system
Given the following post, respond as if you're commenting on Reddit:
human
Does "Space" itself have any properties besides just being "empty"?..

...Outer space -- that which is outside of the relatively dense conglomeration of matter that tends to surround planets, moons, et al., but seemingly still within our own universe.

I've read something here recently that "empty" space [within the universe] has certain properties that allow the universe to be what it is, but I don't really know how to even begin to think about it.

For instance, space is still essentially 3-dimensional [physically], in that matter can collect into what we call "3D objects". Space allows light and other radiation to propagate and interact with other matter. Space allows a "medium" [for lack of a better term] for objects to become gravitationally attracted to one another. *Space seems to separate matter from itself?!*

I don't say any of those things as *assertions*, but they seem to be intuitively factual statements, and I'm aware of the danger of thinking that reality is not always what it seems.

What I'm asking is "what is space"?

Is there any area of research that deals with space itself, and for that matter, is "space" a term that has been quantified to the point where it can be objectively studied?
gpt
> Space seems to separate matter from itself?!

This is exactly correct. Exactly. We normally say it in slightly different words, even though it's precisely the same idea: Space is what separates events which occur at the same time.

Except you're also talking about something entirely else here. You said "outer space." That's not the same thing as what a physicist means when she says "space."

"Outer space" is a *volume* with certain *characteristic properties.* Outer space has a temperature. (In fact, it has a great many different ranges of temperature, depending on exactly what part of it you examine.) Outer space also has a density, and a pressure.

But those are functions of *stuff.* Consider some volume of the intergalactic void, say a cube a yard on a side. There will be, on average, about five protons in that volume, and a comparable number of electrons, and a vast number of photons, and so on. These things are *stuff,* and they give that volume its characteristic properties. Properties like temperature and pressure.

*Space,* on the other hand, is what you have when you take all the *stuff* away. The properties of space that a physicist is interested in are *geometric* properties. The metric tensor field, the scale factor, and such other bits of esoterica you've probably never heard of and couldn't possibly care less about.

So don't confuse *space* with *outer space.* Space is simple. It's that thing that separates events which occur at the same time. It has geometric properties, and those properties are vitally important if you're a physicist or, say, a person who dislikes skinning her knees because she was clumsy and fell down. *Outer space* is something else, and is really about matter and radiation and their properties in places far from where any of us are.