Light.
A marvelous phenomenon.
Its resting mass is zero, which is precisely its most wondrous attribute.
Because zero, as a number, contains infinite mysteries.
The speed of light is the speed of light—it sounds like a repetitive verification, but in reality, it has a more rigorous explanation within.
Firstly, the universe's spacetime permits a maximum value for the 'speed of causality transmission'.
And light, because its resting mass is zero, so its speed remains constant at this maximum value.
Thus, the speed of light equals the limits of the universe.
Yet fascinatingly, light with zero mass does not lack impact force.
It sounds peculiar—
An object with zero mass, no matter how fast, how could it possess kinetic energy, how could it have impact force?
But in fact, the reason is straightforward.
Light can carry energy.
Light with sufficiently powerful energy can even form 'radiation pressure', with force substantial enough to sustain vast celestial structures.
