In front of the blackboard.

After listening to Xu Yun's speech.

Old Su slightly bent down, picked up a small pebble from beside him, and released it after picking it up.

A soft plop was heard.

The pebble naturally fell back to the ground.

"Gravity..."

Then Old Su repeated the word, with a thoughtful expression:

"The name is quite interesting, but Xiaowang, how should the 'heavy' in gravity be explained?"

Xu Yun thought for a moment and then said the answer he had prepared long ago:

"The 'heavy' in gravity refers to the weight of an object.

That is to say, any object with mass, no matter how small, will generate gravity along with it."

Old Su squinted his eyes slightly, simulating in his mind by replacing the stone with sand grains.

After accepting the concept, he continued to ask:

"So, Xiaowang, where does gravity come from?"

Upon hearing this question, Xu Yun's expression became slightly solemn, and he said:

"It is said that a long time ago, a sage named Shi Hao from the Wind Spirit Moon Shadow sect was extraordinarily wise from a young age.

He discovered that in all things in the world, every object with mass exerts a force on other objects with mass.

He named this force universal gravitation, and gravity is a component of this gravitational force on Earth.

Gravity acts vertically downward, causing all objects to freely fall once they are unsupported."

After saying all this.

Xu Yun looked at Old Su, fearing that this master would ask, "What is the essence of universal gravitation?"

After all, the essence of universal gravitation is a concept that even future generations have never had a definitive conclusion on.

As is well known.

Although Little Niu discovered universal gravitation, what he actually found was the mathematical definition of it, rather than the physical concept of universal gravitation.

The physical concept's understanding had to wait until Einstein proposed the theory of relativity, only then did the world gain a further understanding of universal gravitation:

The essence of universal gravitation is the curvature of space—or rather, it is not only space but the four-dimensional spacetime that is curved.

According to the theory of relativity.

Spacetime is a whole, and using physics terminology, flat spacetime is called "Minkowski space."

Curvature means the metric of the flat Minkowski space has changed.

At the same time.

General relativity posits that it is the mass of materials that causes spacetime to curve.

And objects move along the shortest path in four-dimensional spacetime (geodesic motion), their movement form appearing as universal gravitation.

Let's take Xu Yun's previous example.

Xu Yun lands.

Little Niu says:

Oh, that's because Earth's gravity pulls Xu Yun to the ground.

Einstein says:

No, it's because Earth's mass causes the surrounding four-dimensional spacetime to curve, so Xu Yun falls down.

Xu Yun (in physics terms, called a test particle) falls from a tree, and from the perspective of four-dimensional spacetime, Xu Yun travels the shortest distance from the tree at a certain time to the ground at a certain time.

This means Xu Yun is moving along the four-dimensional geodesic.

The projection of this four-dimensional geodesic movement in three-dimensional space is Xu Yun's falling trajectory from the tree.

Therefore, in later generations, the explanation of universal gravitation in the three theoretical mechanics is completely different.

In Little Niu's classical mechanics, gravity is considered a basic force generated by mass.

Einstein's general theory of relativity attributes gravity to spacetime curvature.

Quantum mechanics suggests gravity arises from the exchange of gravitons between substances.

Which viewpoint is right or wrong is unknown, and debates continue to this day—Science magazine, on its 125th anniversary in 2018, still listed this issue as one of the 125 most challenging scientific questions.

Indeed.

Just like bicycles, it's a topic mentioned in middle school physics that still lacks a conclusive answer in modern science.

Among the three points above, classical mechanics' qualitative range of gravity is relatively limited, and it often can't even be applied.

So, some people always claim that general relativity has negated classical mechanics, which is quite an unreasonable statement.

Even in a novel written by Xu Yun in a previous life, where the protagonist time travels and meets Little Niu, there are comments like 'Why look for Newton, classical mechanics is nonsense.'

In reality.

The way to judge whether classical mechanics is correct is actually very simple—just see if it has real-world applications.

It allows you to build planes, drive tanks, construct buildings tens or even hundreds of meters tall, and dive very deep underwater.

It can be applied in various fields of our macroscopic world, so it is very successful.

At least in the macroscopic world we can currently observe, you can absolutely trust it.

Remember the pendulum experiment:

Replace the pendulum with a beheading knife, then release it.

Put your neck 0.01 cm higher than the original position; would you dare to do it?

Anyway, Xu Yun dared and actually did it, using an iron ball the size of a human head.

Compared to classical mechanics, relativity and quantum mechanics discuss a broader scope.

That is the high-speed micro world, often involving powers of ten to the eight or nine, so classical mechanics doesn't work well.

In summary.

Each of the three has its adaptation fields, and classical physics remains strong and necessary in life.

Considering the Northern Song Dynasty period, it might still be possible to understand the micro world, but to understand relativity and quantum mechanics... well...

Unless Old Su can survive poetically to 130 years old, otherwise it's simply impossible.

Therefore, Xu Yun's idea was to discuss gravity within the domain of classical mechanics, conveying knowledge based on classical mechanics as the foundational framework.

Returning to reality.

Perhaps because he didn't have a direct concept of gravity.

After listening to Xu Yun's explanation, Old Su didn't further explore the origin of this 'force.'

Instead, he was filled with a new sense of wonder.

He opened his palm, waved it beside Xiaozhao, then looked at Xu Yun:

"Xiaowang, according to you, there is a gravitational force between objects.

Then why do I not feel any attraction even though Xiao Zhao and Xiaoli are so close?"

Xu Yun looked up at Xiaoli and Xiaozhao, who appeared a bit confused, and smiled as he explained:

"Master, according to Shi Hao, universal gravitation is the weakest class of force in the world.

This force is hard to detect, even less perceptible than a summer mosquito bite, so it's normal for you not to feel it."

Hearing Xu Yun's explanation, Old Su hadn't reacted yet, but Xiaozhao, who hadn't spoken much, couldn't help but say:

"Mr. Wang, since this so-called 'gravity' is too small to detect, how do you know it truly exists?"

Xu Yun immediately gave an approving look to this backup singer and then pointed to a simple device assembled beside him:

"Of course, it's through this."

This set of equipment is about forty or fifty centimeters high, made from tools found by Mr. Xie.

Earlier, when Old Su was trying out chalk, Xu Yun was tinkering with this thing.

And it was quickly assembled because its materials weren't too complicated:

The main part of the equipment is a torsion balance that is not exactly like the ones from the later era, but serves a similar function—yes, a torsion balance:

The prototype of the modern torsion balance was created by Father Michel of the Royal Society of the United Kingdom in the 16th century, optimized in the 18th century, and finalized as the modern torsion balance.

But before that, various ancient civilizations had relatively primitive torsion balances.

For example, the torsion balance invented by Old Su during the Song Dynasty.

At the time, Old Su, while supervising the Water Transporting Celestial Observation Platform, designed a classical torsion balance for calculating torque.

His tool diagram was also included in the "Shao Sheng Astronomical Instrument Manual."

And at this moment.

This torsion balance was attached to a stand by a strong steel wire, with two iron balls, one large and one small, placed at its ends.

There was a small mirror on the steel wire, not very precise but capable of normal reflection.

Indeed.

Seeing up to this point.

Students whose physics teachers hadn't fainted might have already guessed.

The experiment Xu Yun was about to conduct is the famous...

Cavendish torsion balance experiment.

....

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