In ancient Greece,Plato tried to think an talk his way to the truth in extended dialogues with his disciples.Today physicists such as Leonard Mandel of the University of Rochester operate in a somewhat different fashion.He and his students,who are more likely to wear t-shirts and laser proof goggles than robes and sandals,spend countless hours bent over a large metal table trying to align a laser with a complex network of mirrors,lenses, beam splitters and light detectors.
Yet the questions they address in their equipment-jammed laboratory
are no less profound than those contemplated by Plato in his grassy glade.What
are the limits of human knowledge? Is the physical world shaped in some sense
by our perception of it? Is there an element of randomness in the universe,or
are all events predetermined?
Mandel,being inclined toward understatement,offers a more modest
description of his mission."We are trying to understand the implications
of quantum mechanics," he says,"The subject is very old,but we are still
Still,quantum theory has deeply disturbing implications.For
one,it shattered traditional notions of causality.The elegant equation devised
Schrödinger in 1926 to describe
the unfolding of quantum events offered not certainties,as
Newtonian mechanics did,but only an undulating
wave of possibilities. Werner
Heisenberg's uncertainty principle then showed
that our knowledge of nature is fundamentally limited - as soon as we grasp
one part,another part slips through our fingers.
Until recently, the prevailing attitude of most physicists
has been utilitarian: if the theory can foretell the performance of a doped
gallium arsenide semiconductor, why worry about its epistemological implications?
In the past decade or so, however, a growing cadre of researchers has been
probing the ghostly underpinnings of their craft. New technologies, some
based on the very quantum phenomena that they test, have enabled investigators
to carry out experiments Einstein and Bohr could only imagine. These
achievements, in turn,have inspired theorists to dream up even more challenging
- and sometimes bizarre - tests.
The goal of the quantum truth-seekers is not to build faster
computers or communications devices-although that could be an outcome of
the research. And few expect to "disprove" a theory that has been confirmed
in countless experiments. Instead their goal is to lay bare the curious reality
of the quantum realm. "For me, the main purpose of doing experiments is to
show people how strange quantum physics is," says
Anton Zeilinger of the University of
Innsbruck, who is both a theorist and experimentalist ."Most physicists are
very naive; most still believe in real waves or particles."
So far the experiments are confirming Einstein's worst fears.
Photons, neutrons and even whole atoms act sometimes like waves, sometimes
like particles, but they actually have no definite form until they are measured.
Measurements, once made, can also be erased, altering the outcome of an
experiment that has already occurred. A measurement of one quantum entity
can instantaneously influence another far away.This odd behaviour can occur
not only in the microscopic realm but even in objects large enough to be
seen with the naked eye.
But more recent two-slit experiments suggest that
Newton was also right. Modern photodetectors
(which exploit the photoelectric effect explained by Einstein) can show
individual photons plinking against the screen behind the slits in a particular
spot at a particular time-just like particles. But as the photons continue
striking the screen, the interference pattern gradually emerges,a sure sign
that each individual photon went through both slits, like a wave.
Actually, wave-particle duality is even more baffling than this
explanation suggests, as John A. Wheeler of Princeton University demonstrated
with a thought experiment he devised in 1980. " Bohr used to say that if
you aren't confused by quantum physics then you haven't really understood
it," remarks Wheeler who studied under Bohr in the 1930s and went on to become
one of the most adventurous explorers of the quantum world.
Five years after Wheeler outlined what he called the delayed-choice
experiment, it was carried out independently by groups at the University
of Maryland and the University of Munich. They aimed a laser beam not at
a plate with two slits but at a beam splitter, a mirror coated with just
enough silver to reflect half of the photons impinging on it and let the
other half pass through. After diverging at the beam splitter the two beams
were guided back together by mirrors and fed into a detector.
Then the workers installed a customized crystal called a Pockels
Cell in the middle of one route. When an electric current was applied to
the Pockels Cell, it diffracted photons to an auxiliary detector. Otherwise,
photons passed through the cell unhindered. A random signal generator made
it possible to turn the cell on or off after the photon had already passed
the beam splitter but before it reached the detector as Wheeler had specified.
To underscore the weirdness of this effect, Wheeler points
out that astronomers could perform a delayed-choice experiment on light from
quasars, extremely bright, mysterious objects found near the edges of the
universe. In place of a beam splitter and mirrors the experiment requires
a gravitational lens, a galaxy or other massive object that splits the light
from a quasar and refocuses it in the direction of a distant observer, creating
two or more images of the quasar.
