Students, hello everyone. Welcome to this old man's Introduction to Time Machines. I am your instructor for this course, Hu Chedan.

An introduction is just an introduction, meaning it's only intended to give you a basic understanding of time machines, which is why this course is merely an elective.

I love to use a quote from my teacher, Mr. Xia Shuo's book, as the prologue for this course:

Time machines are not a product of physics; time travel is. Time machines are a product of sociology.

Where to begin? To understand a thing, one must understand its history. Generally speaking, we divide the history of time travel into three eras:

I: The Era of Speculation

II: The Era of Chaos

III: The Era of Society

Speculation

In the early First Technological Era—I assume I don't need to explain the era divisions to those of you taking this course—humanity was limited by the weakness of its productive forces and could only cast its vast thoughts into the realm of speculative space-time. Time travel was one such speculation. Philosophers of that time relied on time travel to create many works that are still highly imaginative even by today's standards. However, they remained trapped by problems that are obvious to us now, such as:

• If time reversal exists, what is the scope of that reversal?
• If you go back to the past and kill your past self who is preparing to travel back in time, what would happen?

In a world where thoughts could not be realized, human imagination could still run wild. World-line theory, entropy theory, and other theories of the time were also quite interesting. If I keep going, this will turn into a Medieval History class; interested students can look into them after class.

What truly made humanity realize the possibility of time travel was a great experiment by Luan Biande—quark separation. This experiment is mentioned in elementary school textbooks. Using a brilliant algorithm, he performed a "violent disassembly" of quarks to encode them. The computing power of the computer used for this experiment was sufficient to calculate the migration trajectories of all living creatures on the planet where humans lived at the time, yet the experiment still lasted forty-two years.

When the experiment ended, Luan Biande was already old and frail. The fourth generation of researchers brought 58PB of garbled data to him. In fact, he had already guessed from the data extracted during the experiment that perhaps everything he had done was in vain. But under conditions where the government had long since stopped providing funding, he persisted in finishing the experiment at his own expense. Upon seeing the final experimental data, he touched the SSD, and closed his eyes.

After the world calmly sent Luan Biande off, Xu Gou, the last young man remaining in the Luan Laboratory, began to ponder: Luan's algorithm was so exquisite that it shouldn't be wrong, so why did such a small amount of matter store so much data? This didn't even satisfy the law of conservation of energy. After organizing the data, Xu Gou discovered that it could be used with Hilbert... never mind, we won't cover this part in an elective course. In short, he was astonished to find that this data was derived from a core algorithm: the first line was a coordinate value, followed by an algorithm. By plugging the first line into the following algorithm, one could obtain another coordinate—the coordinates of the particle in the next Planck time. Luan's decoding method was not wrong; he had simply calculated all coordinates with a precision of 100s, thus obtaining the coordinate trajectory of the particle for the 12.4 billion years since its creation.

This algorithm is the core algorithm of time travel—the Luan-Gou Algorithm.

Some students might be wondering: using coordinates to calculate previous positions, isn't that something that could be done thousands of years ago? No. The essence of this algorithm is not calculation, but activation. Because we cannot precisely calculate the coordinates of every quark that makes up even a simple subatomic pair of glasses from the previous second. But we can make the quarks themselves output those coordinates.

Chaos

Quarks themselves are the folding of countless data, and data itself is a form of matter. Perhaps it is better to say that matter is essentially data. Xu Gou created a model of a time machine on his computer and then sealed it away permanently. Until the very last moment before his death, he wrote a letter and sent it to Luan Biande's grave, which read: "I really wish I could see my teacher one more time."

When teaching this chapter on chaos, I want students to remember one sentence:

In an infinite sample size, if an event is possible, the probability of it happening is one hundred percent.

Even though Xu Gou sealed the time machine away, forty-two years later, the scientist Elnia independently assembled a time machine from that pile of garbled data. And successfully sent a banana bought from the store back to the store.

Humanity fell into war from that day on.

Let me leave you with a thought-provoking question: how do you think a time war should be fought?

A student in the back raised his hand.

"By changing the opponent's past to make their existence disappear, while ensuring one's own present remains unchanged."

Well said. Then how do you ensure your own present remains unchanged?

"backup"

Which department are you from?

"Time Machine Management"

Get out of here. If you don't study your own major properly, don't come to my elective course.

