ELI5: Explain Like I'm 5

Rotations in 4-dimensional Euclidean space

Imagine you are holding a toy car and you want to show it to your friend who is standing next to you. You can rotate the car in different ways, like turning the wheels or spinning the car around. In the same way, in 3-dimensional space, you can rotate objects around different axes, like the x, y and z axes.

Now, let's take this to the next level and imagine we have a 4-dimensional space. It's like having a toy car that can move not only left and right, up and down, and front and back, but also in a fourth direction that we cannot see. To rotate an object in 4-dimensional space, we need to consider four axes instead of three.

Think about a shape in 4-dimensional space, let's say a cube. The cube has its vertices, edges, and faces just like a regular 3-dimensional cube, but it exists in a higher dimension. To rotate the cube, we need to use a 4-dimensional rotation matrix, which has 4 rows and 4 columns, instead of a 3-dimensional rotation matrix.

The rotation matrix tells us how much to rotate the object around each axis. In 4-dimensional space, we can rotate an object around any of the four axes by a certain angle. For example, we can rotate the cube around the x-axis, then rotate it around the y-axis, and the z-axis and finally the 4th axis. This will give us a unique orientation for the cube in 4D space.

One way to think about 4D rotations is to imagine a 3D object being projected onto a 2D surface, such as a piece of paper or a screen. When we rotate a 3D object, its projection on the 2D surface also rotates accordingly. Similarly, in 4-dimensional space, when we rotate a 4D object, its projection on the 3D space that we can see also changes accordingly.

In summary, rotations in 4-dimensional Euclidean space are like rotating objects around four axes instead of three. We use a 4-dimensional rotation matrix to determine how much to rotate the object around each axis. This allows us to change the orientation of objects in a higher-dimensional space, such as 4D, which is not possible in our ordinary 3D world.