Reflecting on quantum mechanics some 60 years ago, the British
physicist Sir Arthur Eddington complained that the theory made as much sense
as Lewis Carroll's
poem "Jabberwocky" in which "slithy toves did gyre and gimble in the wabe."
Unfortunately, the jargon of quantum mechanics is rather less lively. An
unobserved quantum entity is said to exist in a
"coherent superposition" of all the possible
"states" permitted by its "wave function." But as soon as an observer
makes a measurement capable of distinguishing between these states the wave
function "collapses", and the entity is forced into a single state.
Most experiments do in fact involve intrusive measurements.
For example, blocking one path or the other or moving detectors close to
the slits obviously disturbs the photons passage in the two-slit experiment
as does placing a detector along one route of the delayed-choice experiment.
But an experiment done last year by Mandel's team at the University of Rochester
shows that a photon can be forced to switch from wavelike to particlelike
behaviour by something much more subtle than direct intervention.
This design does not permit an observer to tell which way any
single photon went after encountering the beam splitter. Each photon therefore
goes both right and left at the beam splitter, like a wave, and passes through
both down-converters, producing two signal wavelets and two idler wavelets.
The signal wavelets generate an interference pattern at their detector. The
pattern is revealed by gradually lengthening the distance that signals from
one down - converter must go to reach the detector. The rate of detection
then rises and falls as the crests and troughs of the interference wavelets
shift in relation to each other, go in and out of phase.
Now comes the odd part. The signal photons and the idler photons,
once emitted by the down-converters, never again cross paths; they proceed
to their respective detectors independently of each other. Nevertheless,
simply by blocking the path of one set of idler photons, the researchers
destroy the interference pattern of the signal photons. What has changed?
The comparison of arrival times need not actually be performed
to destroy the interference pattern. The mere "threat" of obtaining information
about which way the photon travelled, Mandel explains, forces it to travel
only one route. "The quantum state reflects not only what we know about the
system but what is in principle knowable," Mandel says.
Several groups working with optical interferometry, including
Mandel's, claim to have demonstrated what Scully has dubbed a "quantum eraser."
The group that has come closest, according to Scully, is one led by Raymond
Y. Chiao of the University of California at Berkeley.
Then the workers added a device to the interferometer that shifted
the polarization of one set of photons by 90 degrees- If one thinks of a
ray of light as an arrow, polarization is the orientation of the plane of
the arrowhead. One of the peculiarities of polarization is that it is a strictly
binary property; photons are always polarized either vertically or
horizontally.The altered polarization served as a tag; by putting polarization
detectors in front of the simple light detectors at the end of the routes,
one could determine which route each photon had taken. The two paths were
no longer indistinguishable, and so the interference pattern disappeared.
Such possibilities provoke consternation in some quarters. Edwin
T. Jaynes of Washington University, a prominent theorist whose work helped
to inspire Scully to conceive the quantum eraser, has nonetheless dubbed
it "medieval necromancy." Scully was so pleased
by Jaynes's remark that he included it in a recent article on the quantum
But because of their common origin, the properties of the protons
are tightly correlated, or "entangled." For example, through simple conservation
of momentum, one knows that if one proton heads north, the other must have
headed south. Consequently, measuring the momentum of one proton instantaneously
determines the momentum of the other proton- even if it has travelled to
the opposite end of the universe. Einstein said that this
"spooky action at a distance" was incompatible with
any "realistic" model of reality; all the properties of each proton must
be fixed from the moment they first fly apart.
Until the early 1960s, most physicists considered the issue
entirely academic, since no one could imagine how to resolve it experimentally.
Then, in 1964, John S. Bell of CERN, the European laboratory for particle
physics, showed that quantum mechanics predicted stronger statistical
correlations between entangled particles than the so-called local realistic
theory that Einstein preferred. Bell's papers triggered a flurry of laboratory
work culminating in a classic (but not classical) experiment performed a
decade ago by Alain Aspect of the University of Paris.
Instead of the momentum of protons, Aspect analysed the
polarization of pairs of photons emitted by a single source toward separate
detectors. Measured independently, the polarization of each set of photons
fluctuated in a seemingly random way. But when the two sets of measurements
were compared, they displayed an agreement stronger than could be accounted
for by any local realistic theory-just as Bell had predicted. Einstein's
spooky action at a distance was real.