He was right. After Elnia submitted the paper on the time machine, the government keenly realized that this thing could not only calculate the past, but it must also be able to calculate the future. So, a research group belonging to the government performed time travel on a national flag inside the safest underground nuclear security facility on the planet. Ten thousand years later, that flag was still fluttering in the glass cabinet underground. The authorities then calculated for twenty thousand, fifty thousand, and one hundred thousand years. And then announced that human civilization would shine throughout the universe.

One week later, in the eyes of a lab assistant, the recorded image of the first ten-thousand-year experiment changed in an instant from a fluttering red flag to nothingness. Suddenly, his listless gaze turned into a firm look, and the news headlines outside also changed to:

Alien civilization declares war on us through time.

In fact, these records never existed. This is why this period of history is called the Era of Chaos, because history changed through time travel is synchronized to all points in time at the moment it is changed in the past. It only leaves faint memories in the minds of certain special groups of people. Our current data is basically retrieved from those people's minds through hypnosis after the war.

After discovering that history had been changed, the government's first decision was—to frantically repeat the ten-thousand-year experiment. They needed hope—hope that future humans might win.

We don't know if what cheered humanity up was the red flag that finally returned successfully, or some impassioned speech. In any case, half a year after the experiment, humanity began to build a time defense line. In fact, today, for humanity to launch a crusade against a civilization that has just invented a time machine and is already recklessly rewriting time, it doesn't even take half a year. A mature time-traveling civilization must possess the means to attack a target on multiple timelines simultaneously. So that half a year is also what we call the half-year God lent to humanity.

Humanity's method of building a time defense line was simple: BACKUP. Time before the time defense line is unsafe; we must ensure that even if all previous time is destroyed, humanity can still reach this point in time. So we archived and backed up all important nodes in history. The cruelty of the war makes it so that we cannot even be sure today whether our original home really came from the Sun in the Milky Way, or if this is just a backup.

The civilization we were at war with was also extremely clumsy. They used nuclear bombs to bombard the home planet and induced stellar helium flashes. People at the time must have seen stellar ruins more than once. But remember my initial sentence: in an infinite sample size, if an event is possible, the probability of it happening is one hundred percent.

As long as the time defense line is set, even if the star is destroyed and humanity only has a slim chance of survival, it will definitely survive and then return to the timeline to rebuild civilization. Later, we discovered that it is impossible to completely make a civilization disappear just by throwing bombs at each other. Even in the later stages, before the spaceship could reach the opponent's home planet, more advanced defensive spaceships were already waiting outside the system.

So we traced back to earlier times, attempting to influence the enemy's civilization.

At the same time, we discovered that history was deceiving us. In the hypnotic records of people with special abilities, there were even instances where calculation symbols in math textbooks kept changing. The best way to block the development of a civilization is to block its mathematical development. We gathered all the backed-up mathematics and found that they were completely different, yet self-consistent within their respective systems at the time. This also triggered the Fourth Mathematical Crisis: what is the essence of mathematics?

Society

Interestingly, we don't know when we won.

We still exist here, which proves we haven't failed yet. But even the smallest probability cannot be ignored on the infinite scale from the past to the future.

After continuously observing the future for forty-two years, we announced humanity's victory. Then we reconstructed the time defense line, steadily pushing it back to three million years ago. All time serves the present. Three thousand years have passed since the victory, and we are still searching all of time for factors that could trigger a war. We also understand why the enemy declared war on us.

To the universe, time is too fragile, such that even a weak civilization might cause unpredictable consequences due to the casual manipulation of time, even if the probability is so small that one cannot count how many zeros there are. Humanity also slowly understood the sociological attributes of time machines during the search process—a time machine only appears when a society has developed enough to accept it.

In fact, another explanation is that civilizations that discovered time machines before they had the capacity to accept them have all disappeared.

And civilizations that discover time machines naturally have to shoulder the mission of guarding fragile time, ensuring that the trajectory of social development is the same as the trajectory of their own development to the present.

Only by obeying history can we have a future.

A girl in the front row raised her hand, and I signaled for her to ask.

"Teacher, doesn't this mean fatalism? We must live according to the existing trajectory."

I checked my watch; there wasn't enough time to finish the content of this lesson, so I might as well answer this question.

Rather than calling it fatalism, it's better to call it instinct. Survival is the instinct of life; everything we do is for that red flag to flutter hundreds of thousands of years later. For an individual, we never restrict any rights other than changing time. You can be anyone you want as an individual, but for humanity as a whole, we must survive.

Only the destiny of the collective can allow for the freedom of the individual.

Alright students, that's all for this class. In the next class, I will explain in plain language why the scope of time travel spreads at the speed of light, the paradox problems in classical time travel, and what coordinates actually are.

Class dismissed.