if atoms are mostly empty space, then is the vacuum of space the same kind of emptiness? Or are they different?


Also, how can atoms be solid and opaque if they are mostly nothingness?

In: Physics

It’s more appropriate to say that atoms are mostly almost-empty space, with some areas of less-empty space.

Remember, at that scale, particles don’t hang out at specific points. They’re fuzzy clouds distributed across regions of space, in the sense that they are more likely to be in some areas than others. It’s true that atoms are much larger than the particles that make them up, but “empty”/”not empty” isn’t a sharp distinction the universe actually makes. Even the deepest vacuum isn’t totally “empty”, it’s full of the same fluctuating fields whose bumps we call “particles” (or to interpret this another way, particles are constantly appearing and disappearing even in high vacuum). An analogy here is that even a very calm ocean is moving and flowing, even though “waves” as distinct objects are mostly noticeable along the shoreline.

Atoms aren’t “solid” – that notion only makes sense for collections of atoms. It’s better to say that atoms are kind of “squishy” (in the sense that one atom can’t easily enter space very close to another), but that it takes quite a bit of force to squish them by very much. They’re opaque because their electrons interact with light – remember, opaque doesn’t mean “physically blocks movement”, it means “light can’t pass through”. At that scale, light is “as big as” anything else.

You are correct that atoms are mostly empty space, and also correct that the vacuum of space is also empty. That said, there’s a big difference between the two. If you imagine one cubic meter of air, there are about 2.7 * 10^25 molecules of air in that space, each one with about 1-3 atoms depending on what molecule it is. That’s 27,000,000,000,000,000,000,000,000 molecules (27 septillion). Even if each molecule only contains a small amount of matter, that’s still a whole lot of molecules contributing that small amount of matter. In contrast, one cubic meter of outer space contains, on average, about 10 atoms. That’s about as close to a true vacuum as you’ll ever find.

As to how atoms “touch” each other when they’re mostly empty space, the answer is that they don’t. The simplest way to put it is that the electrons orbiting an atom push each other away the same way that two magnets do. When you have a lot of electrons spread out over a small space, all pushing more and more they closer they get, it creates the feeling of touch. How the electrons interact with each other and how the substance is held together determines the way an object or substance “feels”.

That’s not an entirely accurate explanation, but it should kind of help you understand. Electron interactions are what create a lot of the properties we observe in different materials.

When you get down to the size of atoms, concepts like “space with matter in it” and “empty space” kind of break down. We can sort of measure the “sizes” of electrons, and atomic nuclei, but it’s also valid to think of them as single points in space that have certain properties, and whose charges affect a certain area of space.

Two solid objects don’t pass through each other because of repulsion between the electrons in both objects.* It’s got nothing to do with objects that exist in 3 dimensions hitting each other physically. We do “collide” particles in particle accelerators, but it’s more accurate to say that we shoot them close enough that they interact through fundamental forces. All basic collisions that we imagine, like a billiard ball hitting another, are Electromagnetic interactions.

Light, likewise, does/doesn’t pass through something because of how the atoms in the material interact with the photons (or light waves) hitting it. A pane of glass allows light to pass through it without changing its frequency (color) or scattering it because of its crystal structure – it doesn’t have less “stuff” than an opaque sheet of paper. Atoms in glass may absorb and re-emit that light a bunch of times, but it’s transparent to us because the light coming out still has mostly the same information as the light that went in. The paper just reflects (or absorbs and re-emits at a different frequency) most of the light hitting it, destroying whatever information the light had.

So…. the idea of matter as stuff that exists in otherwise empty space is really something we perceive at our scale, and not really something fundamental about existence. Matter and energy aren’t fundamentally different either, and most of the mass of your body comes from the energy holding your atoms together. Crazy, huh?

*To push the electron clouds of the atoms in your skin into the electron clouds of the atoms in the floor you stand on would require the kind of force that creates neutron stars. On the other end, neutrinos stream through earth and your body by the trillions all the time, affecting nothing because they have no EM charge and just don’t interact with your atoms.