(July 7, 2022) In binary systems states are represented by 1 or 0, that is, a high voltage or low voltage generated by a switch. Looked at over time this produces a step function as shown at the top of the figure. The actual change in voltage levels is represented by an impulse which is also called the Dirac Delta function. This is properly found by taking the directional (Dini) derivative of the step function.
In practice a global clock signal changes the state in all binary devices and that signal is a pulse (an up step then down step). The state actually changes during the leading or lagging edge of that pulse. This shows the impulse is the thing which actually changes state.
Present day mathematics is built upon continuous functions and not discontinuous functions such as the step function. This left the impulse without a good mathematical definition yet it was needed by quantum physics. Paul Dirac (Nobel prize winning) was forced to do some ad-hoc definitions which is why the impulse came to be called the Dirac Delta. He presented two definitions showing just how uncomfortable he was with his derivations. (Dirac 1930)
The first definition was to present the impulse as the integral of some arbitrary shaped area with the area set equal to one. First this arbitrary shape is shrunk along the x axis to its limit (becomes so narrow that any further shrinkage has no further effect) so that it becomes like a vertical line. Once that is done the mathematical operation of integration is used define the area which is then arbitrarily set equal to one. The problem with this is that it ends up using two limits in sequence which is a contradiction of the whole idea of a limit. Once something becomes so small that it is meaningless it cannot become meaningless again by shrinking it in a different way. In this case one limit was for the integral area and the other one was for the impulse shape. Because of this, Dirac went on to call impulses “improper” functions:
“(x) is not a function of x according to the usual mathematical definition of a function, which requires a function to have a definite value of each point in its domain, but is something more general which we may call an ‘improper function’ to show up its difference from a function defined by the usual definition. Thus (x) is not a quantity which can be generally used in mathematical analysis like an ordinary function, but its use must be confined to certain simple types of expression for which it is obvious that no inconsistency will arise.” (page 58 or the 4th edition)Because of this rather inelegant approach he tried to derive the impulse again using a method called “integration by parts” which is only valid for continuous and differentiable functions as that technique originates out of the differential chain rule of calculus. Because the impulse is not a continuous function this second approach is also invalid.
Still, his approaches are the best continuous mathematics can do leaving one or the other of Dirac’s derivations to be repeated in textbooks to this day. Of course, by not using directional derivatives this cripples the formulation of physical theories.
Dirac, P. (1930) Principles of Quantum Mechanics, Fourth Edition (Oxford University Press, Oxford, 1958, first published in 1930)