
superpositions—to create the classical one we observe. Quantum mechanics has
within it its own demise, and the process is too rapid to ever see.
Zeilinger's group, which has tested decoherence, does not believe there is a
fundamental limit on the size of an object to observe superposition.
Superpositions should exist even for objects we see, similar to the infamous
example of Schrödinger's cat. In fact, Gröblacher now spends his nights
testing larger-scale quantum mechanics in which a small mirror is humanely
substituted for a cat.

Brukner and Kofler had a simple idea. They wanted to find out what would
happen if they assumed that a reality similar to the one we experience is
true—every large object has only one value for each measurable property that
does not change. In other words, you know your couch is blue, and you don't
expect to be able to alter it just by looking. This form of realism,
"macrorealism," was first posited by Leggett in the 1980s.

Late last year Brukner and Kofler showed that it does not matter how many
particles are around, or how large an object is, quantum mechanics always
holds true. The reason we see our world as we do is because of what we use to
observe it. The human body is a just barely adequate measuring
deviimplications of all their work for reality and our world. Like others in
their field, they had focused on entanglement and decoherence to construct
our future information technology, such as quantum computers, and not for
understanding reality. But the group's work on these kinds of applications
pushed up against quantum mechanics' foundations. To repeat a famous dictum,
"All information is physical." How we get information from our world depends
on how it is encoded. Quantum mechanics encodes information, and how we
obtain this through measurement is how we study and construct our world.

I asked Dr. Zeilinger about this as I was about to leave his office. "In the
history of physics, we have learned that there are distinctions that we
really should not make, such as between space and time... It could very well
be that the distinction we make between information and reality is wrong.
This is not saying that everything is just information. But it is saying that
we need a new concept that encompasses or includes both." Zeilinger smiled as
he finished: "I throw this out as a challenge to our philosophy friends."

A few weeks later I was looking around on the IQOQI website when I noticed a
job posting for a one-year fellowship at the institute. They were looking for
a philosopher to collaborate with the group.