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David Bohm (1917–1992)

Bohm showed that quantum mechanics does not require indeterminacy. His 1952 hidden-variable theory — now called Bohmian mechanics or the de Broglie-Bohm theory — provides a deterministic account of quantum phenomena in which particles always have definite positions, guided by a “pilot wave” that is the wave function of the system. The theory reproduces all the predictions of standard quantum mechanics without invoking collapse or the measurement problem. The price is non-locality: the pilot wave connects distant particles instantaneously, so the measurement of one particle affects the trajectory of another regardless of the distance between them. Bell’s theorem (1964) showed that any hidden-variable theory reproducing quantum predictions must be non-local; Bohm’s theory pays the price explicitly. The theory was ignored for decades — partly because von Neumann had published a (flawed) proof that hidden-variable theories are impossible, partly because the Copenhagen interpretation’s dominance discouraged alternatives. It has been revived since the 1990s as a serious interpretive option, defended by a minority of physicists and philosophers who regard its determinism and realism as virtues that outweigh the non-locality cost.

Life

Born 20 December 1917 in Wilkes-Barre, Pennsylvania, to a family of Jewish immigrants from Eastern Europe. Undergraduate at Penn State, then graduate work at Caltech and the University of California, Berkeley, where he completed his PhD (1943) under J. Robert Oppenheimer. Worked on the Manhattan Project at the Berkeley Radiation Laboratory, though he was denied security clearance for the classified work at Los Alamos.

Assistant professor at Princeton (1947–51). Published Quantum Theory (1951), a textbook that gave one of the clearest expositions of the Copenhagen interpretation — and then immediately began developing the hidden-variable alternative that contradicted it. Einstein, then also at Princeton, encouraged Bohm to pursue the alternative; the two had extended conversations about the incompleteness of quantum mechanics.

Called before the House Un-American Activities Committee in 1950 because of his former association with Communist Party members at Berkeley. He refused to testify against colleagues, was arrested for contempt of Congress, and was acquitted — but Princeton did not renew his contract. Unable to find a position in the United States, Bohm emigrated: first to the University of São Paulo, Brazil (1951–55), then to the Technion in Haifa, Israel (1955–57), then to the University of Bristol (1957–61), then to Birkbeck College, University of London (1961–87), where he spent the rest of his career.

The political exile shaped Bohm’s intellectual trajectory: cut off from the American physics establishment, he pursued unfashionable questions — hidden variables, wholeness, the relationship between physics and consciousness — that the mainstream regarded as unscientific. Fellow of the Royal Society (1990). Died 27 October 1992 in London.

Bohmian mechanics

The 1952 papers (“A Suggested Interpretation of the Quantum Theory in Terms of ‘Hidden’ Variables,” Parts I and II, Physical Review) present the theory. The structure:

Particles and the wave function. A quantum system consists of particles that always have definite positions, plus a wave function ψ that evolves according to the Schrödinger equation (exactly as in standard quantum mechanics). The wave function is not a probability amplitude — it is a real physical field that guides the particles.

The guidance equation. Each particle’s velocity is determined by the wave function: the particle moves along the gradient of the phase of ψ. The trajectory is deterministic — given the initial position of the particle and the wave function, the future trajectory is fixed. The randomness in quantum mechanics arises not from indeterminacy but from ignorance of the initial positions, which are distributed according to |ψ|² (the quantum equilibrium hypothesis).

Non-locality. The wave function of an entangled system is defined on configuration space (a high-dimensional space with one set of coordinates for each particle), not on ordinary three-dimensional space. A change in the wave function due to a measurement on one particle instantaneously affects the guidance equation for all other particles in the system, regardless of distance. This is the non-locality that Bell’s theorem requires of any hidden-variable theory that reproduces quantum predictions.

No collapse. The wave function never collapses. What appears as collapse in the Copenhagen picture is explained by the dynamics: after a measurement, the wave function develops branches that are effectively decoupled from each other (decoherence), and the particle follows one branch. The other branches persist in the wave function but have no further effect on the particle’s trajectory. The “empty” branches are sometimes called “zombie waves.”

Louis de Broglie had proposed a similar pilot-wave theory in 1927, presented it at the Solvay conference, and abandoned it under criticism from Pauli. Bohm independently reinvented the theory in 1952 and developed it further. The theory is now called the de Broglie-Bohm theory to acknowledge the dual lineage.

The implicate order

Bohm’s later work — Wholeness and the Implicate Order (1980) and subsequent books — moved from the technical foundations of quantum mechanics to a broader philosophical programme. The central concept: the “implicate order” — an underlying, enfolded order from which the “explicate order” (the world of separate objects and events that we observe) unfolds. The implicate order is not a hidden layer behind appearances; it is the deeper reality of which appearances are projections, in the way that a hologram encodes a three-dimensional image in a two-dimensional pattern.

The programme was influenced by extended conversations with Jiddu Krishnamurti, the Indian philosophical teacher, from the 1960s onward. Bohm saw a connection between the fragmentation of scientific thought (the division of reality into separate domains — physics, biology, psychology — that then cannot be reunited) and the fragmentation of human consciousness. The implicate-order programme was an attempt to heal both fragmentations simultaneously.

The reception was mixed. Physicists generally regarded the implicate-order work as speculative and insufficiently formalised — suggestive as metaphor but not productive as physics. Philosophers were more receptive, though opinions divided on whether the programme was a genuine philosophical contribution or an overextension of physical intuition. The work has influenced the foundations-of-physics community less than Bohmian mechanics and more than its critics suggest.

Where Bohm stops

Bohmian mechanics reproduces all the predictions of standard quantum mechanics but does not generate new ones. The theory is empirically equivalent to the standard formalism — it makes the same predictions for every experiment. This has been both its strength (it cannot be refuted by any experiment that confirms standard quantum mechanics) and its weakness (it cannot be confirmed by any experiment either). Whether empirical equivalence is a serious objection depends on what one demands of a physical theory. Bohm’s defenders argue that the theory is preferable because it provides a clear ontology (particles with definite positions) and a clear dynamics (the guidance equation) without invoking collapse. Critics argue that the non-locality is too high a price — that the instantaneous influence of distant measurements on local trajectories is at least as problematic as the measurement problem it solves.

The quantum equilibrium hypothesis — that particle positions are distributed according to |ψ|² — is an additional assumption that does not follow from the dynamics. Why should the initial conditions of the universe produce a |ψ|² distribution rather than some other distribution? Antony Valentini has explored the possibility that quantum equilibrium is not exact — that deviations (“quantum non-equilibrium”) might exist and would produce experimentally detectable violations of standard quantum predictions. If found, such violations would distinguish Bohmian mechanics from standard quantum mechanics. They have not been found; the hypothesis remains speculative but keeps the theory empirically open in principle.

The implicate-order programme has not been formalised into a testable physical theory. Bohm intended it as a framework within which new physical theories could be developed, not as a theory itself. Whether the framework is fertile — whether it has led or will lead to new physics — is an open question. The algebraic approaches to quantum gravity that draw on non-commutative geometry have some structural affinity with the implicate-order ideas, but the connection is suggestive rather than demonstrated.

Key works

See also: Heisenberg · Einstein · Born · Relational quantum mechanics