My son wants to know how we can perceive light and it not take up any space at the same time.
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This is hard to understand because there is nothing you can physically relate it to in everyday life. Your son understands the concept of “things” and “space” and that two “things” can’t occupy the same “space.” But he only understands this because he can touch and feel those things – he can directly experience it, and so in his mind there is a model of how the world works that matches these experiences. Photons aren’t “things” in this model because they do not take up space. Two photons can be in the same space at the same time.
Why?
Maybe a better question is “Why not?” We take for granted that “things” exclude each other from the space they occupy, but why should they? Why is that more intuitive than them not doing it? The reason is that we readily accept the idea just because we experience it directly. It’s no more sensible than saying that things should be able to occupy the same physical space.
If you can accept that both situations are equally non-intuitive or strange, then I think the next question is “so why _do_ things take up space?” And the answer is – they don’t, at least not really. All that is happening is that the particles that those things are made up of exert forces on other similar particles. In this case, it’s electrons. Because things are made of atoms, and atoms have electrons around them, and electrons push other electrons away, two things can’t occupy the same space. The electrons push each other apart.
In other words, everything we experience is actually just tiny little invisible particles exerting forces on each other. When you put your hand on the table, the table’s electrons push against the electrons in your skin. Those push on the electrons on the outside of your neurons, which compresses proteins, which trigger electrical discharges (which are again electrons pushing on each other), which push electrons along the nerve, all the way to your brain where more electrons push more electrons through more nerve connections in a complicated pattern we call the “sense of touch.”
Photons work the same way. They don’t occupy space on their own. Unlike electrons though, they don’t exert force on other photons. When they hit an electron and they have the right frequency, however, they cause that electron to gain energy. When that happens to electrons inside of special proteins in your retina, and specifically the electrons in a specific bond inside of those special proteins, the change in energy causes the molecule to change shape…. which causes its electrons to push on more electrons in a long chain all the way up to your brain where they create the pattern called “sense of sight.”
So the reason that photons can be seen even though they don’t take up space is that taking up space is not relevant to our senses. What is relevant is electrons being pushed around. Sometimes that happens when other electrons run into them, but sometimes it happens when a molecule changes shape because it absorbed the energy from a photon.
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