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"Primordial gravitational waves?"

This time.

When he heard Chen Ning Yang bring up the concept, Huang Kun didn't look nearly as confused as before.

Instead,

a flicker of understanding crossed his face.

Gravitational waves.

The term should really be understood as two parts: "gravity" and "waves."

So why would gravity have waves?

The answer obviously wasn't that gravity was female. It was because spacetime had structure. The motion of matter we usually observed all took place within spacetime.

In a sense, matter could be thought of as the actors, and spacetime as the stage on which they performed.

For ordinary waves, such as water waves, sound waves, and electromagnetic waves, the actors moved while the stage remained still.

Gravitational waves, on the other hand, were the movement of the stage itself.

In Little Ox's Newtonian mechanics,

spacetime was an unremarkable stage, because time flowed uniformly and space extended uniformly.

No matter how much matter there was or how it moved, it had no effect on this stage. So there could be no waves. This was the absolute conception of space and time mentioned earlier.

But in Old Einstein's theory of relativity, the stage had very unusual properties.

In general relativity, Old Einstein's description of gravity became far more complicated than Little Ox's inverse square law. It took a rather roundabout form:

Mass caused spacetime to curve, and objects moving through curved spacetime appeared to be acted upon by gravity.

Imagine a flat sheet of paper in front of you. Its curvature was zero.

On this sheet, the interior angles of a triangle added up to 180 degrees, and the circumference of a circle equaled 2π times its radius, among other things. The theorems of Euclidean geometry—all the plane geometry you learned in middle school—held true.

If you deformed the paper,