After the simple certificate-issuing ceremony, Michelson said to Li Yu: "According to tradition, we have to conduct a physics seminar with new members."
Li Yu said: "No problem, president please."
Michelson thought for a while and said: "How about I use the field of optics, which I am good at. You must know that the current physics community has not been able to explain the constant speed of light. Even the British physics guru Lord Kelvin called it a
A dark cloud."
Michelson raised this question very appropriately, after all, this dark cloud was brought about by his Michelson-Morley experiment.
Li Yu said: "It is indeed an incredible dark cloud, because the speed of light cannot be explained by classical mechanical theory because the speed of light remains unchanged."
Michelson said: "This is exactly where the problem lies. Many people can vaguely feel that there seems to be some mystery of nature hidden behind it, so many people try to explain or prove it theoretically."
Li Yu said: "I think the speed of light cannot be explained or proven at all, because it is the most basic physical quantity, just like Boltzmann's constant."
Michelson said: "But according to Mr. Maxwell's equations, the speed of light can be derived."
The derivation described by Michelson is to use Maxwell's equations to calculate the speed of light.
The process involves some relatively advanced mathematical content, but overall it is not particularly difficult. Students from the physics department can complete it without having to specialize in mathematics.
After all, the physics major of later generations cannot do without important mathematical tools.
In short, after a series of substitutions such as tensors and curls, a very simple expression for the speed of light can be obtained:
C=1/εμ.
(Limited by the format, the root sign in the denominator 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 dielectric constant ε and the vacuum permeability μ.
Since the last two are constants, the speed of light is naturally also a constant.
Maxwell is only 0.1 mm away from special relativity.
But on the eve of the birth of special relativity, Maxwell failed to figure out why the speed of light became a constant. This was a super difficult problem.
But in fact, modern people understand that in a strict sense, we cannot say that Maxwell's equations have derived 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 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 cannot be proven since ancient times."
"Can't prove it?" Michelson originally wanted to refute, but after all, he had been immersed in the measurement of the speed of light for too many years and knew that there was nothing wrong. Maybe he couldn't prove that it was really the answer.
Li Yu continued: "In Mr. Conan Doyle's "The Sherlock Holmes", Sherlock Holmes said this to Watson, when you eliminate all impossible situations, the rest, no matter how unbelievable, is
It’s a fact.”
Those who have watched related movies or TV series must be very impressed. This sentence is very cool to say in English.
Michelson said: "I have read this excellent collection of mystery novels, and your quote makes sense. If, as you said, it cannot be proven that the speed of light remains unchanged, then wouldn't it become an axiom or a postulate?"
Li Yu solemnly said: "That's right!"
Michelson took 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.
Postulates are basic principles that do not require proof.
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
②A finite line segment can continue to be extended
③A circle can be drawn with any point as the center and any distance.
④All right angles are equal to each other
⑤A 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 the two right angles, then the two straight lines will intersect on this side after being infinitely extended.
The problem lies in the fifth postulate.
Because it is obviously too long compared to the first four.
Even Euclid himself was not satisfied with it and wanted 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 room for sloppiness.
Later, mathematics was axiomatized, and a long proof process was even conducted for 1+1=2.
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 started writing "Principles of Mathematics" seven years later, in order to solve various paradoxes and then use logic to explain the axioms.
At first they took a big detour and proved that 1+1=2 took 379 pages!
This book looks like a heavenly book.
Of course, there is no need to read it, because they make too many detours and the language is too verbose.
Many people have complained about it, and the emergence of Gödel's incompleteness theorem has basically declared the failure of Russell's theory.
But people have always made exploratory attempts.
In fact, people in later generations can use Peano's axiom system and prove that 1+1=2 in two pages.
Coming back to the fifth postulate, it actually not only profoundly affected the world of mathematics, but also had a profound impact on the world of physics, especially the birth of the theory of relativity, so it has to be said.
Because the fifth postulate directly led to the birth of non-Euclidean geometry.
The Russian mathematician Lobachevsky and the German Riemann respectively completed their own non-Euclidean geometry deductions.
The two are not exactly the same, but the processes are both revolutionary events in the history of mathematics.
In short, once the basic axioms or postulates are wrong, the entire mathematical edifice must be revised, and then a more magnificent edifice will be born.
The world of mathematics has already undergone such a large-scale change.
As a precursor, mathematics has completed the revolution, and disciplines such as physics can use mathematical tools to carry out revolution.
Because the theory of relativity is based on Riemannian geometry, that is, on non-Euclidean geometry.
Later, theories such as "gravity causes space-time to bend" that became familiar to people are inseparable from Riemann's geometry.
In addition, gravity seems to be the easiest to understand, 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 and will be discussed later.
Anyway, now Michelson instantly understood the meaning of Li Yu's words: "You mean, the constant speed of light should be regarded as an axiom and a huge deduction should be carried out?"
Li Yu nodded: "Compared with the two constants electrical conductivity and magnetic permeability used in Maxwell's equations to derive the expression of the speed of light, the speed of light in vacuum is a more basic 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 was a little conservative, because it was later proved that Maxwell's equations are also applicable to the relativistic system.
