8 2 D O C . 7 7 E S S E N C E O F Q U A N T U M M E C H A N I C S The following considerations are common to both ways of conceiving of the theory: There are De Broglie waves that impinge roughly normally on S and are diffracted by the opening O.[2] After passing through O, they form spherical waves that reach the film P, and their intensity at P is responsible[3] for what happens there. We can now characterize the two points of view as follows: 1. Concept I.— The De Broglie-Schrödinger waves do not correspond to a sin- gle electron, but rather to a cloud of electrons spread out in space. The theory gives no information at all about individual processes but instead only about the whole of an infinity of elementary processes. 2. Concept II.— The theory claims to be a complete theory of individual pro- cesses. Each particle that moves toward the screen, as far as can be determined by its velocity and its position, is described by a wave packet of De Broglie- Schrödinger waves of short wavelengths and with a small angular spread. This wave packet is diffracted and afterwards some part of it arrives at the film P in a state of resolution. According to the first point of view, which is purely statistical, expresses the probability that at the position considered, there exists one certain particle of the cloud, for example, at a given position on the screen. According to the second point of view, expresses the probability that at the moment in question, the same particle is to be found at a given position (on the screen, for example). Here, the theory treats individual processes, and claims to de- scribe everything that is governed by laws. The second concept goes further than the first, in the sense that all the informa- tion that results from II also results from the theory by virtue of II, but the converse is not true.[4] It is only by means of II that the theory contains the consequence that the conservation laws hold for the elementary process it is only with II that the the- ory can deduce the result of the Geiger-Bothe experiment[5] and that it can explain the fact that in a Wilson cloud chamber, the droplets produced by an particle are found roughly along continuous lines. However, on the other hand, I have some objections to raise against concept II. The diffuse wave propagating toward P does not possess a privileged direction. If were seen simply as the probability that at a given position, a given particle could be found at a given instant, it might happen that the same elementary process would produce an action at two or at several positions on the film. But the inter- pretation that expresses the probability that this particle is to be found at the given position supposes a quite particular mechanism of action at a distance, which prevents the wave, extended continuously over space, from producing an action at two positions on the film.[6] [p. 255] 2 2 2 2