Chapter 132 Newton: So love will disappear, right?
in front of the blackboard.
After listening to what Xu Yun said.
Lao Su bent down slightly, picked up a pebble from beside him, picked it up and let go.
Just listen to a click.
The stone fell back to the ground naturally.
"Gravity."
Then Lao Su repeated the word with a thoughtful expression:
"The name is quite interesting, but Xiao Wang, how should we explain this heavy word "gravity"?
Xu Yun thought for a moment and said the answer he had prepared long ago:
"The heavy word gravity means the weight of an object.
That is to say, any object with mass, no matter how small it is, will generate gravity. ”
Lao Su's eyes blurred, and he simulated it in his mind by replacing the stones with grains of sand.
After accepting this concept, he continued to ask:
"So Xiao Wang, where does gravity come from?"
Hearing this question, the expression on Xu Yun's face could not help but become a little solemn, and he just heard him say:
"It is rumored that a long time ago, Fengling Yueying Sect gave birth to a sage named Shi Hao, who was extremely smart since he was a child.
He discovered that among 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 gravity on Earth.
Gravity is vertically downward, so that all objects will fall freely after being separated from the support. ”
Having said this.
Xu Yun couldn't help but look at Lao Su, fearing that the boss would ask again, 'What is the nature of gravity?'
After all, the nature of gravitation is a concept that has never been concretely determined by later generations.
As everyone knows.
Although Maverick discovered gravity, what he discovered was actually the mathematical definition of gravity, not the physical concept of gravity.
The answer to physical concepts had to wait until Einstein proposed the theory of relativity, and then the world would have a better understanding of universal gravitation:
The essence of universal gravity is the curvature of space—or in other words, it is not just space that is curved, but the four-dimensional space-time.
For the theory of relativity.
Space-time is a whole, described in terms of physics, and flat space-time is called "Minkowski space".
The meaning of curvature is that the metric of the straight Minkowski space has changed.
at the same time.
General relativity believes that it is the mass of matter that causes space-time to curve.
The object moves along the shortest path of four-dimensional space-time (geodesic motion), and its motion is manifested in the form of universal gravitation.
Let’s take Xu Yun as an example.
Xu Yun landed.
Mavericks said:
Oh, this is because the earth's gravity pulled Xu Yun to the ground.
Einstein said:
No, it was the mass of the earth that curved the four-dimensional space-time around the earth, so Xu Yun fell.
Xu Yun (called test particle in physics terminology) fell from the tree. From the perspective of four-dimensional time and space, Xu Yun walked the shortest distance from a certain point on the tree at a certain time to a certain point on the ground at a certain time.
In other words, Xu Yun is moving according to the four-dimensional geodesic.
The projection of this four-dimensional geodesic motion in the three-dimensional space is the trajectory of Xu Yun's fall from the tree.
Therefore, in later generations, the explanations of gravitation in the three mechanics are completely different.
In Maverick's classical mechanics, gravity is considered a fundamental force produced by mass.
Einstein's general theory of relativity believes that gravity is caused by the curvature of space-time.
Quantum mechanics believes that gravity is generated by the exchange of gravitons between matter.
It is unknown whether the above views are right or wrong, and they are still being debated today - "Science" celebrated its 125th anniversary in 2018. This question was also classified as one of the 125 most challenging scientific questions.
That's right.
Just like bicycles, this is an issue that will be mentioned in junior high school physics, but the scientific community still has no specific conclusion.
Among the above three points, the qualitative scope of classical mechanics for gravity is relatively limited, and in many cases classical mechanics is not even used.
Therefore, some people always claim that general relativity denies classical mechanics. This is actually a very rogue statement.
Even in a novel written by Xu Yun in his previous life, the protagonist met Mavericks in time travel, but some people commented, "Why are you looking for Newton? Classical mechanics is nonsense."
Actually.
The way to judge whether classical mechanics is correct or not is actually very simple - just see whether it has practical value.
It allows you to build airplanes, drive tanks, build tens or even hundreds of meters tall buildings, and dive very deep into the water.
It can be used in various fields of our macro world, so it is very successful.
At least in the macroscopic world we can see now, you can absolutely believe it.
Do you still remember the pendulum experiment:
Replace the pendulum with a beheading knife and release it.
Then put the neck at its original position and raise it 0.01 cm higher. Do you dare to do it?
Anyway, Xu Yun dared to do it, and he actually did it, but he used an iron ball as big as a human head.
Compared with classical mechanics, relativity and quantum mechanics discuss a broader category.
That is to say, in the high-speed microscopic world, it is always a matter of ten to the power of eight or nine, so classical mechanics is not easy to use.
all in all.
Each of the three has its own applicable field, and classical physics is still strong and necessary in life.
Considering that during the Northern Song Dynasty, it might be possible to understand the microscopic world, but if you want to understand the theory of relativity and quantum mechanics
Unless Old Su could recite poetry until he was 130 years old, it would be impossible.
Therefore, Xu Yun's idea is to discuss gravity in the category of classical mechanics, and all knowledge will be taught according to the basic framework of classical mechanics.
His eyes returned to reality.
Maybe it’s because I don’t have an intuitive concept of gravity.
After listening to Xu Yun's explanation, Lao Su did not delve further into the origin of this 'power'.
Instead, there was a new wave of doubt.
I saw him open his palm, put it next to Xiao Zhao and wave it, and then looked at Xu Yun:
"Xiao Wang, according to what you said, there will be a gravitational force between objects.
So why am I not aware of any attraction when we are so close to Xiao Zhao and Xiao Li? ”
Xu Yun looked up at Xiao Li and Xiao Zhao who were a little confused, and explained with a smile:
"Master, according to what Shi Hao said, gravity is the weakest force in the world.
The force is difficult to detect, even less tactile than a summer mosquito bite, so it's normal that you don't feel it. ”
Hearing Xu Yun's explanation, Lao Su didn't respond yet, and Xiao Zhao, who had been silent until now, couldn't help but speak:
"Your Majesty, since this so-called 'gravity' is too small to be detected, how do you know that it actually exists?"
Xu Yun immediately cast an approving look at the admirer, then he pointed to a assembled simple prop beside him and said:
"Of course it depends on this."
This set of props is about forty to fifty centimeters high and was also made using tools found by Mr. Xie Duguan.
When Lao Su was trying chalk, Xu Yun was tinkering with this thing.
And since it can be assembled in a short time, the materials of this prop are naturally not that complicated:
The main body of the prop is a twisting scale that is different from later generations, but has similar functions - yes, twisting scale:
The prototype of the modern torsion scale was produced by Father Michael of the Royal Society in the 16th century. It was optimized in the 18th century and finally became the modern torsion scale.
However, before that, various ancient civilizations had some relatively primitive torsion scales.
For example, the torsion scale invented by Lao Su in the Song Dynasty.
When Lao Su was supervising the production of the water transport instrument platform, in order to calculate the torque, he designed a classical torsion scale that could be precisely calibrated.
The tool diagram was also drawn by Lao Su in "Shao Sheng Yi Xiang Fa Yao".
And at this moment.
This torsion scale is tied to the bracket through a very tough steel wire, and two iron balls, one large and one small, are placed at both ends of the torsion scale.
There is a small mirror on the steel wire that is not very precise but can reflect normally.
That's right.
See here.
The physics teacher’s classmates who were not mad to death must have guessed it.
The experiment Xu Yun will conduct this time will be famous in later generations
Cavendish torsion balance experiment. (End of chapter)