THE quantum world is famous for its weirdness: its particles live in an eerie
world of uncertainties and ghostly multiple existences. We, on the other hand,
are surrounded by robust and solid certainty. It might be handy to be in two
places at once, but we’ll never manage it. Although this separation of the
microscopic and the everyday might seem perfectly natural, in fact it’s anything
but. According to quantum theory, quantum coexistence is infectious: it should
percolate up from the atomic world to ours, and afflict us all. So why doesn’t
it?
This question has baffled some of the greatest minds in physics:
Schrödinger, Einstein, Dirac and Feynman all failed to make sense of it.
But now, some 70 years after quantum theory first upset the apple cart,
salvation may finally be at hand. In the past few years, some daring physicists
have invented an ingenious new twist to the theory that could finally unite the
two worlds.
Erwin Schrödinger, one of the theory’s founders, was the first to point
out that quantum weirdness should invade the classical world. He illustrated the
point with a famous thought experiment, which arranges a direct link from the
quantum world to ours. It works like this. In a box sits a radioactive nucleus,
a gun and a cat. Because it is radioactive, the nucleus can decay and emit a
neutron. Things are arranged so the neutron will trigger the gun to shoot the
cat.
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If the nucleus remains whole, the cat lives, and if it decays, the cat dies.
But being a quantum particle, the nucleus doesn’t have to choose between its two
possible states. Instead, it develops gradually into a strange combination of
both—called a “superposition”. Because of the link, the split existence of
the nucleus infects the cat as well. So if the nucleus stays in its ghostly
superposition of states, the cat stays in a ghastly coexistence between life and
death.
This conclusion follows unavoidably from the theory. But it seems like pure
nonsense. Cats are either alive or dead—there is no in-between. Isn’t this
just proof that something is dreadfully wrong with the theory? Schrödinger
thought so, and so did Einstein, who quipped that “if quantum physics is
correct, then the world is crazy”. But neither could work out how to fix it.
Meanwhile, it was becoming increasingly obvious in the 1930s that quantum theory
worked very well for atoms and molecules. So physicists devised an artificial
solution. They just tacked an extra rule onto the theory to forbid
superpositions in big objects.
This extra rule—known as the “measurement postulate”—says that
the multiple existences of any object will collapse back to a single existence
whenever the object interacts with a “classical measuring device”. That could be
all sorts of things—a photographic plate, the eye of a human being, or any
other big object. In essence, the measurement postulate says that big things
don’t get into superpositions because superpositions collapse whenever they
encounter big things. It’s a policeman, patrolling the border between classical
and quantum worlds, and keeping multiple existences down where they belong.
This artifice is effective for most practical purposes, but it still leaves a
mighty split between the quantum and classical worlds. The postulate clearly
says that there are some things, such as electrons and protons, that act
according to quantum rules, and others, such as photographic plates and
experimenters, that follow classical (non-quantum) rules. There are two separate
domains with their own distinct laws of physics. So much for a unified theory of
the world.
In their desperation to get rid of the ugly split, physicists have invented
countless schemes designed to show that the extra measurement postulate arises
somehow out of the combined action of the more natural rules of quantum theory.
But it simply cannot. The ordinary quantum rules preserve multiple existences,
whereas the measurement postulate destroys them, so trying to wring one from the
other is hopeless. John Bell, the world’s foremost quantum expert until his
death in 1990, likened the effort to a snake trying to swallow itself by the
tail. “It can be done up to a point,” he said. “But it becomes embarrassing for
the spectators even before it becomes uncomfortable for the snake.”
Radical trio
So what is to be done? If quantum theory can’t make sense of the single
existences of ordinary objects, it clearly needs some help. But the problem is
so staggeringly difficult that for many years only a few physicists even tried
to solve it. Then in 1986, three Italian physicists had a brilliant idea. Aware
of the early concerns of Einstein and Schrödinger, Gian-Carlo Ghirardi of
the University of Trieste, Alberto Rimini of the University of Pavia, and Tullio
Weber, also of Trieste, reckoned that the measurement postulate disguised a
deeper problem with the quantum rules themselves. Change these, they thought,
and perhaps you can drop the measurement rule.
In quantum theory, a “particle” does not sit in just one place, but occupies
many places all at once. Its true position is defined by a fuzzy blob called a
“wave function”, which sets out the probability of finding the particle in
various locations. With time, the wave function of any particle spreads out,
bleeding into an expanding volume of space, as the particle’s multiple
existences proliferate.
