From 3, 4, and 5, a final conclusion can be drawn:

6. This reference frame can only be relative to space itself, thus this objectively existing space is absolute space.

According to the original trajectory.

Little Niu had no doubts about the results of this experiment and thus proposed the concept of absolute spacetime, which became the biggest stain for later critics attacking him.

Everyone should know what happened afterward.

Einstein proposed the theory of relativity, suggesting that spacetime is one entity, and the two are inseparable.

In Einstein's spacetime view.

All things move in spacetime at the same speed, which is light speed.

Move faster in space, slower in time; the total speed always remains at light speed.

The structure of spacetime is a gravitational field, inseparable from each other and also inseparable from the material field,

Space tells matter how to move, matter tells space how to curve.

If compared from a mathematical perspective, Newton's spacetime view is undoubtedly R^3xR.

Which is a flat three-dimensional Euclidean space combined with a one-dimensional time unrelated to it, a very simple structure.

Einstein's spacetime is M^4, four-dimensional curved Riemann space including three-dimensional space and one-dimensional time.

You can imagine it as a four-dimensional curved "surface", binding time and space together; its metric tensor is the gravitational potential.

It's simple, right?

Then Xu Yun thought for a moment.

Instinctively, he planned to take out paper and pen from the backpack beside him.

But soon he laughed at himself—in this space, where would paper and pen come from?

Just as he was preparing to return to reality, suddenly in front of him appeared...

A ballpoint pen and a sheet of paper.

"..."

Xu Yun took the paper and pen, glanced around space a few times, and shouted:

"Boss, how about a table and chair too?"

After a moment.

Whoosh—

A table and chair appeared in front of Xu Yun, with a large sticker of Jia Ran prominently pasted on the front right of the table.

Xu Yun: "..."

This space indeed has issues, doesn't it?

Then he shook his head, forcing himself to eliminate distracting thoughts.

Sitting in the chair, he started to write a reply to Little Niu:

"Dear Mr. Newton,"

"Mr. Newton, long time no see, I am Fat Fish."

"If you still have the drafts I originally wrote to you, you should be able to confirm my identity through handwriting comparison."

"I am very pleased to receive your letter after several years, but I want to first explain to you that in recent years, I have suffered from severe asthma, arranged by my family to vacation in a remote place, and only recently gradually recovered."

"I am currently traveling in Europe, if you need to contact me, you can send letters to the original address.

"My assistant will forward the letters to me, but due to travel issues, my response time may be delayed."

"As for Guinea's dividends, I will leave them with you for now."

After writing this.

Xu Yun paused, reviewed the content once again.

Once ensured all casual matters were addressed, he continued writing:

"The mathematical tools you invented are truly admirable, a height I am far from reaching independently, please allow me to extend my utmost respect to you."

"However, as for the barrel experiment you mentioned, I have another view."

"I can roughly understand your thought process, at first glance it also seems quite ingenious, with six increasing ideas altogether."

"But have you thought about... making a mistake in your thought process on the fifth step?"

Indeed.

In the six derivations created by Little Niu, a fatal flaw appears in the fifth step.

In the fifth step.

Little Niu viewed water, the barrel, and space as one system.

But in reality.

The correct combination of this system should be water, the barrel, and all other matter in the universe.

The inertia of water is the result of interaction among water, the barrel, and all other matter in the universe.

In other words.

The angular velocity Ω of this body of water rotates relative to all other matter in the universe outside the water.

Then Xu Yun paused briefly and attached his rebuttal thought process to the letter as well:

"Let's assume, this universe only contains this barrel of water and this barrel."

"Then the phenomenon of your experiment would become like this: initially, the relative motion of water in the barrel is maximum, the surface immediately becomes concave."

"As the relative motion between water and barrel decreases, the surface gradually becomes flat. The angular velocity calculated at the beginning of the experiment is the speed of the body of water relative to the barrel."

"In other words, your so-called absolute space in the absence of other matter in the universe is actually the barrel's reference frame."

"Next, let's assume we are still in our universe, but the mass of our barrel is rooted... somewhat small, about Earth-sized."

"Then as the barrel starts to rotate, the water surface will also immediately start to change."

"Then the central mass's spin angular momentum and the circumferential mass's orbital angular momentum would conform to a certain function, namely the total rotational inertia of all matter in the universe equals zero."

Friends who have undertaken graduate-level physics should know.

Xu Yun's explanation thought process is very friendly to beginners and is intrinsic reference point theory.

That is, setting the reference frame in the target area's boundary effect region, all entities within the region relatively stationary to this reference frame are defined as this region's intrinsic reference point.

The intrinsic reference point as a whole has common motion, and the accelerations produced by various external forces are also identical.