A physicist (1917-1992) who came up with a radical alternative interpretation of quantum mechanics. One of the fundamentally counter-intuitive ideas in standard QM is that quantum events are ultimately random. Some physicists, such as Einstein, unwilling to accept this, hoped that a deeper underlying mechanism would be found that removed the randomness: they wanted "hidden variables". Their hopes were dashed when in 1932 John von Neumann apparently proved that hidden variables were inconsistent with quantum theory.

David Bohm however showed* that von Neumann's proof made a crucial assumption that events interacted locally: that is, causation propagated through spacetime in the way we intuitively imagine it to, one event causing another right next to it. If events at distant locations can be directly linked, a hidden variables theory of quantum mechanics is mathematically possible. He produced a version of the Einstein-Podolsky-Rosen paradox, which seems to be an instance of non-local causation.

His 1952 paper containing some of these ideas was "A suggested interpretation of the quantum theory in terms of 'hidden' variables". His principal book is Wholeness and the Implicate Order (1980).

This led to his own theory, which I don't understand, and don't believe in, and which barely rates a mention in most physics books, but which is nevertheless an immensely important contribution, in that it is a genuine and serious attempt at an alternative theory, consistent with all known quantum-mechanical observations. If anyone either understands it or at least knows more about it, there's a big welcoming box for you underneath this write-up. (Later: see quantum non-locality for excellent coverage of the whole debate including Bohm's contribution, and which is more correct than mine here, if they differ.)

Bohm called his vision of the universe the implicate order. It may also be termed quantum implicature, I think, unless I just accidentally made that up. The universe is "folded up" in such a way that every part is intimately and immediately connected with any other. (So he is not merely talking about occasional wormholes in an orthodox Kaluza-Klein higher-dimensional spacetime.) Any particle, such as a single photon going through one of the slits in the two-slit experiment, is accompanied by a pilot wave that in effect washes over the entire universe and interacts with every other pilot wave, to produce the wave interference effects seen in standard physics.

David Deutsch, champion of the rival many-worlds hypothesis, says that the immensely complicated calculations required for the working of this wave, while hidden under the modest name of "wave", are in effect the infinite parallel universes of the many-worlds interpretation, and do not simplify anything: they do not save the idea that there is a single universe.

Although Bohm's was a serious scientific theory, and has been taken seriously by many scientists, though usually rejected by them, it had rather more resonance than it ought to have in non-scientific culture. It seems to support a meaningless New Age idea of holism; and indeed Bohm's own later work, venturing into ecology and world peace, did rather go along with this. He believed mind should be taken into account in a quasi-holographic view of the universe. But in essence his real contribution to physics no more supports Buddhism or New-Age-ism than Einstein or Heisenberg's theories support the amateur relativism sometimes drawn from them.

Born in Pennsylvania, he was blacklisted in the McCarthy witch-hunts and abandoned the United States, teaching in England, Israel, and Brazil.


* Afternote. Some time ago Oolong pointed out to me that the nature of Bohm's demonstration was rather different. The situation still seems to be uncertain. What Bohm did was construct a hidden-variables method that was consistent and therefore contradicted von Neumann. It was John Bell who tried to work out what exactly was wrong with von Neumann's argument, and whether locality was the key, therein developing Bell's inequality.

Greater detail at
http://physicsweb.org/article/world/11/12/8 (recommended!)
http://www.mtnmath.com/book/node27.html
http://www.mathpages.com/rr/s9-06/9-06.htm

Bohm came up with his own version of quantum mechanics which (it has now been proven) makes exactly the same predictions as the standard QM formalism, but has a "realistic" interpretation, involving point particles with real paths.

In the two slit experiment, a series of particles are shot, one at a time, towards a detector, with a double-slitted wall interposed.

Notoriously, even if the particles arrive one at a time, the detector shows the distribution of the particles has interference fringes as though the particle had propagated like a wave, going through both slits. Controversy still rages about the significance of this result.

In Bohm's model, there is a moving point particle, accompanied by a "pilot wave", which influences the path taken by the particle. The particle surfs along in particular spots on the wave:


          _         _
         ' `       ' `
    ~.^ /   \  @  /   \ ^.~
       V     \   /     V 
              `_'

When the pilot wave passes through the two slits, and interferes with itself, Bohm showed, it produces a set of possible paths behind the slits which result in the fringed distribution seen in the experiment.

Technically, the Schroedinger equation is used to produce a wave function (the "pilot wave") which then generates a velocity field, determining the motion of the particles. The point particles are taken to have real positions and momenta. The nonlocal twist comes in since the wave function takes place not in "physical" 3-dimensional space, but in a higher dimensional "configuration space" depending on the (simultaneous!) position of all particles in the environment.

This aside, perhaps Bohm's most notable contribution to the science was in formulating a thought experiment - a variant of the Einstein-Podolsky-Rosen paradox, using spin rather than position and momentum - which proved to be experimentally testable, and showed conclusively that the Bell inequalities were violated by straightforward quantum theory, leading to the widespread acceptance of nonlocality in the 1960's. (This work was published in Quantum Theory (1951) and Physical Review 85, pp. 169-193.)

Suspicions about Bohm's political allegiances on the part of the authorities were responsible for his being ejected from the Manhattan Project.


Information from the Introduction to Bohmian Mechanics at:
http://www.mathematik.uni-muenchen.de/~bohmmech/Poster/post/postE.html

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