A Man of Science and Engineering in Troubled Times Wandering in the Late Qing Dynasty

Chapter 297 Subversion
After the simple certificate awarding ceremony, Michelson said to Li Yu: "According to tradition, we will conduct a physics seminar with new members."

Li Yu said: "No problem, please, President."

Michelson thought for a while and said, "If not, just use the field of optics that I am good at. Your Excellency must know that the fact that the speed of light remains constant in today's physics circles has not been able to explain it, and even Lord Kelvin, the British physicist, called it a A dark cloud."

It's a fitting question for Michelson to ask, since his Michelson-Morley experiment brought this dark cloud.

Li Yu said: "It is indeed a terrible dark cloud, because the constant speed of light cannot be explained by classical mechanical theory."

Michelson said: "The problem is exactly here. Many people can faintly feel that there seems to be some mystery of nature hidden behind it, so many people try to explain or prove it theoretically."

However, Li Yu said: "I think the constant speed of light cannot be explained or proved at all, because this is itself the most basic physical quantity, just like the Boltzmann constant."

Michelson said: "But according to Mr. Maxwell's equations, the speed of light can be derived."

Michelson's derivation is to use Maxwell's equations to calculate the speed of light.

The process involves some advanced mathematics content, but overall it is not particularly difficult, and students from the physics department can complete it without a special mathematics department.

After all, the physics majors of later generations are inseparable from important mathematical tools.

In short, after a series of substitutions such as tensor and curl, a very simple expression for the speed of light can be obtained:
C=1/εμ.

(Limited by the format, the denominator has a root sign that is not typed, but it does not affect understanding.)
In short, the final speed of light can be determined by only two constants, namely, the vacuum permittivity ε and the vacuum permeability μ.

Since the latter two are constants, the speed of light is naturally also a constant.

The distance between Maxwell and special relativity is only 0.1 mm.

But on the eve of the birth of the special theory of relativity, Maxwell failed to understand why the speed of light became a constant, which is a super difficult problem.

But in fact, modern people understand that in a strict sense, we cannot say that Maxwell’s equations deduced the speed of light or proved the principle of invariance of the speed of light. The above derivation only gives the relationship between the speed of light and the electrical conductivity and magnetic permeability.

So Li Yu said: "Mr. President, as an excellent physicist, you must know that many of the most basic physical principles since ancient times cannot be proved."

"Can't prove it?" Michelson originally wanted to refute, but after all, he has been immersed in the measurement of the speed of light for too many years, and he knows that there is nothing wrong, and perhaps the unprovable truth is the answer.

Li Yu continued: "In Mr. Conan Doyle's "The Detective Collection of Sherlock Holmes", Holmes said to Watson that when you eliminate all impossible situations, the rest, no matter how unbelievable, are all unbelievable." Is the truth."

Those who have watched related movies or TV dramas must be very impressed. It is cool to say this sentence in English.

Michelson said: "I have read this excellent collection of mystery novels, and your quotation is quite reasonable. If, as you say, the constant speed of light cannot be proved, then it is not an axiom or a postulate?"

Li Yu said solemnly, "That's right!"

Michelson sucked in a breath. The revolutionary Fifth Postulate in the history of mathematics was right in front of him, and he had to express his great shock.

The so-called postulates are basic principles that do not need to be proved.

Euclid used five postulates to deduce the huge Euclidean geometry, which is known as classical geometry. The five postulates seem to be very basic:

①A straight line can be drawn from any point to any other point
② A finite line segment can continue to be extended

③A circle can be drawn with any point as the center and any distance

④ All right angles are equal to each other

⑤ A straight line intersects two other straight lines in the same plane, if the sum of the two interior angles on a certain side is less than the sum of two right angles, then the two straight lines intersect on this side after being infinitely extended.

The problem is the fifth postulate.

Because it is obviously too long compared with the first four.

Even Euclid himself was dissatisfied with it and tried to prove it, but in vain.

——Don’t think that mathematicians are just looking for something to do. The establishment of mathematics has always been the most rigorous, and there is no sloppy.

Later, mathematics was axiomatic, and even a long process of proving 1+1=2 was carried out.

Note that it is "later", that is, at least at the end of 1903, no proof has been given.

