When it comes to the four physical quantities of distance, time, energy and inertia, it’s 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 wouldn’t be physics, and this is a book about physics, so we must find other ways 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 doesn’t change and it doesn’t fly about on its own.
The smallest possible ruler we can have is therefore the electron. Things smaller than an electron move about at the speed of light. They can’t be used as rulers.
Distance is in other word a relative measure of inertial matter.