12 THE PRESIDENTIAL ADDRESS 
It seems to be nothing more than pure good luck. We know of 
no way of increasing the chances of individual electrons ; each just 
takes its turn with the rest. It is a concept with which science has 
been familiar ever since Rutherford and Soddy gave us the law of 
spontaneous disintegration of radioactive substances—of a million 
atoms ten broke up every year, and no help we could give to a 
selected ten would cause fate to select them rather than the ten of 
her own choosing. It was the same with Bohr’s model of the atom ; 
Einstein found that without the caprices of fate it was impossible 
to explain the ordinary spectrum of a hot body; call on fate, and 
we at once obtained Planck’s formula, which agrees exactly with 
observation. 
From the dawn of human history, man has been wont to attribute 
the results of his own incompetence to the interference of a malign 
fate. The particle-picture seems to make fate even more powerful 
and more all-pervading than ever before ; she not only has her finger 
in human affairs, but also in every atom in the universe. The new 
physics has got rid of mechanistic determinism, but only at the price 
of getting rid of the uniformity of nature as well ! 
I do not suppose that any serious scientist feels that such a state- 
ment must be accepted as final; certainly I do not. I think the 
analogy of the beam of light falling on the dirty window-pane will 
show us the fallacy of it. 
Heisenberg’s mathematical equation shows that the energy of a 
beam of light must always be an integral number of quanta. We 
have observational evidence of this in the photoelectric effect, in 
which atoms always suffer damage by whole quanta. 
Now this is often stated in parable form. ‘The parable tells us that 
light consists of discrete light-particles, called photons, each carrying 
a single quantum of energy. A beam of light becomes a shower of 
photons moving through space like the bullets from a machine-gun ; 
it is easy to see why they necessarily do damage by whole quanta. 
When a shower of photons falls on a dirty window-pane, some of 
the photons are captured by the dirt, while the rest escape capture 
and get through. And again the question arises: How are the 
lucky photons singled out? The obvious superficial answer is a 
wave of the hand towards Fortune’s wheel ; it is the same answer 
that Newton gave when he spoke of his ‘ corpuscles of light ’ experi- 
encing alternating fits of transmission and reflection. But we readily 
see that such an answer is superficial. 
Our balance at the bank always consists of an integral number of 
pence, but it does not follow that it is a pile of bronze pennies. A 
child may, however, picture it as so being, and ask his father what 
determines which particular pennies go to pay the rent. The father 
may answer ‘ Mere chance "—a foolish answer, but no more foolish 
