AT ITS deepest level, nature is random and unpredictable. That, most physicists would say, is the unavoidable lesson of quantum
theory. Try to track the location of an electron and you'll find only a probability that it is here or there. Measure the spin of an
atom and all you get is a 50:50 chance that it is up or down. Watch a photon hit a glass plate and it will either pass through or be
reflected, but it's impossible to know which without measuring it.
Where does this randomness come from? Before quantum theory, physicists could believe in determinism, the idea of a world unfolding
with precise mathematical certainty. Since then, however, the weird probabilistic behaviour of the quantum world has rudely
intruded, and the mainstream view is that this uncertainty is a fundamental feature of everything from alpha particles to Z bosons.
Indeed, most quantum researchers celebrate the notion that pure chance lies at the foundations of the universe.
However, a sizeable minority of physicists have long been pushing entirely the opposite view. They remain unconvinced that quantum
theory depends on pure chance, and they shun the philosophical contortions of quantum weirdness. The world is not inherently random,
they say, it only appears that way. Their response has been to develop quantum models that are deterministic, and that describe a
world that has "objective" properties, whether or not we measure them. The problem is that such models have had flaws that many
physicists consider fatal, such as inconsistencies with established theories.
Until now, that is. A series of recent papers show that the idea of a deterministic and objective universe is alive and kicking. At
the very least, the notion that quantum theory put the nail in the coffin of determinism has been wildly overstated, says physicist
Sheldon Goldstein of Rutgers University in New Jersey. He and a cadre of like-minded physicists have been pursuing an alternative
quantum theory known as Bohmian mechanics, in which particles follow precise trajectories or paths through space and time, and the
future is perfectly predictable from the past. "It's a reformulation of quantum theory that is not at all congenial to supposedly
deep quantum philosophy," says Goldstein. "It's precise and objective - and deterministic."
If these researchers can convince their peers, most of whom remain sceptical, it would be a big step towards rebuilding the universe
as Einstein wanted, one in which "God does not play dice". It could also trigger a search for evidence of physics beyond quantum
theory, paving the way for a better and more intuitive theory of how the universe works. Nearly a century after the discovery of
quantum weirdness, it seems determinism may be back.
The debate over quantum theory and determinism started in the 1920s, when physicists Niels Bohr and Werner Heisenberg suggested that
the unpredictability of quantum phenomena reflected an inherent fuzziness in nature. Einstein and others countered that the
unpredictability might instead reflect nothing more than a lack of adequate knowledge. In principle you could predict the outcome of
a coin flip, they argued, if you had perfect knowledge of the coin's initial state and surroundings.
At the historic 1927 Solvay meeting in Brussels, physicist Louis de Broglie tried to further this idea, showing how quantum
randomness might arise in a non-mysterious way. He suggested that quantum particles show wave-like phenomena because they are
accompanied by "pilot waves" that influence their motion in just the right way as to make them obey the Schrödinger wave equation, a
cornerstone of quantum theory. However, most dismissed de Broglie's ideas, citing in particular shortcomings pointed out by the
physicist Wolfgang Pauli.
Yet de Broglie's ideas would not go quietly. In the early 1950s, physicist David Bohm developed a more consistent version of the
pilot-wave model, one based on the same equations as ordinary quantum theory but offering a different interpretation of them. Bohm
found buried within those equations a close link to the mathematics of classical physics, which is based on Newton's laws of motion.
Bohmian mechanics asserts that the outcome of an experiment isn't truly random, but is determined by the values of certain "hidden
variables". For instance, in quantum theory two electrons may be "entangled" such that their states appear to have a kind of spooky
link; measuring the spin of one determines the spin of the other, say. Bohm's theory suggests that they share a hidden variable
governing spin. The theory also shows how probabilistic quantum measurements can always arise from specific particle trajectories.