This matter is more complicated. To put it simply, Russell and his teacher Whitehead began to write "Principles of Mathematics" 7 years later, to solve various paradoxes and then explain the axioms with logic.

At first they made a big detour, proving that 1+1=2 took 379 pages!

This book looks like a heavenly book.

Of course, there is no need to read, because they have too many twists and turns, and the language is too long-winded.

Many people complained about it, and Gödel's incompleteness theorem came out after that, which basically declared the failure of Russell's theory.

But people always tried to explore.

In fact, people in later generations can use the Peano axiom system to prove that 1+1=2 in two pages.

Going back to the fifth postulate, it actually not only deeply affected the mathematics world, but also had a profound impact on the physics world, especially the birth of the theory of relativity, so I have to say.

Because the fifth postulate directly leads to the birth of non-Euclidean geometry.

Russian mathematician Lobachevsky and German Riemann completed their own deduction of non-Euclidean geometry.

The two are not quite the same, but the process is a revolutionary event in the history of mathematics.

In short, once the basic axioms or public postulates are wrong, the entire mathematical building will be revised, and then a more magnificent building will be born.

The world of mathematics has already undergone such a massive change.

As a forerunner, mathematics has completed the revolution, and disciplines such as physics can use mathematical tools to carry out the revolution.

Because the theory of relativity is based on Riemannian geometry, that is, non-Euclidean geometry.

Later, the theories such as "gravity causes space-time bending" that people are familiar with are inseparable from Riemann's geometry.

In addition, gravity is also a seemingly best-understood pit, but it is a super bottomless pit.

Even Einstein stumbled on this in his later years, which directly made it impossible to complete the so-called "grand unified theory".

Because Einstein went in the wrong direction, gravity is actually the hardest and should be included last.

But no wonder Einstein, because at that time people did not know the strong and weak forces among the four major forces.

The relationship is a bit complicated, so I will talk about it slowly later.

Anyway, now Michelson instantly understood the meaning of Li Yu's words: "You mean, the constant speed of light should be used as an axiom for huge deduction?"

Li Yu nodded: "Compared to the two constants of electrical conductivity and magnetic permeability used in deriving the expression of the speed of light from Maxwell's equations, the speed of light in vacuum is a more fundamental physical quantity. Of course, the greatness of Maxwell's equations cannot be denied. It uses A simple way to suggest to us that the speed of light may be constant."

Li Yu said it a little conservatively, because it was later proved that Maxwell's equations are also applicable to the system of relativity.

That's why it is said that Maxwell almost touched the threshold of special relativity, and he was really at the door.

Michelson is different from conventional physicists, he himself is a staunch supporter of the constant speed of light.Who told him to measure the speed of light all his life, no one understands better than him that no matter how the experiment is done, the speed of light remains the same.

This is the speed of light in a vacuum, because it is well known that the speed of light in water slows down.

The reason is simple: the propagation of light itself does not require a medium, and the medium slows down the speed of light.

Michelson pondered for a while, and said: "I have read your books, and all your papers are rigorous mathematics and profound physical explanations in parallel. Today, you say that the speed of light cannot be deduced mathematically, which makes me feel a chill in my heart."

He believed in Li Yu's ability, but what Li Yu said today was too informative.

Li Yu said: "It's nothing surprising. There are too many principles in physics that are not deduced by derivation, but obtained through experiments. Since the experiments are not wrong, it means that the mysteries of nature are like this. Just like apples will fall from the tree. drop down."

Michelson was a little stunned: "I understand what you mean. An apple falling from a tree inspired Newton to discover the magnificent three laws of mechanics; and people did not want to prove why the apple fell from the tree. The speed of light today is not The same goes for changing, no need to prove."

"Sure enough, it's the president! That's the reason," Li Yu said. There are probably very few people in the world who can accept this point of view calmly. He then said, "So even if you ask me why the speed of light is constant, I will only I regret to answer three words, 'I don't know'."

Michelson smiled: "You are very frank."

Li Yu spread his hands: "I can only answer this way. In the final analysis, the constant speed of light is the result of the experiment, not a theoretical deduction."

At least until Li Yu crossed over, why the speed of light remained unchanged, humans really don't know.

To put it more bluntly, it is a kind of artificial regulation, because the experiment is like this.

