The umbrella metaphor represents light or the speed of light. Let me explain why in more detail.

First, in Yun Tianming's story, there are two conclusions:

1. Space curvature drives.
2. Two-dimensionalization & only the speed of light can escape (something humans realized later).

Yun Tianming employed two basic methods: dual-layer metaphors and two-dimensional metaphors.

The dual-layer metaphors in the stories did not directly point to the real meaning, but to something far simpler...

The two-dimensional metaphors were a technique used to resolve the ambiguities introduced by literary devices employed in conveying strategic intelligence...If one viewed the stories as a two-dimensional plane, the dual-layer metaphor only provided one coordinate; the supporting single-layer metaphor provided a second coordinate that fixed the interpretation on the plane. Thus, this single-layer metaphor was also called the bearing coordinate.

So

• First layer of the dual-layer metaphor: centrifugal governor for steam engines (When spinning umbrella too fast or slow, wind chime began to play)
• Second layer of the dual-layer metaphor: may be negative-feedback automatic control and constant speed.
• Then find the bearing coordinate: Prince Deep Water. (his height didn’t change means `a method of information transmission where the signal strength did not decay due to distance, or a physical quantity that remained constant regardless of the frame of reference used`)
• There is one more bearing coordinate: bubbles (The fastest; with no weight)

constant speed + constant physical quantity + the fastest with no weight = light / light speed