Take a key puzzle in quantum theory: explaining how a beam of particles passing through two slits in a screen will create a
wave-like interference pattern, even if the particles are sent one at a time. While mainstream quantum theory insists that you can't
give any account of exactly how a given particle moves, Bohmian mechanics can. It suggests that a quantum wave associated with each
particle goes through both slits and sets up a pattern of constructive and destructive interference - just like the bright and dark
interference bands produced with light. This wave pattern then acts on the particles, driving them towards the "bright" bands of
constructive interference (see Diagram).
In the Bohmian view, the statistical interference pattern arises from individual particles following distinct trajectories. This
does away with any inherent quantum fuzziness, and shows that it's still possible to believe not only in determinism but also in the
intuitive notion that particles really act like particles, having definite positions at all times. "The wave function choreographs
the motion of the particles," says physicist Detlef Dürr of Ludwig Maximilian University in Munich, Germany. "As a result, while
everything is deterministic, the universe evolves in such way that the appearance of randomness emerges, and precisely as described
by the quantum formalism."
Still, the majority of physicists didn't take Bohmian mechanics very seriously, often suggesting that it was contrived and afflicted
with technical problems. One of the biggest of these was that Bohm's original model cannot cope with the physics of Einstein's
special relativity. It worked only for particles with low energies and speeds, where the number of particles in any process remains
fixed. At higher energies, relativistic processes routinely create and destroy particles, as when an electron and positron
annihilate one another, turning their energy into light. The simplest version of Bohm's theory could not handle such processes.
So Goldstein and others have tried to develop modified versions of the theory that can. Their work began in the 1980s and 90s as
part of an effort to develop Bohmian models that describe not only quantum particles but quantum fields as well, which provide the
basic framework of all modern physics. In these models, the universe consists both of particles following precise trajectories and
of continuous fields that, like classical magnetic or electric fields, also evolve in a deterministic way. Over the past decade,
Goldstein, working with Dürr and physicist Nino Zanghi of the University of Genoa in Italy, has shown that this picture gives a
consistent view of relativistic particle processes, while reproducing the accurate predictions of quantum field theory (Physical
Review Letters, vol 93, p 090402).
The most promising result to come out of this framework was published last year by Ward Struyve and Hans Westman, both at the
Perimeter Institute in Waterloo, Ontario, Canada. They developed a Bohmian model that matches one of the most accurate theories in
the history of science - quantum electrodynamics, the theory of light and its interactions with charged particles. In fact, Struyve
and Westman found that a number of Bohmian models can easily account for all such phenomena, while remaining fully deterministic
(Proceedings of the Royal Society A, vol 463, p 3115).
The researchers have not yet ironed out all the wrinkles. In particular, critics contend that Bohmian models still don't satisfy the
fundamental principle of relativity, that all frames of reference are on an equal footing. Nevertheless the models appear to have no
serious difficulty in coping with particles being created or destroyed, as many physicists had thought they would. "This is real
progress that's happened over the past decade," says Tim Maudlin, a philosopher of physics at Rutgers. "The main objections to the
theory have now either been addressed, turned out not to be serious or represent issues for the standard theory as much as the Bohm
Goldstein and others have also solved another nagging problem for Bohmian models: elucidating how a deterministic theory can give
rise to the fuzziness observed in quantum experiments in the first place. The uncertainty principle of quantum mechanics states that
measuring the position of a quantum particle limits your knowledge of its momentum, and vice versa. The standard explanation is that
the particle's state is undetermined until you measure it, but in Bohmian mechanics the state is always well defined. The trick,
Goldstein says, is that measuring one variable stirs up uncertainty in the other due to interactions between the measuring device
and the particle, in a way that matches the uncertainty principle.
Even so, most physicists are not yet ready to embrace the new models, because one crucial problem remains: Bohmian theory, critics
point out, doesn't make any predictions that differ from those of ordinary quantum mechanics. "The theory is successful only because
it keeps standard wave mechanics unchanged," says Dieter Zeh of the University of Heidelberg in Germany. He adds that the rest of
the theory is biased towards the ideas of classical physics and is "observationally meaningless".
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