The experiment began with a laser firing into a down-converter
which produced pairs of correlated photons. Each of these photons then passed
through a separate two-slit apparatus and thence to a photon detector.Through
conservation of momentum, one could determine the route of each photon if
one knew the route of its partner. But the arrangement of mirrors and bean
splitters made it impossible to determine the actual route of either
Next, the workers slightly lengthened one of the four routes,
as Chiao did in his quantum eraser experiment. Although the rate at which
photons struck each detector did not change, the rate of simultaneous firings
recorded by a coincidence counter oscillated, forming a telltale interference
pattern like the one observed by Chiao. Such a pattern could occur only if
each photon, the one on the left and the one on the right, was passing through
both slits to its respective detector, its momentum fundamentally undefined
and yet still correlated with the momentum of its distant partner.
Still more ambitious EPR experiments have been proposed but
not yet carried out. Greenberger, Zeilinger and Michael Home of Stonehill
College have shown that three or more particles sprung from a single source
will exhibit much stronger nonlocal correlations than those between just
two particles. Bernard Yurke and David Stoler of AT& T Bell Laboratories
have even suggested a way in which three particles emitted from separate
locations can exhibit the EPR effect.
A die-hard realist might dismiss the experiments described above,
since they all involve that quintessence of ineffability, light But electrons,
neutrons, protons and even whole atoms-the stuff our own bodies are made
of-also display pathological behaviour. Researchers observed wavelike behaviour
in electrons through indirect means as early as the 1920s, and they began
carrying out two- slit experiments with electrons several decades ago.
Since the mid-l970s various workers have done interference
experiments with neutrons, which are almost 2,000 times heavier than electrons.
Some 15 years ago, for example, Samuel A. Werner of the University of Missouri
at Columbia and others found that the interference pattern formed by neutrons
diffracted along two paths by a sculpted silicon crystal could be altered
simply by changing the interferometer' s orientation relative to the earth's
gravitational field. It was the first demonstration that the Schrödinger
equation holds true under the sway of gravity.
Pritchard says physicists may one day be able to pass biologically
significant molecules such as proteins or nucleic acids through an
interferometer.In principle, one could even observe wavelike behaviour in
a whole organism, such as an amoeba. There are some obstacles,though: the
amoeba would have to travel very slowly, so slowly, in fact, that it would
take some three years to get through the interferometer, according to Pritchard.
The experiment would also have to be conducted in an environment completely
free of gravitational or other influences-that is, in outer space.
While physicists may never nudge a philosopher into a superposition
of states, they are hard at work trying to induce wavelike behaviour in objects
literally large enough to see. The research has rekindled interest in a famous
thought experiment posed by Schrödinger in 1935. In a version altered
by John Bell, the EPR theorist, to be more palatable to animal lovers, a
cat is placed in a box containing a lump of radioactive matter, which has
a 50 per - cent chance of emitting a particle in a one-hour period. When
the particle decays, it triggers a Geiger counter, which in turn causes a
flask of milk to pour into a bowl, feeding the cat. (In Schrödinger
's version, a hammer smashes a flask of poison gas, killing the cat.)
Common sense dictates that a cat cannot have a stomach both
empty and full. But quantum mechanics dictates that after one hour, if no
one has looked in the box, the radioactive lump and so the cat exist in a
superposition of indistinguishable states; the former is both decayed and
undecayed, and the latter is both hungry and full.
But since the early 1980s Anthony J.Leggett, a theorist at the
University of Illinois, has argued that one should be able to observe a
superconducting quantum interference device, more commonly called a SQUID,
in a superposition of states. A SQUID, which is typically the size of a pinhead
and therefore huge in comparison with atoms or other quantum objects, consists
of a loop of superconducting material, through which electrons flow without
resistance, broken by a thin slice of insulating material called a Josephson
junction. In a classical world the electrons would be completely blocked
by the insulator, but the quantum indefiniteness of the electrons' positions
allows hordes of them to "tunnel" blithely through the gap.
Inspired by Leggett's calculations, Claudia D. Tesche of the
IBM Watson center proposed an experiment that could show the superposition
quite directly. Given certain conditions, Tesche notes, the current in a
SQUID has an equal chance of flowing in either direction. According to quantum
mechanics, then, it should flow both ways, creating an interference pattern
analogous to the one formed in a two-slit experiment.
