When it comes to the four physical quantities of distance, time, energy and inertia, it is tempting to treat them as if they are somehow outside of physics. We can imagine a god in the heavens holding a ruler and a clock, distributing energy and bestowing inertia onto matter.
However, that would not be physics, and this is a book about physics, so we must find another way to define these quantities if we are to include them in our model.
The way we can do this is as follows:
To measure distance, we need a ruler of some kind. In our daily lives, the ruler we use is ourselves. We measure everything relative to our own size.
However, when we want to be precise about our measurements, we use a carefully crafted ruler.
Such a ruler is something we can carry around with us. Its length does not change and it does not fly about on its own.
The smallest possible ruler we can make is therefore the electron. Things smaller than an electron moves about at the speed of light and can therefore not be used.
Distance is in other word a function of matter in the open state.