## How can the universe be "flat"? We're 3D! (Beginner)

1. You say the that the observational universe is flat how can this be if we live in 3D?

2. I cannot wrap my brain around the meaning of a FLAT universe. I've read hundreds of definitions on the thing and cannot make sense from it. The ant on the balloon does not make sense to me because there is space above and below the ant on the surface of the balloon. If the universe is physically flat, then what is above and below it? I have tried and tried to understand what 'flatness' means but it never makes sense. Space and the things in it are found in every direction so how is the universe flat? Is it flat like a coin or a tyre or flat like a sheet of atoms? Does it actually just mean that if the universe was filled with adjacent cubes, all the corner angles of all the cubes would equal 90degrees? or something like that?

It's great when our readers answer their own questions! #2 is totally correct - when we say the universe is flat it is not in the same sense that a piece of paper is flat, but rather means that the geometry of the universe is such that parallel lines will never cross, the angles in a triangle will always add up to 180 degress, and the corners of cubes will always make right angles. We call this kind of geometry (the kind you learned in school) Euclidean geometry.

It's easy to make examples in 2D space (ie. a flat piece of paper vs. a curved piece of paper, or the surface of a balloon). It's not so easy to illustrate flat 3D space - since we are 3D! So it's totally understandable that the concept is confusing

This is refering to the theory that the whole universe is flat. I believe that the universe as a whole likely has some three-dimentionality to it, but I'm curious as to why it is that asteroid belts, the milky way, and even the planets in our solar system all seem to be flat and fall in line with one another. For instance why isn't mars above us, and why is it that all the planets seem to be spaced out and lined up.

The orbits of the planets and the shapes of spiral galaxies has to do with the way they form and conservation of angular momentum, and nothing to do with the geometry of the universe as a whole. As the solar system formed from a giant spherical gas cloud, the cloud started to rotate. That meant that it was easier for material to fall in along the poles than around the 'equator', because around the equator it had to battle against the centripetal acceleration trying to push it out again. The cloud therefore slowly collapsed into a disk like structure from which the planets formed. The same exact process explains the disks of spiral galaxies.