The Uncertainty Principle

Abstract

MR. W. L. COWLEY¹ raises a point of some importance to a popular presentation of quantum theory. The understanding of the Uncertainty Principle is confused by there being two quite distinct ideas associated with the one name. Mr. Cowley's discussion of Heisenberg's uncertainty principle relates to the form of it which could be derived from the old quantum theory of Planck, Einstein and Bohr (one may call it'uncertainty I'). As he remarks, books intended for wide circulation usually introduce the principle in this form, by describing the ‘microscope experiment', and often take it no further. However, the connexion between uncertainty I and uncertainty II (which depends on the new quantum theory) is slight. The old quantum theory only recognized the so-called ‘wave-particle duality’ of light, and not of matter ; uncertainty I is a rather artificial statement about the precision of observation practicable with an optical microscope. In the new quantum theory of de Broglie and others, however, the ‘duality’ is formulated for matter also, and uncertainty II expresses a consequence of this.