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There is no doubt about it: quantum mechanics is the most spectacularly
successful theory in the history of science. In the eighty years since its
invention, it has helped to explain phenomena ranging from the glow of radium
to the detonation of stars, and yielded such potent technologies as lasers
and computer chips. We have paid a price for these advances, however: the
abandonment of our most basic assumptions about reality.

According to the orthodox interpretation of quantum mechanics (although
‘orthodox’ seems an odd description for such a radical world view), subatomic
entities such as electrons or photons are either waves or particles – depending
on how the physicist chooses to observe them. Actually, until they are observed,
quantum entities have no real existence; they exist in a probabilistic limbo
of many possible ‘superposed’ states. The physicist John Wheeler of Princeton
University has likened a photon travelling through an interferometer to
a ‘great smoky dragon’ whose head becomes visible as it bites a detector
but whose body is nothing but smoke.

No physicist was more sensitive to these conundrums or worked harder
to resolve them than David Bohm of the University of London’s Birkbeck College.
Bohm, who died of a heart attack last October at the age of 74, had spent
40 years promoting an alternative to the orthodox interpretation. Sometimes
called the pilot-wave interpretation, it preserves all the astonishing predictive
power of quantum mechanics. But it eliminates many of the most paradoxical
aspects of the orthodox interpretation, such as the schizophrenic character
of quantum entities and their dependence on observers for existence. It
puts scales and flesh back on the body of the dragon.

The pilot-wave theory, however, requires one strange quantum phenomenon:
nonlocality, or the ability of particles to influence each other instantaneously
across vast distances. As a consequence, Bohm’s interpretation has long
been ignored or scorned by physicists. He was accused – not entirely without
justification, as even his admirers admit – of being obscure or overly speculative
in his writings. But the pilot-wave theory has attracted increasing attention
recently. The revival was sparked by John Bell of CERN, the European laboratory
for particle physics. Bell was a quantum theorist who has attained an almost
mythical status among physicists since his death three years ago (‘The man
who proved Einstein was wrong’, ¿ìè¶ÌÊÓƵ, 24 November 1990). Bell called
Bohm’s theory ‘a revelation’ and asked: ‘Why is the pilot-wave picture
ignored in textbooks? Should it not be taught, not as the only way, but
as an antidote to the prevailing complacency?’

Since then, Bohm’s theory has been prominently featured in numerous
other papers and books, such as Quantum Mechanics and Experience, a book
by the philosopher David Albert of Columbia University that was published
last year. ‘Bell drew attention to Bohm’s theory, but then it could speak
for itself,’ Albert says. ‘It is a very explicit and powerful counterexample
to all these pronouncements about the end of determinism and the subjective
nature of reality.’

Sheldon Goldstein, a mathematician at Rutgers University in New Jersey,
agrees. He notes that Bohm’s approach, because it eliminates the need for
an observer to define a quantum phenomenon, is proving to be particularly
useful to quantum cosmologists. After all, Goldstein explains, when one
is trying to describe the entire Universe in quantum terms, who, and where,
is the observer? The future of Bohm’s broader philosophical work, Goldstein
adds, is less clear. ‘I’ve never met anyone who really understands the implicate
order,’ he comments, referring to a holistic view of reality that Bohm set
forth in two popular books, Wholeness and the Implicate Order, published
in 1970, and Science, Order and Creativity, cowritten with David Peat and
published in 1987.

From metaphysical to mainstream

Unlike some physicists known for their metaphysical bent, Bohm also
made important contributions to mainstream physics. A coordinate system
he invented early in his career became a standard tool for modelling the
behaviour of plasmas. His 1951 classic text, Quantum Theory, is still considered
one of the clearest accounts of quantum mechanics. In 1958, together with
Yakir Aharanov, then his student, Bohm predicted that a magnetic field could
alter the route of an electron even if the two were separated by an impenetrable
shield. This nonclassical phenomenon, now known as the Aharanov-Bohm effect,
has been amply confirmed by experiment.