Ghirardi, Rimini and Weber proposed a subtle change in the quantum rules that
determine how wavefunctions evolve. Suppose, they said, wave functions usually
spread out according to normal quantum rules, but very rarely—once every
100 million years or so—the wavefunction of a single particle collapses
and becomes localised to a tiny region. This change scarcely affects single
particles, but has a huge effect on big things.
A cat or any other object of similar size contains some 1027 particles. And
even though the wave function of any one is likely to take 100 million years to
collapse, there are so many particles that it is overwhelmingly likely that the
wave function of at least one particle will collapse within just 10-12
seconds. What’s more, because the particles in an object interact with one
another, their wavefunctions are entangled. The normal quantum rules then demand
that the collapse in one particle instantaneously triggers a collapse in all the
others. The collapse of one particle’s wave function drags the whole lot into a
definite state.
So in the scheme of Ghirardi, Rimini and Weber, electrons and protons act as
they should, and remain in superpositions for long times, but weird living-dead
cats are—within a mere trillionth of a second—either spared or put
out of their misery. All this follows naturally from the theory, without any
extra rules slapped on. There is no need to divide the world into separate sets
of laws.
This is an impressive achievement. And yet, the GRW theory has some big
problems of its own. After all, it doesn’t begin to explain what would make a
wave function collapse, nor why it should happen only every 100 million
years.
Also, according to Ian Percival, a physicist at Queen Mary and Westfield
College in London, the idea flies in the face of the way nature usually works.
He points out that in virtually all processes in the physical world, changes
over longer time intervals come about by the accumulation of changes over
shorter intervals. But in the GRW scheme, the interruptions on long times that
lead to collapse don’t arise naturally from any processes over shorter times. So
it’s difficult to imagine what might cause them.
Still unpalatable
This makes the GRW scheme almost as unpalatable as the ordinary quantum
theory with its bolted-on measurement rule. But in the past few years, some new
ideas have emerged that show how these problems might be solved. Most notably,
Percival, along with Nicholas Gisin of the University of Geneva, has developed
“quantum state diffusion theory”, which stands the GRW picture on its head.
Percival’s and Gisin’s idea was born of an analogy with an old problem in
physics—Brownian motion. If you peer through a microscope at a dust
particle floating in water, you’ll see that it bounces around erratically,
rather like a ball in a pinball machine. This “Brownian motion” is all down to
molecules. What happens is that in a liquid, the molecules move about violently,
zinging this way and that. A speck of dust endures a constant barrage of such
molecules, and the knocks it receives at their hands cause its erratic
jitter.
A dust particle in the air does much the same thing, but in between molecular
collisions, gravity relentlessly drags it down (see
Diagram). Over
very short periods of time, the irregular, “noisy” part of this motion is most
evident as the dust particle flits to and fro. But over long times, the many
irregular motions add up, and out of the erratic jitter emerges the particle’s
downward drifting motion.
What does this have to do with quantum theory? Percival and Gisin see the
natural and continuous spreading motion of a quantum particle’s wave function as
a kind of drift, albeit of a more abstract kind. In normal quantum theory, this
drift is all there is. But in the GRW scheme, the wave function’s continuous
drift (spreading) is interrupted every 100 million years or so by a sudden,
random event that drives it to collapse again to a small volume. These random
hits are rather like the molecular collisions of Brownian motion, but the GRW
picture doesn’t quite fit the analogy. In the GRW model, random collapse events
tend to be separated by long periods of time, during which a great deal of drift
occurs. But the erratic events in Brownian motion happen very frequently, and
drift emerges as these rapid events accumulate.
To develop a more natural theory, Gisin and Percival suggest that the random
fluctuations happen over very short periods, so that the state of a quantum
system follows a sort of Brownian motion. Over very short periods, the irregular
part of the motion is most important, and the wave function fluctuates
haphazardly. But over longer periods, the fluctuations add up to give a steady
development, and the wave function spreads as expected from normal quantum
theory.
But Percival and Gisin also include another element in their equations which
spell the end for multiple existences. This property of the equations, known as
“nonlinearity”, arms the quantum world against itself. In effect, the
nonlinearities force the different partial existences of an object to struggle
against one another for supremacy, until all but one have been eliminated, and
the wavefunction has collapsed.
