Imagine, if you will, that we can peer deep into a solid object, and looking very, very closely, smaller than the eye can actually see, we can begin to make out the internal structure of that object, and zooming even closer in, we find what appear to be the smallest things that make up this object.
Let’s call these things ATOMS.
That’s a bit of Ancient Greek (or Jain) philosophy, the theory of the atomisation of matter, the idea that there are small things that make up large things, and that these small things cannot be made any smaller. This is a fantastic theory, as it explains a lot of things that happen around us. In several thousand years, nobody has seriously been able to challenge this theory; however, it has undergone continuous refinement.
Besides ordinary solid objects there are also things that you can’t hold in your hands; things like light and heat and the little electric shocks you get when you comb really dry hair.
Heat appears to be what happens to ordinary solid objects when the tiny atomic parts are moving wider or further or faster. Most things expand but stay holding together when they get hotter, which confirms this idea. There are special cases, like water, which expands as it freezes, but that can be explained by thinking about how atoms move in relation to each other…
Anyway, back to light.
Light is a different matter altogether. It appears to be able to pass through some objects without changing radically. It travels very long distances without being stopped. It seems to cause ordinary matter to heat up. It doesn’t rest anywhere.