36 SECTIONAL ADDRESSES 



emission of neutrons (the neutron w x being a particle whose mass-number 

 is unity and nuclear charge zero). 

 Hence we have 



A1 27_l_„4_^.p 30 I „ 1 

 A1 13 I a 2 ^ r 15 I "0 > 



the resulting product being an isotope of phosphorus. But if the bom- 

 bardment ceases we find that positrons are emitted, the positron (p) 

 being a particle of negligible mass and unit positive charge. The isotope 

 of phosphorus produced is in fact radio-active and the nuclear equation 

 gives 



Pl5 3 °-^Si u 30 +A 



the final product being an isotope of silicon. Bombardment by protons, 

 neutrons, or deuterons may produce disintegration products which are 

 unstable ; the unstable products resulting from bombardments by 

 oc-particles or deuterons pass over into stable species, sometimes with the 

 emission of positrons, sometimes with the emission of electrons ; this 

 latter species of decay — -the p-active species — is often accompanied by 

 y-radiation, so that artificially produced radio-active substances behave 

 in the manner characteristic of natural p-active substances. Neutron 

 bombardment, when it produces radio-elements, produces elements which 

 are p-active. 



By nothing has the world-picture of to-day been so transformed from 

 that of a generation — nay of a decade — ago than by the introduction of 

 the uncertainty principle and by its effect on our notions of causality. 



It can be shown that of two conjugate quantities — time and energy, 

 or position (x) and momentum (/>)— the product of their uncertainties 

 of determination can never be less than the quantum h. Thus an increase 

 in the accuracy of the determination of one quantity necessitates a corre- 

 sponding decrease in the accuracy of the conjugate quantity, and in 

 particular the exact determination of one quantity leaves the other com- 

 pletely undetermined. An attempt to determine the position of a 

 particle involves its illumination by light of suitable wave-length, and 

 decrease of the wave-length in order to improve the definition of its 

 position involves an increase in the magnitude of the recoil due to the 

 Compton scattering process. 



Following a suggestion of Dr. Flint, let us fix our attention on the 

 quantities position and momentum and consider a co-ordinate system in 

 which momentum (p) is plotted along one axis and position (x) along 

 the other. The co-ordinate space gives us the possible simultaneous 

 values of x and p. Suppose this space divided into rectangles each of 

 area h. Then the uncertainty principle, which asserts that the product 

 (SxSp) of the uncertainties of the determination of position and momen- 

 tum can never be less than h, may be illustrated by resuscitating Maxwell's 

 demon and permitting him to push a point about at will within any one 

 of the rectangles. The movement of the point, that is, the corresponding 

 changes of position and momentum, will not be detected, for they do 

 not correspond to any detectable change in the world of sense. 



Unfortunately the word ' indeterminism,' which has other connotations, 

 has become associated with the statement of the principle. Many of us- 



