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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I don’t know a good way to explain this to a literal child, but here’s my best attempt:
Light comes in individual little bits that we call photons. Photons are little points, literally so small they have no length or width or depth. However, despite being little points, they still carry energy. When they hit things, they transfer that energy. For example, photons from the sun will hit your skin and you’ll feel that energy as heat. Your eyes are just fancy machines that uses all the little photons hitting it to paint a picture of the world.
In a very very oversimplified statement, only things with Mass can interact with space.
Things without mass like photons can only interact with objects in space.
Light is made of energy (Electromagnetic radiation) so it doesn’t have any matter (atoms) to occupy space
No. In fact, most things don’t actually occupy space.
Electrons refuse to share space with other electrons, but a photon or neutrino can pass right by.
The idea that a thing may be solid and occupy space makes sense to us because it seems true in the world of big things, but in the world of very small things it isn’t true anymore.
Light has a size (the wavelength), but the pieces of light are allowed to overlap with each other. So it exists in space, but doesn’t “take up” the space in the sense of preventing other light from being there too.
Insofar as this question makes any sense on a particle physics scale, the answer is “no”. Photons are an example of a *boson*, a type of particle that can have more than one of the same exact particle in the same place.
Normal materials “take up space” because of the electrical repulsion between the electrons in the material, though, not because there’s no more space for particles.
This is a very good question that I’ve never considered before. It really comes down to quantum mechanics and the wave particle duality. Light is simply a disturbance in the electromagnetic field, which permeates the entire universe and therefore all photons are everywhere at once until you observe it, thus collapsing the wave function (like Schrodinger’s cat). Of course the nature of the wave means there’s a location where the photon most likely is, and that can become quite a large area, but can always be collapsed down again by making an observation. However for a given photon, the probability that it is at an arbitrary point is never 0, no matter how infinitely small that probability is. The only way to get that probability to 0, would be to have a barrier it takes infinite energy to cross over, or else quantum tunnelling could take it through. Keep in mind, these non zero probabilities are really really close to 0. Small enough that a photon existing outside of where it is very likely to have been found probably hasn’t happened in the history of the universe. Acknowledging the possibility is more of a thought experiment than anything.
To look more into this, consider the double slit experiment with a single photon (also called single photon interference).
Schrodinger’s cat, but you’ve probably already heard of this one, but you can also look into the schrodinger wave equation and wave collapse if you want to get a little more advanced.
Quantum tunneling, which is basically proof that the peak of the wave equation isn’t necessarily where the particle is. And we know it exists because we’ve observed it and everything from basic chemistry to stars wouldn’t work if it didn’t exist. It was actually discovered because the wave equation predicted it.
This is some high level quantum mechanics shit and someone could probably get their PhD writing about the size of a photon. Sorry I couldn’t quite ELI5.
Light pushes objects all the time. However, the light that do the majority of the pushing are not detected by our eyes. The light that our eyes can detect have too low energy to do much pushing; but the amount isn’t really 0, just way too small to be noticeable. The light that do the bulk of the actual pushing is what we perceive to be “object occupying space”. When you push a wall and feel a resistance, a huge part of that is the light from the wall pushing against your hand.
Whilst all those explanations are correct I want to keep ELI5:
Take a puddle of water.
Drop a rock in it.
The waves don’t occupy space.
They move through it.
You can see the height at which they move the water and the distance between them but the wave itself does not occupy space.
Same with light.
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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