Flat vs round explained. Without light being bent by gravity the fourth and third dimensions would be a consistent flat series of sheets overlapping one another. Without gravity bending light there would be no third dimension.

Here's an analogy I gave my students to help them understand (or to make things more confusing...)

A one dimensional universe would be like a line... That line can either extend forever (open), can have a fixed length with a beginning and end, or can or can loop back on itself in a circle (closed). An object could move along that closed loop, but if it travels far enough it'll eventually circle back to its starting point. But that one dimensional object cannot discern the curve in its linear universe and perceives that it is traveling along a straight line.

A two dimensional universe is a flat plane; That plane can extend forever in all directions, or have edges (I use a piece of graph paper to visualize this, since the students are used to graphs with a 0,0 origin that extend to infinity in all directions). Or, that plane could be curved into a 3D sphere... much like the surface of the earth.

If you set out from your home and travel in a straight line around the globe, eventually you can circle the whole world and return back home. If you've ever taken an intercontinental flight, or mapped one out, you might have encountered what's called the "great circle" concept: because maps of the 3D earth surface have to be distorted to project them in 2D, and because the surface of earth appears flat from our perspective, straight lines can be curved on maps. Fly in a perfectly straight line from Atlanta to Moscow and you'll start our traveling north, then east, and finally south, even though you never turned.

Even simpler, if you've ever looked at a BAF on the intel map you know that the fields have curves for that same reason...

To extend this into 3D, our universe either has an edge, or is infinite, or... loops back on itself in a 4th dimension. But just as a 1D being can only imagine life along a line, and a 2D being cannot think in 3D, our brains have no way to comprehend what this 4th dimension would actually be like. Just like it's very difficult to stand on the surface of the earth and perceive that we are standing on a massive sphere and not a flat plane, it's also very difficult to detect whether our universe has a 4D curvature. But if it does, we could set out from earth in a straight line and, eventually, end up back where we began.

## Comments

Is flatter than Earth!

`flat`

is something about 2-dimensional,`ball`

means the same but in 3-dimensional world.IMO our universe is quite like that but spread in 4 (or more) dimensions

PS

it can be kinda flat but still closed: https://en.wikipedia.org/wiki/Möbius_****

I thought it was doughnut shaped.

Flat vs round explained. Without light being bent by gravity the fourth and third dimensions would be a consistent flat series of sheets overlapping one another. Without gravity bending light there would be no third dimension.

Here's an analogy I gave my students to help them understand (or to make things more confusing...)

A one dimensional universe would be like a line... That line can either extend forever (open), can have a fixed length with a beginning and end, or can or can loop back on itself in a circle (closed). An object could move along that closed loop, but if it travels far enough it'll eventually circle back to its starting point. But that one dimensional object cannot discern the curve in its linear universe and perceives that it is traveling along a straight line.

A two dimensional universe is a flat plane; That plane can extend forever in all directions, or have edges (I use a piece of graph paper to visualize this, since the students are used to graphs with a 0,0 origin that extend to infinity in all directions). Or, that plane could be curved into a 3D sphere... much like the surface of the earth.

If you set out from your home and travel in a straight line around the globe, eventually you can circle the whole world and return back home. If you've ever taken an intercontinental flight, or mapped one out, you might have encountered what's called the "great circle" concept: because maps of the 3D earth surface have to be distorted to project them in 2D, and because the surface of earth appears flat from our perspective, straight lines can be curved on maps. Fly in a perfectly straight line from Atlanta to Moscow and you'll start our traveling north, then east, and finally south, even though you never turned.

Even simpler, if you've ever looked at a BAF on the intel map you know that the fields have curves for that same reason...

To extend this into 3D, our universe either has an edge, or is infinite, or... loops back on itself in a 4th dimension. But just as a 1D being can only imagine life along a line, and a 2D being cannot think in 3D, our brains have no way to comprehend what this 4th dimension would actually be like. Just like it's very difficult to stand on the surface of the earth and perceive that we are standing on a massive sphere and not a flat plane, it's also very difficult to detect whether our universe has a 4D curvature. But if it does, we could set out from earth in a straight line and, eventually, end up back where we began.