Why does gravity seem so weak? Why can a small magnet attract or repel objects that are much larger than its own weight? There is a theory that may solve this mystery for us, but before we can do so, we need to explore some basic concepts.

The theory of quantum gravity, which is attracting much attention today, sees all elementary particles as vibrating strings, and these strings exist in a huddled high-dimensional space, not the three-dimensional space we experience every day. So, what exactly is the "dimension" of space?

The line is a geometric entity as we know it, it can be fully described by its length alone, and it is a symbol of one-dimensional space. Imagine that the trajectory formed by moving a point is a line. Then, consider a polygon, which has a length and width, and requires two measurement steps to determine its position, so it is two-dimensional. Similarly, we can form a polygon by moving the line in a direction perpendicular to itself.

In the same way, three-dimensional shapes such as cubes have their own generation principles. It has three dimensions: length, width, and height, and it takes three measurements to determine its position. By moving the square in a direction perpendicular to its plane, we get the cube. And things like cylinders, spheres, etc., are also examples of three-dimensional objects.

As the dimensions increase, it becomes more complex to understand. The concept of a curled high-dimensional space is mathematically feasible but intuitively unimaginable. So, how can we perceive these extra dimensions?

We can think in terms of the diffusion of light. In three-dimensional space, the brightness of a point light source decreases with the square of the distance from the light source because the area illuminated by the light increases with the square of the distance. However, this law only works in three-dimensional space.

在四維空間中，光的亮度會隨著距離的三次方而降低；在五維空間中，亮度與距離的四次方成反比。到了十維空間，亮度則與距離的九次方成反比。

Going back to string theory, assuming that this membrane represents the three-dimensional universe we live in, then particles like quarks and electrons can actually be seen as strings with both ends fixed to this membrane. However, the graviton, as the carrier of gravity, has the form of a closed circle with the ends not fixed within our three-dimensional space, but can move freely in additional dimensions.

This is probably why gravity appears so weak, because we can actually only feel a small part of the gravitational pull, and most of it has leaked into other dimensions. So, where exactly are these extra dimensions hidden?

One theory is that these dimensions are actually ubiquitous, but they are so tiny that we are difficult to perceive. Looking at a wire from a distance long enough, it appears to be just a one-dimensional line. But when you look closer, you see that it actually has an extra dimension.

At the tiniest scale, each point of space contains an additional dimension. If you can shrink it down to a small enough size, you will find that space is actually a complex multidimensional structure, which is exactly what "Kachu space" describes, named after the famous Chinese mathematician Yau Chengtong.

With equipment such as CERN's Large Hadron Collider (LHC), scientists can accelerate protons and antiprotons to extremely high energy levels that can produce gravitons in a collision. If this happens, we may be able to witness the formation of gravitons in three dimensions and watch them disappear quickly into those hidden extra dimensions.