Although Bohm had been afflicted with heart disease for several years,
he continued to refine his ideas up until his death. Just last year, he
finished writing a book on quantum mechanics and the implicate order with
his long-time collaborator, Basil Hiley of Birkbeck College. Hiley expects
the book to be published by Routledge within a year.

But Bohm always saw his ideas not as ends in themselves, but rather
spurs for further research and thought. He emphasised this during an interview
last August at his home in Edgware in the northwest of London. The pilot-wave
theory, for example, was merely one of many possible interpretations of
quantum mechanics. ‘It’s almost like the old Japanese movie Rashomon, where
there are so many interpretations for the same facts,’ he said. ‘That may
lead us to suspect that maybe we’re not looking at it right, or that maybe
we should go further.’

During our interview, Bohm was pale, but he seemed suffused with a nervous
energy. He spoke in a low, urgent monotone for more than two hours about
his life-long quest for understanding the Universe. An American, he received
his doctorate from the University of California at Berkeley and then went
to Princeton. But he left the US in 1951, during the height of the anti-communist
hysteria, after refusing to answer questions from the Committee on Un-American
Activities about whether he or any of his scientific colleagues were communists.
After staying in Brazil and Israel, he settled in England in the late 1950s.

The pattern of Bohm’s career was established early. His investigations
of plasmas at Berkeley and Princeton provided tools for controlling and
predicting their behaviour, but Bohm was always more concerned with the
meaning of his findings. ‘I became very interested in how relatively autonomous
particles went their own way but still work together to create the collective
behaviour.’ These ideas contributed to the holistic views of reality he
developed later (which in turn anticipated the antireductionist concepts
now being developed by students of chaos and complexity).

Increasingly, Bohm questioned the underpinnings of all his research,
quantum mechanics itself. In the late 1940s he began writing his book on
quantum mechanics because, he said, ‘I wanted to understand it’. The book
proceeded smoothly enough until the last chapter, in which Bohm attempted
to explain the orthodox interpretation of quantum mechanics. Also called
the Copenhagen interpretation, it had been set forth in the 1920s by the
great Danish physicist Niels Bohr.

The keystone of Bohr’s interpretation was the concept of complementarity,
which held that wave-particle duality is a paradox that cannot be resolved.
Bohr also ruled out the possibility that the probabilistic behaviour of
quantum systems was actually the result of underlying deterministic mechanisms
called hidden variables. Reality was unknowable because it was intrinsically
indefinite, Bohr insisted.

Particles are always particles

In trying to explain Bohr’s approach, Bohm became dissatisfied with
it. ‘The whole idea of science so far has been to say that underlying the
phenomenon is some reality which explains things,’ he explained. ‘It was
not that Bohr denied reality, but he said quantum mechanics implied there
was nothing more that could be said about it.’ Such a view, Bohm decided,
reduced quantum mechanics to ‘a system of formulas that we use to make predictions
or to control things technologically. I said, that’s not enough. I don’t
think I would be very interested in science if that were all there was.’

In 1952 Bohm defied Bohr’s prohibition against hidden-variable explanations
in a classic two-part paper in Physical Review entitled ‘A suggested interpretation
of the quantum theory in terms of ‘hidden’ variables’. He proposed that
particles are indeed particles – and at all times, not just when they are
observed. Their behaviour is determined by an unusual field or wave consisting
both of classical versions of forces such as electromagnetism and an entirely
new force – which Bohm called the quantum potential – that is responsible
for nonclassical effects. The positions of particles in turn serve as the
hidden variables determining the nature of the pilot wave.

Bohm’s interpretation was causal, or deterministic. Particles always
had a distinct position and velocity, but any effort to measure these properties
precisely would destroy information about them by physically altering the
pilot wave. Bohm gave the uncertainty principle a purely physical rather
than metaphysical meaning. Bohr had interpreted the uncertainty principle,
Bohm explained, as meaning ‘not that there is uncertainty, but that there
is an inherent ambiguity’ in a quantum system.

