What is quantum mechanics?

Belying its reputation for arcane irrelevance, quantum mechanics addresses problems of great importance for practical human activity. It explains, for example, the rigidity of chairs and tables through the quantum properties of the atoms from which they are built. It was found in the early years of this century that an atom comprises a small nucleus surrounded by orbiting electrons, akin to a mini solar system.

This created a theoretical problem, for the classical physics of Newton, which had received its final formulation by the end of the 19th century, predicts that such a model cannot be stable - the electrons will radiate energy in the form of light and the structure should collapse. The classical theory therefore contradicts experience.

The difficulty was resolved by proposing that transformations of a mechanical system such as an atom can occur only via the emission of a definite "quantum" of light energy; radiation of unlimited amounts is forbidden. Quantum mechanics thus implies that atoms will be stable, and so will the things they compose.

Quantum mechanics is a theory describing the interaction of atoms and light. Apart from the problem of stability, it embraces an array of natural phenomena and devices, such as superconductivity and lasers. But it is potentially much more than a means of coherently organising diverse atomic phenomena. It compels us to confront problems such as the nature of reality.

To appreciate why the explanation offered by quantum mechanics is problematic it is necessary to understand the critical role played by language in scientific theories, itself a tricky subject. Roughly speaking, the discourse of physics is carried on at two interconnected levels: a formal or mathematical level that allows precision in predictions and comparison with experiment, and an informal system of concepts that expresses what the theory says about the world, that is, the meaning it attributes to a word such as "atom". In pre-quantum physics both aspects evolved together into a unified world view. But in quantum mechanics only the mathematical side received a clear formulation.

It was argued, notably by Niels Bohr, that in the quantum domain we meet a new situation in which we can no longer form mental images of physical processes but must be content with predicting the outcomes of experiments. The job of the physicist is simply to gain proficiency in manipulating mathematical symbols. We must be content with knowing how to drive a car and not ask how it works. A major source of confusion has been that Bohr retained the informal language of classical physics but changed the meaning of the terms.

For him, the word "energy" is shorthand for the outcome of a complex experiment and not an attribute of an objective system. Ambiguity was seen as an irreducible feature of nature.

The dominance of Bohr's views and a corresponding culture of science as prediction was rapidly established in quantum physics but physicists such as Schrodinger, Einstein and de Broglie resisted these ideas. Einstein developed a sophisticated critique of "Bohr's tranquillising philosophy" that inspired a generation of quantum dissenters who believed conceptual clarity in science is not optional philosophical baggage.

The problem of interpreting the quantum theory is a counter-example to those who claim that scientific knowledge emerges from a pure activity divorced from the society in which scientists work. That one very special view of quantum mechanics has dominated is the outcome of complex social, political and linguistic issues. History pervades quantum science.