... like I'm 5 years old
When we talk about dimensions, we're referring to the different ways we can move or measure things in our world. Let's start with a point. A point has zero dimensions because it doesn't have length, width, or height. If we take a step forward from the point, we move along a line, which is a one-dimensional space. If we move sideways from the line, we create a square, which is a two-dimensional space. Likewise, if we move upwards from the square, we get a cube, which is three-dimensional. But what about the 4th dimension? To understand that, we need to think about time. Imagine the cube as a snapshot of time, like a photograph. If we take multiple photos of the cube over a certain period and stack them together, we get a 4-dimensional object, which has length, width, height, and duration.
Imagine you're reading a book. The words on the page represent a two-dimensional space. You can move left, right, up, and down on the page. The book itself represents a three-dimensional space. You can move through it by flipping the pages. Now, imagine reading the book from start to finish. That represents a journey through the fourth dimension, time.
... like I'm in College
The 4th dimension can be understood in two contexts: space-time and spatial. In Einstein's theory of relativity, the 4th dimension is time, combined with the three spatial dimensions to form a four-dimensional manifold known as space-time. This is the framework where all physical phenomena take place.
On the other hand, mathematically or in theoretical physics, the 4th dimension can be seen purely as a spatial dimension. Here, the fourth dimension is no different than the other three, just another direction in which one can move. This concept is quite abstract and difficult for us to visualize as we live in a three-dimensional world.
Imagine we're using Lego bricks to represent dimensions. A single Lego brick can represent a point. If we line up several bricks, we have a one-dimensional line. If we arrange them into a square, we have a two-dimensional plane. If we stack these squares, we create a three-dimensional cube.
To represent the fourth dimension, we would need multiple Lego cubes. Each cube would represent the same 3D space at different moments. So, if we imagine stacking several cubes that depict the Lego structure at different points in time, we've created a simplified model of the 4th dimension in terms of space-time. This is a helpful way to imagine how time can be considered as another dimension, just like length, width, and height.
... like I'm an expert
In the realm of theoretical physics and mathematics, the 4th dimension is often conceptualized as a spatial dimension, independent of time. This concept is fundamental to various theories such as string theory and M-theory.
The 4th spatial dimension is often described using the concept of a hypercube or tesseract, a four-dimensional analogue of the cube. It consists of an inner cube, an outer cube, and the eight lines connecting the vertices of the two. This is a projection of the tesseract into three dimensions, just as the drawing of a cube on paper is a projection of the 3D cube into two dimensions.