Bohm sent out preprints of the paper and was quickly informed that
his interpretation was an old one, proposed 25 years earlier by Louis de
Broglie. De Broglie had abandoned the pilot-wave concept after Wolfgang
Pauli pointed out that, when applied to systems involving more than one
particle, it led to ‘some very strange behaviour’. This strange behaviour
referred to by Pauli, Bohm realised, was nonlocality.

Actually, nonlocality was a feature intrinsic to all quantum theories,
not just Bohm’s. Einstein had demonstrated this fact back in 1935 in an
effort to show that quantum mechanics must be flawed. Working together with
Boris Podolsky and Nathan Rosen at Princeton, Einstein proposed a thought
experiment involving two particles that spring from a common source and
fly in opposite directions. According to the standard model of quantum mechanics,
neither particle has a definite position or momentum before it is measured;
but by measuring the momentum of one particle, the physicist instantaneously
forces the other particle to assume a fixed position – even if it is on
the other side of the Galaxy. Deriding this effect as ‘spooky action at
a distance’, Einstein argued that it violated both common sense and the
theory of relativity, which prohibits the propagation of effects faster
than the speed of light; quantum mechanics must be an incomplete theory.

Perhaps because he had always had a holistic view of reality, Bohm was
not disturbed by nonlocality. ‘I must have tacitly been feeling all along
that quantum mechanics was nonlocal,’ he said. In Quantum Theory, Bohm even
suggested an experiment that could demonstrate nonlocality more clearly
and easily than the one proposed by Einstein, Podolsky and Rosen. Bohm called
for measuring not the momentum and position of two particles from a common
source but rather their spin.

Bohm’s spin experiment became the basis for a brilliant mathematical
proof by Bell in 1964 showing that no local hidden-variable theory could
replicate the predictions of quantum mechanics. In 1982, a group led by
the French physicist Alain Aspect at the University of Paris-South, carried
out Bohm’s experiment, demonstrating once and for all that quantum mechanics
does indeed require spooky action. (The reason that nonlocality does not
violate the theory of relativity is that one cannot exploit it to transmit
information faster than light or instantaneously.) Bohm said he never had
any doubts about the outcome of the experiment: ‘It would have been a terrific
surprise to find out otherwise.’

Ironically, Bell’s theorem and the Aspect experiment were widely thought
to rule out all hidden-variable theories, including Bohm’s. It was Bell
who pointed out years later that Bohm’s theory, since it was nonlocal, was
not ruled out by his theorem. According to Bohm’s model, nonlocality was
mediated through the pilot wave: any localised physical act, such as the
measurement of a particle, would instantaneously alter the shape of the
entire pilot wave, affecting all particles under its influence.

Bohm continued to develop the pilot-wave theory through the 1980s with
the help of collaborators such as Hiley. In its latest version, the Bohmian
pilot wave is quite distinct from the one posited by de Broglie. De Broglie
conceived of the pilot wave as a kind of mechanical force which pushed particles
this way and that through the transmission of energy. Bohm’s pilot wave
is more subtle: it guides particles not through its amplitude but through
its form – much as the form rather than the amplitude of a flight-controller’s
radio transmission controls a plane’s behaviour. The wave’s ability to influence
particles therefore does not diminish with distance, as classical waves
do.

In the last decade, Bohm also became absorbed in another perennial puzzle:
why quantum effects are generally limited to very small-scale phenomena.
Two recent efforts to explain this mystery left him unimpressed. One of
these, proposed by Gian Carlo Ghirardi of the University of Trieste and
others, holds that as a quantum entity propagates through space, its multiple,
possible states converge into a single state that behaves in a classical
way. Roger Penrose of the University of Oxford presented another possibility
in his book The Emperor’s New Mind: quantum effects disappear in systems
containing so much mass that gravity – which is usually negligible at subatomic
scales – becomes a factor.

Bohm favoured what he felt was a much simpler explanation: heat. Various
lines of evidence – notably the fact that superconductivity, which relies
on quantum effects, occurs only at very low temperatures – suggest that
thermal energy swamps quantum effects. To completely resolve the issue
of the limits of quantum effects, Bohm contended that: ‘It would be required
to connect thermodynamics and quantum mechanics in a deep fundamental way
rather than the present superficial way, which is that you start with quantum
mechanics and then apply statistics. It may be that thermal properties
are just as essential as quantum properties, or there’s something deeper
than both.’