Just as in the GRW theory, collapse happens very slowly for single particles,
but very quickly for big ones. It works in much the same way. On average, the
struggle between the partial existences of any single particle takes a very long
time. But because of the random fluctuations it can
sometimes—rarely—happen quickly. Given the huge number of particles
in an ordinary object, it is overwhelmingly likely that at least one of them
will have collapsed back to a single existence in a tiny fraction of a second.
This collapse drags the entire collection of particles with it, so the whole
object reverts to a single existence.
Field in flux
This theory certainly seems to do the trick. But what could be causing the
fluctuations? One intriguing hypothesis is that they reflect irreducible
fluctuations in the very fabric of space-time itself. Tentative attempts by
physicists to build a quantum version of Einstein’s general
relativity—which views gravity as curvature in the geometry of
space-time—suggest that the Universe’s gravitational field should
fluctuate rapidly over distances and times of about 10-35 metres and
10-44 seconds. So it may be that these very fluctuations are popping up
in Percival and Gisin’s theory. If so, it would seem that tangible effects of
quantum gravity are all around us, prohibiting multiple existences in big
objects and keeping Schrödinger’s cat in one piece.
Even more remarkably, Percival and Gisin believe that it may soon be possible
to detect these fluctuations in the laboratory. Not directly, to be sure. But
they should have measurable effects on delicate interference experiments.
Imagine a beam of particles split into partial existences which are sent
along different paths (see
Diagram). According to quantum theory, each particle
is like a clock that oscillates with a characteristic frequency. So the number
of cycles it goes through by the time it gets to the screen depends on how long
it takes to get there. When they arrive, the partial existences interfere with
one another, forming a pattern that depends on small differences in the number
of cycles each clock has gone through.FIG-mg20795001.GIF
But space-time fluctuations along the paths could disturb these
relationships—because the fluctuations should make the clocks speed up or
slow down erratically as they travel. So the clock settings of the two partial
existences at the screen will vary randomly and the expected pattern will be
destroyed.
In 1992, Mark Kasevich and Steven Chu of Stanford University directed two
beams of sodium atoms along different paths some 15 centimetres long, and found
the pattern expected from normal quantum theory. So the fluctuations—if
present—didn’t have noticeable effects. These experiments would be
sensitive enough to detect the fluctuations if they take place in around
10-44 seconds.
But the fluctuations may well be more rapid yet. One way to improve the
sensitivity of the experiments would be to allow the beams of atoms to travel
over longer distances before they interfere with each other. This is trickier,
because external noise would be harder to eliminate. But it would give the
effects of the fluctuations more time to accumulate, and should provide a more
sensitive probe within the next few years.
If the fluctuations are detected, these new theories will undoubtedly
displace ordinary quantum theory. Theoretical physicist Roger Penrose of Oxford
University believes that this would be an important step forward. But he
suspects that a more radical break with the ideas of ordinary quantum theory is
needed. “I think there has to be a very major revolution in the way we look at
quantum mechanics,” he says.
He points out, for example, that any superposition of states necessarily
leads to a superposition of universes with different space-time geometries. And
that spells trouble. Consider the cat when it is suspended between life and
death. In one existence the cat lies dead on the floor. In the other, it prowls
its cage. So the Universe exists in a superposition of states with different
mass distributions.
Using the details of the general theory of relativity, Penrose argues that
this situation would undermine the very notion of energy—rather than being
a well-defined quantity, energy would become vague and uncertain. In such a
world, the crucial principle of the conservation of energy would be in
trouble.
Penrose suggests that a Universe in this dilemma would be unstable, and would
fall naturally into one state or the other, eliminating the superposition. And
he suggests that the decay would be more rapid for superpositions involving more
widely differing distributions of mass—for bigger particles, for instance,
or for objects involving many particles. These ideas would achieve the same
sewing-up of the classical and quantum worlds as Gisin and Percival’s theory,
but would also make a real connection to the theory of gravity. “I am not
proposing a theory,” says Penrose. “I am only saying that this is the kind of
level at which something new has to come in.”
These are exciting times for quantum physics. After 70 years,
Schrödinger’s and Einstein’s worries have finally borne fruit, and the ugly
divide between the quantum and classical worlds looks likely to be bridged.
Whatever the details of the ultimate theory, it may turn out to be both more and
less bizarre than Schrödinger and Einstein suspected.