with the equality sign applying to the case of a Gaussian wave packet. Since there is a difference of a factor 4π between expressions (18) and (19), students may be confused as to which is more ‘correct’. The difference arises due to the way that x and p are defined in each case. Heisenberg’s definitions were based on actual experiments, whereas later it became
customary to identify the quantities with the standard deviations on a statistical measurement of the values of x and p, as defined in statistical theory by x = (x −x)21/2, for example In our applications here, we choose to use the Heisenberg expressions (17) and (18), as being more suited, perhaps, to practical situations.