Sometimes theories / that have been out of fashion for some while / can come back into consideration / in view of later developments.

A case in point is an idea / that Lord Kelvin put forward / in about 1867, / in which atoms (the elementary particles of his day) / were to be regarded as being composed of tiny knot-like structures.

This idea attracted some considerable attention / at the time, / and the mathematician J. G. Tait began a systematic study of knots / on the basis of this.

But the theory did not lead to any clear-cut correspondence / with the actual physical behaviour of atoms, / so it became largely forgotten.

However, more recently, / ideas of this general kind / have begun to find favour again, / partly in view of their connection / with string-theoretic notions.

The mathematical theory of knots / has also encountered a revival, / since around 1984, / starting with the work of Vaughan Jones, / whose seminal ideas had their roots / in theoretical considerations within quantum field theory.

The methods of string theory / were subsequently employed by Edward Witten / to obtain a kind of quantum field theory / which, in a certain sense, / encompasses these new developments / in the mathematical theory of knots.