Crossing the Quantum Frontier
Time Tunnel

There is a mysterious boundary between the familiar predictability of ordinary objects and the spooky uncertainties of the quantum world. Now physicists are on the verge of discovering what happens there, says Mark Buchanan

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.

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.

Quantum Coexistence is infectious:it should afflict us all.
So why doesn't it?

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 wave functions 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 wave- function 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 wave- functions 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.

Apples 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.

Brownian Motion DiagramA dust particle in the air does much the same thing, but in between molecular collisions, gravity relentlessly drags it down (see Diagram, opposite). 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 wave- function 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.
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.

The effects of quantum gravity are all around us,keeping Shrödinger's cat in one piece

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 fluctuations-if 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. []




Chaos Quantum Logic Cosmos Conscious Belief Elect. Art Chem. Maths

New Scientist 26 Apr 1997 File Info: Created Updated 22/8/2006 Page Address: