Thinking in four dimensions is very very tricky. We are three dimensional beings and our reality is bounded in three dimensions. Imagine a one-dimensional civilization that lives on a line and cannot see left or right or up or down. Only straight ahead and straight backwards. In other words, their reality is composed of points on that line. Now try explaining a square to these folks.
Similarly, a two-dimensional civilization that lives on a plane and cannot see up/down will be perfectly fine with squares, but will have a horrid time imagining a cube.
We humans, being more awesome than the 2D people, can see up/down. I am looking up right now, and...eeek...a spider...
Ahem...moving right along.
So we can see up and down. But that is where the awesomeness stops. Our perception is limited to three mutually perpendicular axes: X, Y, and Z. It is impossible to sketch a fourth axis that is mutually perpendicular to all three. Hint: don't even try.
To add to the confusion, some people will tell you that Time is the fourth dimension.
Four-dimensional objects have nothing at all to do with time. Here is how I try to visualize four-dimensional things. Like I said, this is very very tricky. I can barely get my mind around the idea.
Here is one way.
Take a square piece of paper. This is a two-dimesional object to you, for practical purposes. Now hold this piece of paper perfectly flat and parallel to the floor and near a wall. The shadow of this paper on the wall will be a line, if you are holding the paper perfectly flat. We can say that the projection of a two-dimensional object is a one-dimensional object. 'Cuz we fancy like dat.
Similarly, the projection of a three-dimensional object is a two-dimensional object. Like the shadow of a sphere on a floor, or the shadow of your head on a wall. See below.
Taking that further, a three-dimensional object is simply the shadow or projection of a four-dimensional object.
The implication of this line of thinking is profound. It means that every object around us is situated in four dimensions and what we are observing, touching, and interacting with are the three-dimensional shadows of these objects. That cellphone you are clutching is a shadow of the "real" cellphone in 4D space. Indeed we ourselves are shadows of our four-dimensional selves. If we ever encountered a more endowed civilization who can see in 4D, they would see us very, very, very differently from how we ourselves see us. And we would only see their shadows.
Also, why stop at four dimensions? There may be an infinite number of dimensions each projecting onto a lesser dimension. That four-dimensional cellphone that we cannot see in 4D space, but only touch the shadow of in 3D space, is itself a shadow of its five-dimensional self. Reality is a bloody hall of mirrors. Most of that hall...we are effectively blind to, except through mathematics and imagination. And even imagination fails us here. Or fails me at least. I can barely think about four dimensions. Thinking in five dimensions? Forget it.
This isn't pop science-fiction either. There is indeed real mathematics around this. Serious people have serious jobs working with this shit.
Bizarre, no?
Here is another way to visualize four dimensions that I find helpful.
Wait, come back.
Imagine a perfectly flat and square piece of paper. It has four edges. So four lines make up the four faces of a square; you can think of it like that. A cube has six square faces. Generalizing that, we may say that every N-dimensional object has a certain number of (N-1)-dimensional faces.
So it goes that a four-dimensional object has eight three-dimensional faces. Now imagine an object with eight cubic faces, and you have just imagined a four-dimensional object.
Unhelpful Note: In case you are inclined to think so, this is not as easy as imagining a cube with eight other cubes hanging off it like geometrically perfect tumors. That is not a four-dimensional object.
Laters.
Imagine a perfectly flat and square piece of paper. It has four edges. So four lines make up the four faces of a square; you can think of it like that. A cube has six square faces. Generalizing that, we may say that every N-dimensional object has a certain number of (N-1)-dimensional faces.
So it goes that a four-dimensional object has eight three-dimensional faces. Now imagine an object with eight cubic faces, and you have just imagined a four-dimensional object.
Unhelpful Note: In case you are inclined to think so, this is not as easy as imagining a cube with eight other cubes hanging off it like geometrically perfect tumors. That is not a four-dimensional object.
Laters.



