Stevin's leaden ball experiment, the theme that runs through this story, has now broken up into two parts: the equal acceleration of the two different masses, and the structure and behaviour of matter. The equal-acceleration behaviour of ‘gravity’ is explained by the properties of space-time discovered by Einstein. Next on the agenda are those leaden balls or, more generally, any matter.

To understand the structure and behaviour of matter, we must delve a bit more deeply into the world of particle physics that is too small to be seen with an optical microscope. This will give us more insight into the nature of forces, and the possible beginning of a connection between the space-time world at large with the particle world on small scales.

The interaction of light with matter, such as in the case of the glass of a windowpane, takes place via an infinity of alternatives. These alternatives, when added together by means of the rolling-wheel process, produce effects collectively known as interference. For example, the ‘choices’ that light rays have in reflection-and-transmission at surfaces (such as glass or water) can produce visible interference, as they do when producing the shimmering colours in a soap bubble.

With a little effort it can be shown that, in empty space, the most probable path of a quantum is a straight line. At least this aspect of Huygens's relativity is nicely built into quantum mechanics. But what about curved paths? How are accelerations introduced?

Let us go back to the presentation of the rolling phase wheel associated with the motion of a quantum. The shape of the most probable path is determined by the addition of the phases of all possible paths between two points. It stands to reason, then, to introduce accelerations (and thus curved paths) by means of a prescription of the phases along the paths.

If the shape of the most probable path is curved, Huygens would have said that the object moving along that path has been accelerated. In the quantum world, the shape of the most probable path is governed by the arrow-addition of the phases of the quantum wheel along all possible paths. Thus, what we experience as an acceleration in our large-scale world is due to the behaviour of the phases of the quanta.