#### David Bohm on Quantum Theory

(Bohm’s ontological interpretation of quantum theory)

#### Against Orthodoxy

Bohm challenged the dominant assumptions of science and of scientists, and this disposition was not well received. After writing a textbook on quantum theory that was released in 1951 that was praised by Albert Einstein among others, Bohm followed up in 1952 with a journal article titled **A Suggested Interpretation of the Quantum Theory in Terms of “Hidden” Variables**.

#### Hidden Variables

From the abstract of this article Bohm states the following:

“The usual interpretation of the quantum theory is self-consistent, but it involves an assumption that cannot be tested experimentally, viz., that the most complete possible specification of an individual system is in terms of a wave function that determines only probable results of actual measurement processes. The only way of investigating the truth of this assumption is by trying to find some other interpretation of the quantum theory in terms of at present “hidden” variables, which in principle determine the precise behaviour of an individual system, but which are in practice averaged over in measurements of the types that can now be carried out. In this paper and in a subsequent paper, an interpretation of the quantum theory in terms of just such “hidden” variables is suggested.”

#### John Bell

The physicist John Bell, known for his Bell’s theorem, had the following to say about Bohm’s 1952 article:

“But in 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into non-relativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the ‘observer,’ could be eliminated. But why then had Born not told me of this ‘pilot wave’? If only to point out what was wrong with it? Why did von Neumann not consider it? More extraordinarily, why did people go on producing ‘impossibility’ proofs, after 1952, and as recently as 1978? Why is the pilot wave picture ignored in text books? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show us that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice?”

#### Ontology

“The reason why I published that paper was not because I was worried about a lack of determinism or the lack of causality in quantum physics. Rather, I was worried because there was no ontology…” — Basil Hiley quoting David Bohm

#### The Work Continued

Bohm continued to work on this other interpretation, which went from being referred to as ‘hidden variables’ to the ‘ontological interpretation’, for 40 years until his death in 1992. In Bohm’s book **Wholeness and the Implicate Order**, first published in 1980, we see the following introduction in a chapter on hidden variables in quantum theory:

“The question of whether there are hidden variables underlying the quantum theory was thought to have been settled definitely in the negative long ago. As a result, the majority of modern physicists no longer regard this question as relevant for physical theory. In the past few year, however, a number of physicists, including this author, have developed a new approach to this problem, which raises the question of hidden variables again.”

Bohm’s last book, published posthumously and coauthored with Basil Hiley went into great detail about this new ontological interpretation. On the back cover of this book, The Undivided Universe, we find an excellent review that makes the key point about this interpretation:

“Any physicist with an interest in foundational issues should read this book. But it would be a mistake to think of Bohm’s theory as just another interpretation of quantum mechanics, with appeal only to those who like to indulge in idle philosophical thought. Bohm’s theory is a theory for physicists, indeed more so than quantum mechanics, for it does what conventional quantum mechanics patently fails to do: it attempts to tell us the world is like.” – Constatine Pagons, Physics World

#### Benefits

Bohm and Hiley’s 1986 paper **An Ontological Basis for the Quantum Theory** provides the following list of benefits of the Ontological Interpretation:

“The main advantages of this ontology are the following:

(1) It permits an objective description of quantum processes, in which neither measurement nor preparation of a states plays a fundamental part. This enables us to understand intuitively what the quantum theory means.

(2) It avoids the need for introducing a ‘collapse’ of the wave function that would violate the basic laws of physics, such as Schrodinger’s equate.

(3) It gives a clear discussion of the time transition, and of the ‘watched dog’ effect.

(4) It gives a clear notion of the meaning of the classical limit, as arising in a very simple way whenever the quantum potential is negligible compared with other terms in the Hamilton-Jacobi equation. It also makes it clear that the classical limit is not always valid on a macroscopic scale, because, under certain special conditions, the quantum potential can still be large in such systems.

(5) A more detailed analysis shows that it covers the whole range of quantum theory. In this connection, it is important to note that one can extend the casual interpretation to quantum fields, and thus deal with all bosonic systems. We shall in fact give a more detailed treatment of this question in the next paper, where we shall also discuss how fermions are to be incorporated into the causal interpretation.

(6) It has important advantages for cosmology, especially because it can discuss the universe apart from the questions of ‘the collapse of the wave function’ and of the absolute need for the presence of observers, which are characteristic of other interpretations.

#### Further Reading

Wholeness and the Implicate Order

The Undivided Universe

An Ontological Basis for the Quantum Theory

Some Remarks on the Evolution of Bohm’s Proposals for an Alternative to Standard Quantum Mechanics (A paper by Basil Hiley)