That's why it is said that Maxwell almost touched the threshold of special relativity. He was really at the door.
Michelson is different from conventional physicists. He himself is a firm supporter of the constant speed of light. He has been measuring 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 constant.
What we are talking about here is the speed of light in a vacuum, because it is well known that the speed of light slows down in water.
The reason is simple: the propagation of light itself does not require a medium, but the medium will slow down the speed of light.
Michelson pondered for a while and said: "I have read your books, and all of your papers are rigorous mathematics and profound physical explanations. 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 Li Yu's words today contained too much information.
Li Yu said: "There is nothing surprising. There are many principles in physics that are not derived through derivation, but are 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.
Fall down."
Michelson was a little stunned: "I understand what you mean. The apple falling from the 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
It is the same if it changes, no need to prove it.”
"It is indeed the president! That's the truth," Li Yu said. There are probably only a 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 can only
I can regretfully answer in three words, 'I don't know'."
Michelson smiled and said: "You are very candid."
Li Yu spread his hands: "I can only answer this. In the final analysis, the constant speed of light is the result of the experiment, not a theoretical deduction."
At least before Li Yu traveled through time, humans really didn’t know why the speed of light remained unchanged.
To put it more bluntly, it is an artificial rule because the experiment is like this.
If one day experiments show that the speed of light can change, it is estimated that more than half of the entire physical world will collapse by then.
Because this kind of subversion is like an apple falling from the earth and suddenly flying into the sky.
At that time, the entire physical theory will have to be rebuilt.
Physics is like this. Once you have experiments and then explain the phenomena, you cannot doubt that there are errors in nature.
But this kind of thing should be impossible, at least not in the current universe.
(Unless a Trisolaran sophon intervenes.)
Michelson said: "I feel more and more that it would be a wise choice for you to join physics. We met briefly at the White House dinner last time, and today we have an in-depth discussion. I admire your knowledge of mathematics and mathematics."
Li Yu smiled and said: "It's like falling to the ground."
Since the release of his Star Wars series, Li Yu has discovered that many people like to say some "Chinese proverbs", but the idioms are really difficult to say.
Michelson said: "I will compile the conversation today and publish it in the conference paper tomorrow."
——
Kitty Hawk, North Carolina, USA.
Many people's first impression of North Carolina should come from Jordan.
Jordan came out of the University of North Carolina team, and now he is also the owner of the Charlotte Hornets in North Carolina.
The cold wind at this time was already quite biting.
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, there were only five people watching at the scene.
The Wright brothers rushed over excitedly: "Mr. Li Yu, why are you here?"
Li Yu smiled and said: "I told you, I want to see you fly into the sky with my engine."
Brother Wilbur Wright said: "I really didn't expect that as the world's top scientist, you would be optimistic about our flight test."
Li Yu generally told them the story of Wanhu.
His younger brother Orville Wright said: "It turns out that the Chinese had such a dream hundreds of years ago. I always thought that it was Mr. Leonardo da Vinci who originally had the idea of flying."
Leonardo da Vinci was also an all-rounder. He did design an ornithopter in his manuscripts, and even a helicopter and a parachute.
It feels like a time traveler.
Newton has a famous saying: "If I have seen farther than others, it is because I stand on the shoulders of giants."
Although this sentence was originally directed at Hook, it does make sense.
The same applies to the Wright brothers.
In fact, in the decades before the Wright brothers, many people had done a lot of work and achieved many results in aircraft structure, lift and drag research, stability and control, gliding flight, etc. Without them as pioneers, the Wright brothers would have
Impossible to succeed.
The Wright brothers happily took Li Yu to visit their aircraft "Flyer 1" today.
This aircraft doesn't look too small, with a wingspan of more than 12 meters, which is about 14 to 15 meters for a modern fighter jet.
However, the take-off weight is only a pitiful 360kg. For comparison, the take-off weight of the J-15 or F18 is as high as more than 30 tons.
If it is a civil airliner such as a Boeing 737, it will be even higher, directly exceeding 50 tons.
Brother Wilbur Wright looked at the weather: "It's time to start."
His younger brother Orwell Wright took out a coin, and the two guessed the coin and finally decided that his younger brother Orwell would drive first.
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 then push it to taxi.
This... is actually a bit inappropriate.
Li Yu felt it necessary to introduce his electronic ignition system to them.
Using a joystick to start a car is already ungentlemanly, but starting an airplane with a joystick is really awesome.
Hand-operated tractors call you an expert!
But overall, after so many wind tunnel tests, the aircraft's attitude control is pretty good.
After picking up speed, Wilbur let go of his hand, and Flyer 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.
Flyer 1 flew for only 12 seconds and landed 36 meters away.
Then I switched to my brother as the driver and the flight took 59 seconds and 260 meters.
The data sounds inferior to later generations of wingsuit flight, but as the first powered flight for humans, it is significant enough.
The two brothers were having fun and wanted to continue the test flight, but a strong wind blew over the plane and caused serious damage, so they had to stop the flight test.