 as a natural exponential
Consider again the general form of the exponential function, . There are two ways in which we can transform any function of the form into a natural exponential function.
- Firstly, we can choose a suitable variable transformation (, where ) so that we may write as .
- Alternatively, if we don’t want to adjust the variable, we may rewrite as (for ) and then let merge with into some other constant so that we end up with
Suppose for example that a biologist, let’s call her Fleur, studies a colony of bacteria in a Petri dish. Fleur finds that the number of bacteria in the Petri dish grows exponentially with a linear increase in time. Say she counts the time, , in seconds and formulates the function to describe the number of bacteria. If then n(t) is cumbersome to work with mathematically. If Fleur could rewrite in terms of a natural logarithm then any differential equation with in it would become much easier to solve.
Suppose Fleur indeed wants to rewrite in terms of a natural logarithm. She has measured the number of bacteria at and found it to be . It would be a bad idea for Fleur to use the second method (2) of naturalising because that would change and then would conflict with her initial measument. Fleur, being the clever biologist that she is, realises that and therefore opts for method (1). Suppose that she sees that doubles every 2.5 seconds. Then she might try substituting the variable for a variable for which each unit-step is not 1 second but 2.5 seconds (). The function can now be rewritten into a function which has a natural exponential form: . This will make Fleur’s life much easier.
Now we may ask the question whether the natural exponential growth-rate is a characteristic of the system that is studied or a characteristic of the way we describe that system. The facts that is a characteristic of the definition of derivative and that any exponential function can be written as a natural exponential might suggest that is a characteristic of our number system and not of nature. Such a conclusion, however, disregards a distinguishing feature of logarithmic functions.
But not every phenomenon can be described by an exponential
The distinguishing feature of logarithmic functions is that their rate of growth at a certain moment scales with the value of the function at that moment. A natural logarithmic function is a special case of this in that the scaling is very simple: the rate of growth is at every moment exactly equal to the value of the function at that moment.
Any dataset that can be described in terms of exponentials can also be described in terms of natural exponentials (any exponential function can be transformed such that its base is ), but we have stumbled upon an objective fact about nature (as it comes to us in our observations) whenever we discover that some phenomenon allows a description in terms of exponentials at all.
The fact that we can describe nature in terms of natural logarithms is a consequence of the way we describe nature, but the fact that we can use logarithms at all tells us something about nature’s dynamics.