To arrive at such a theory, physicists might need to jettison some basic
assumptions about the organisation of nature. ‘Fundamental notions like
order and structure condition our thinking unconsciously, and new kinds
of theories depend on new kinds of order,’ he said. During the Enlightenment,
he noted, thinkers such as Rene Descartes and Isaac Newton replaced the
ancients’ concept of order with a mechanistic view. Although the advent
of relativity and other theories has brought about modifications in this
order, Bohm said, ‘the basic idea is still the same: a mechanical order
described by coordinates’.

Bohm himself began formulating what he called the implicate order several
decades ago. His ideas were inspired in part by a simple experiment he saw
on television, in which a drop of ink was squeezed onto a cylinder of glycerine.
When the cylinder rotated, the ink diffused through the glycerine in an
apparently irreversible fashion; its order seemed to have disintegrated.
But when the direction of rotation was reversed, the ink gathered into a
drop again.

Bohm made this simple experiment into a metaphor for all of reality.
Underlying the apparently chaotic realm of physical appearances – the explicate
order – there is always a deeper, implicate order that is often hidden.
Applying this concept to the quantum realm, Bohm proposed that the implicate
order is the quantum potential, a field consisting of an infinite number
of fluctuating waves. The overlapping of these waves generates what appear
to us as particles: these constitute the explicate order. Even such seemingly
fundamental concepts as space and time may be merely explicate manifestations
of some ‘nonlocal, deeper implicate order’, according to Bohm.

Bohm hoped the implicate order could even point the way to a resolution
of that perennial conundrum of philosophy, the mind-matter problem. His
belief was based on hints and rough analogies rather than on any concrete
evidence. For example, he compared the way a pilot wave guides a particle
to the way thought guides the movements of a dancer. ‘The movement of the
body is coming from thought, and the movement of the electron is coming
from something very subtle, this wave. So there are similarities, which
should make it possible to relate them.’

Despite his own enormous ambition as a truth seeker, Bohm rejected the
possibility that scientists can ever bring their enterprise to an end by
reducing all of nature to a single fundamental phenomenon (such as infinitesimal
particles called superstrings). ‘At each level we have something which is
taken as appearance and something else is taken as the essence which explains
the appearance. But there’s no end to this. What underlies it all is unknown
and cannot be grasped by thought.’

Indeed, scientists’ belief that they are on the verge of a final theory
may prevent them from seeking deeper truths. ‘It’s like fish,’ Bohm elaborated.
‘If you have fish in a tank and you put a glass barrier in there, the fish
learn to keep away from it. Then if you take the barrier away the fish never
cross the barrier.’ ¿ìè¶ÌÊÓƵs who are frustrated at the thought that ultimate
truths are unattainable should consider the alternative. ‘They are going
to be very frustrated if they get the final answer and then have nothing
to do except be technicians,’ Bohm said.

Science, Bohm believed, is sure to evolve in totally unexpected ways.
He expressed the hope, for example, that future scientists will be less
dependent on mathematics for modelling reality and will draw on new sources
of metaphor and analogy. ‘We have an assumption now that’s getting stronger
and stronger that mathematics is the only way to deal with reality,’ Bohm
said. ‘Because it’s worked so well for a while we’ve assumed that it has
to be that way.’

Indeed, like some other scientific visionaries, Bohm expected that science
and art would someday merge. ‘This division of art and science is temporary,’
he said. ‘It didn’t exist in the past, and there’s no reason why it should
go on in the future.’ Just as art consists not simply of works of art but
of an ‘attitude, the artistic spirit’, so does science consist not in the
accumulation of knowledge but in the creation of fresh modes of perception.
‘The ability to perceive or think differently is more important than the
knowledge gained.’

No matter how history treats Bohm’s specific ideas, this, surely, will
be one of his greatest legacies: his ability to make the rest of us perceive
and think differently.

John Horgan is a senior writer for Scientific American.