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XIII. i] 
173 
qualitatively in all the ways known to geometry but not quan 
titatively; the uniformity of the electronic charge must in 
some way represent an absolute restriction on the measure of 
the crumpling. 
“ Each particle of matter—each electron let us say—occupies 
one point of space at any one instant of time, and the succession 
of these points will form a line in the four-dimensional space- 
time continuum—the * world-line ’ of the electron. In the 
neighbourhood of this world-line there is a deformation of 
the continuum due to the existence of the electron. 
“ The near approach of two electrons or of any two charged 
particles is represented by a near approach of their world-lines 
in the four-dimensional continuum. Each world-line is sur 
rounded by its associated deformation, and in regions in which 
the world-lines are near to one another the adjacent regions of 
the continuum will be doubly deformed. 
“ A priori there are two possibilities open. The first is that 
the two deformations are merely additive, just as, when two 
ships approach, each making its own wash (or deformation of 
the surface of the sea), the height of wash at any point is the sum 
of the heights of the washes made by the two ships independently. 
The second possibility is that, as there have been found to be 
restrictions on the amount of deformation associated with the two 
separate world-lines, there may be a further restriction on the 
deformation arising from their combination. 
“ In actual fact the former alternative appears to prevail 
when one or both of the charged particles are ‘ free ’ electrons, 
but the latter alternative when they are ‘ bound ’ together; 
that is, when they are permanently describing orbits about one 
another. It is these latter restrictions that have given rise to 
the theory of quanta. Just as the restrictions associated with 
single world-lines give rise to an atomic constant e, the charge 
on an electron, so the restrictions associated with pairs of world 
lines give rise to a second atomic constant. This is generally 
taken to be h, Planck’s constant, but in many respects it is more 
appropriate to regard the product he as the second constant, 
where c is the velocity of light. It is significant that he is of the 
same physical dimensions as e 2 and so may be regarded as being 
the same thing as e 2 except for a numerical multiplier. Thus 
while the restrictions connected with one world-line introduce e, 
those connected with two world-lines, depending only on e 2 , 
introduce no essentially new constant, whence it may reasonably 
be suspected that the two sets of restrictions are merely different 
aspects of one and the same set. It looks as though the atomicity 
of the quantum theory is only another aspect of the atomicity 
of electric charges.”