Those who travel back in time

"Five times," Chen Qing said.

"Chen Qing, right?" Fang Xiande picked up the water on the coffee table and took a sip. "Didn't my future self tell you never to come looking for me?"

"I told you, but I really can't do anything about it. I can't escape the time loop, and you are the person who understands time best."

"Do you know how dangerous it is for my past self to influence my future? A careless remark from you could cause my life to go astray. I might never appear in that hospital you mentioned again, I might never meet you again, or I might die prematurely. If you really want to change the end of the world, you shouldn't have come to contact my past self."

"Dr. Fang, in the normal timeline, I would never come to contact you, but now it's a time loop. I'll be back in 20 minutes, and our meeting will be erased."

"Remember what you just said." Dr. Fang sat down again. "After getting trapped in the time loop, why did you try to find the cause in the hospital? Shouldn't the first thing to think of regarding the time anomaly be a time machine?"

"Although it's all a matter of time, this isn't time travel. This time loop is completely different from the time travel I've experienced before. If this were time travel, where did my past self go? I've looped more than ten times; there should be more than a dozen versions of myself. Even assuming this time loop really was caused by a time machine, the motivation doesn't make any sense. Why would your future self trap me in this loop?"

"Your future self is trapping you in a cycle? Chen Qing, is it definitely me who's orchestrating this?"

"Ah Jie?"

"Who is A-Jie?"

"There's another person in the base who's learning how to use the time machine from you."

"What I mean is, does it necessarily have to be human-operated? Couldn't it be the time machine itself? It's hard to find a motive for trapping you in a time loop, but if the time machine itself traps you in a loop, that makes sense. Machines don't think, they don't need motives."

"A time machine operating itself?"

"My wording was inaccurate. What I meant was that the time machine malfunctioned. Although a time machine sounds very high-tech, it's still a machine, and it's normal for machines to malfunction, right? Perhaps there's a problem with the time machine's program. I can think of a fault model right now: it's a typical case of recursive stack overflow."

Fang Xiande then found paper and a pen, and wrote down two function calls: arrive(20250630, 09:17) and initiate_rollback(). He picked up the paper and said to Chen Qing, "Each 'arrival' of the time machine is a function call within the system, which will occupy some 'call stack' resources. After a normal time travel, this function will return normally and release the resources. However, in your mission, the function arrive(20250630, 09:17) responsible for handling your arrival in 2025 has mistakenly called another function, initiate_rollback()."

At this point, Fang Xiande put down the paper and wrote down another `arrive()` function: "And the function of this rollback function is precisely to reset your state to the initial state when the `arrive()` function was first called, and then execute it again. Therefore, this leads to..."

"Dr. Fang, Dr. Fang!" Seeing Fang Xiande getting more and more excited, Chen Qing quickly interrupted him: "I believe it. I believe it's a problem with the time machine. You can stop now."

"What I just said might not be true. I've never seen a time machine; it was just a hypothesis. I have three more hypotheses; would you like to hear them?"

"Dr. Fang, just tell me how to break out of this cycle."

"Since the time loop was caused by a malfunction in the time machine, then fixing the time machine will solve the problem."

"It seems there's no way to tell your future self the message so he can fix the time machine."

"I'm talking about repairing the current time machine. Just like changing your past will change your future, changing the past of the time machine will also change the future time machine. Of course, the probability of trying to repair it twenty years ago, hoping that the repair will affect it twenty years from now, and that the effect will be able to correct the mistakes twenty years from now, is extremely small."

"The time machine in this timeline does not exist, or rather, we don't know where it is. The time machine suddenly appeared in the morgue of Dawn Hospital in 2039."

"Appeared suddenly?"

"That day, the lights in the base suddenly flickered. When your future self went to check, you found that a machine had suddenly appeared in the morgue, which was originally used as a warehouse."

"A time machine is essentially a machine that manipulates time and space, and it naturally has the function of time and space transmission, which makes sense. Do you know who sent the time machine to 2039?"

"Completely clueless."

"Back to your question. If this timeline doesn't have a time machine, then the situation is more complicated."

"How complicated is it?"

"This line is your timeline, from your birth to your death." Fang Xiande found a thin thread and placed it on the table, straightening it. He held one end in his left hand and the other in his right. "Let's assume the left hand represents your birth, and the right hand represents your death." He then pinched a point slightly to the left with his right hand: "This is a moment in your past. When your past changes..." He began to move his hand holding the thread to the right: "This change will be transmitted along this line all the way to your future. No matter if this line is straight or curved, this transmission will not change."

This explanation is relatively easy to understand, unlike the previous ones which involved functions and formulas, making them obscure and difficult to comprehend.

"If this point is 9:17 AM in 2025, and this point is 10:43 AM in 2025," Fang Xiande said, pinching two points on the thin thread with each hand. He then slowly brought his hands together, folding the two points into one, creating an arc around the thread. "Now, the point 9:17 and the point 10:43 have merged into one, creating a closed loop right next to your timeline. You're now trapped in this circle, only able to move from 9:17 to 10:43 and then back from 10:43 to 9:17, repeating this cycle endlessly. Any changes you make within this circle only circulate within the circle itself and don't reach the main timeline. This is the current time loop."

At this point, Chen Qing understood.

"To people in the future, all they can see is this straight line." Fang Xiande first pointed to the straight line, then to the folded arc next to it. "They can't see this circle at all, because it's just a point on the straight line. To people in the future, your timeline hasn't changed at all. They don't know you were in trouble in the past; they think everything is normal. How could they possibly take any action?"