If one day the experiment measures that the speed of light can change, it is estimated that more than half of the entire physical world will collapse at that time.

Because this kind of subversion is like falling an apple on the earth and suddenly flying into the sky.

At that time, the entire physical theory has to be rebuilt.

Physics, that’s it. With experiments and then to explain phenomena, you can’t doubt that there are mistakes in nature.

But this kind of thing should be impossible, at least not in the current universe. ,

(Unless sophons from Trisolarans intervene.)
Michelson said: "I feel more and more how wise it is to let you join the Physics Society. We met briefly at the White House dinner last time, and today we discussed in depth. I admire your knowledge of mathematics and physics."

Li Yu laughed and said, "It's five-body projection."

Since the publication of his Star Wars series, Li Yu found that many people like to say some "Chinese proverbs", but idioms are really bad.

Michelson said: "I will organize the conversation today and publish it in the journal tomorrow."

-

Kitty Hawk, North Carolina, USA.

Many people's first impression of North Carolina should be from Jordan.

Jordan came out of the University of North Carolina team, and now he is the owner of the Charlotte Hornets in North Carolina.

At this time the cold wind is already relatively bitter.

When Li Yu appeared on the empty beach with a few people, the Wright brothers saw them at a glance.

Because there were very few people present today, except for Li Yu and his party, only five people were watching.

The Wright Brothers rushed over excitedly: "Mr. Li Yu, why are you here?"

Li Yu smiled and said, "I've said it all, it's up to you to fly into the sky with my engine."

Brother Wilbur Wright said: "It's really unexpected that you, as the world's top scientist, would be optimistic about our flight test."

Li Yu basically told them the story of Wanhu.

Brother Orwell Wright said: "It turns out that the Chinese had such a dream hundreds of years ago. I always thought that it was Mr. Da Vinci who had the idea of ​​flying at first."

Leonardo da Vinci is also an all-rounder. He did design a flapper in his manuscript, and even designed a helicopter and a parachute.

Feel like a time traveler.

Newton had a famous saying: "If I have seen further than others, it is because I stand on the shoulders of giants."

Although this sentence was originally aimed at Hooker, it does make sense.

Same goes for the Wright Brothers.

In fact, in the decades before the Wright Brothers, many people have done a lot of work on aircraft structure, lift and drag research, stability and control, gliding flight, etc., and achieved many results.Without their pioneers, the Wright Brothers could not have succeeded.

The Wright brothers showed Li Yu a tour of their aircraft "Aviator 1" with great interest.

This plane does not look small, with a wingspan of more than 12 meters, which is about [-] or [-] meters for modern fighter jets.

However, the take-off weight is only a poor 360kg.In comparison, the take-off weight of the J-15 or F18 is as high as more than 30 tons.

If it is a civil aviation airliner such as a Boeing 737, it will be higher, directly exceeding 50 tons.

Brother Wilbur Wright looked at the weather: "It's time to start."

The younger brother Orwell Wright took out a coin, and the two finally decided that the younger brother Orwell would drive first by guessing the coin.

Orwell climbed onto the lower wing of the plane, holding the wooden joystick in his hand, and said to Wilbur, "Brother, start it!"

Yes, this plane cannot be started by one person, Wilbur needs to start the engine under the plane and push it to taxi.

This... is a bit inappropriate.

Li Yu felt the need to introduce his electronic ignition system to them.

It's not gentlemanly to start a car with a joystick, but it's really awesome to start an airplane with a joystick.

Hand-cranked tractors call experts!
But generally speaking, after so many wind tunnel tests, the attitude of the aircraft is well controlled.

After gaining speed, Wilbur let go of his hand, and Aviator 1 flew out tremblingly.

Fortunately, the thrust-to-weight ratio of this aircraft is only 0.03, otherwise it would be impossible to push the aircraft manually.

Aviator 1 flew for only 12 seconds and landed 36 meters away.

Then I changed to my brother and flew for 59 seconds, 260 meters.

The data sounds not as good as the wingsuit flight of later generations, but as the first powered flight of human beings, it is meaningful enough.

The two brothers played with great interest and wanted to continue the test flight, but a gust of wind blew up and overturned the plane and suffered serious damage, so they had to stop the flight test.

(End of this chapter)