Orthodoxy under Attack
All the recent experiments, completed and proposed, have hardly
led to a consensus on what exactly quantum mechanics means. If only by default,
the "orthodox" view of quantum mechanics is still the one set forth in the
1920s by Bohr. Called the Copenhagen interpretation, its basic assertion
is that what we observe is all we can know; any speculation about what a
photon, an atom or even a SQUID "really is" or what it is doing when we're
not looking is just that-speculation.
Bell's exhortations helped to revive interest in a realistic
theory originally proposed in the 1950s by Bohm.In Bohm's view, a quantum
entity such as an electron does in fact exist in a particular place at a
particular time, but its behaviour is governed by an unusual field, or pilot
wave, whose properties are defined by the Schrödinger wave function.The
hypothesis does allow one quantum quirk, nonlocality, but it eliminates another,
the indefiniteness of position of a particle. Its predictions are identical
to those of standard quantum mechanics.
Unlike Bohm's pilot-wave concept, the theory of Ghirardi's group
offers predictions that diverge from those of orthodox quantum physics, albeit
subtly. " If you shine a neutron through two slits, you get an interference
pattern," Pearle says. " But if our theory is correct, the interference should
disappear if you make the measurement far enough away." The theory also requires
slight violations of the law of conservation of energy. Zeilinger of the
University of Innsbruck was sufficiently interested in the theory to test
the neutron prediction, which was not borne out. "This approach is one of
those dead end roads that has to be walked by someone," he sighs.
Yet another view currently enjoying some attention, although
not as a result of Bell's efforts, is the many-worlds interpretation, which
was invented in the 1950s by Hugh Everett III of Princeton.The theory sought
to answer the question of why, when we observe a quantum phenomenon, we see
only one outcome of the many allowed by its wave function. Everett proposed
that whenever a measurement forces a particle to make a choice, for instance,
between going left or right in a two-slit apparatus, the entire universe
splits into two separate universes; the particle goes left in one universe
and right in the other.
Although the theory was long dismissed as more science fiction
than science, it has been revived in a modified form by
of the California Institute of Technology and James B. Hartle of the
University of California at Santa Barbara.They call their version the
many-histories interpretation and emphasize that the histories are
"potentialities" rather than physical actualities. Gell-Mann has reportedly
predicted that this view will dominate the field by the end of the century.
The It from Bit
A different kind of paradigm shift is envisioned by Wheeler.The
most profound lesson of quantum mechanics, he remarks, is that physical phenomena
are somehow defined by the questions we ask of them. " This is in some sense
a participatory universe," he says. The basis of reality may not be the quantum,
which despite its elusiveness is still a physical phenomenon, but the bit,
the answer to a yes-or-no question,which is the fundamental currency of computing
and communications. Wheeler calls his idea "the it from bit."
Meanwhile theorists at the surreal frontier of quantum theory
are conjuring up thought experiments that could unveil the riddle in the
enigma once and for all. David Deutsch
of the University of Oxford thinks it should be possible,
at least in principle, to build a "quantum computer,"
one that achieves superposition of states. Deutsch has shown that if different
superposed states of the computer can work on separate parts of a problem
at the same time, the computer may achieve a kind of quantum parallelism,
solving certain problems more quickly than classical computers.
Taking this idea further, Albert - with just one of his minds
- has conceived of a quantum computer capable of making certain measurements
of itself and its environment. Such a "quantum automaton" would be capable
of knowing more about itself than any outside observer could ever know-and
even more than is ordinarily permitted by the uncertainty principle.The automaton
could also serve as a kind of eyewitness of the quantum world, resolving
questions about whether wave functions truly collapse, for example. Albert
says he has no idea how actually to engineer such a machine, but his calculations
show the Schrödinger equation allows such a possibility.
If that doesn't work, there is always Aharonov's time machine.The
machine, which is based not only on quantum theory but also on general
relativity, is a massive sphere that can rapidly expand or contract Einstein's
theory predicts that time will speed up for an occupant of the sphere as
it expands and gravity becomes proportionately weaker, and time will slow
down as the sphere contracts. If the machine and its occupant can be induced
into a superposition of states corresponding to different sizes and so different
rates of time, Aharonov says, they may "tunnel" into the future.The occupant
can then disembark, ask physicists of the future to explain the mysteries
of quantum mechanics and then bring the answers-assuming there are any-back
to the present. Until then, like Plato's benighted cave dwellers, we can
only stare at the shadows of quanta flickering on the walls of our cave and
wonder what they mean.