Upon hearing this explanation, Xiao Yu suddenly understood why this voice had sounded so familiar to him.
Because… it was clearly his own voice, from the Earth era!
“A future me, from five hundred thousand years later… because I did something foolish right now…” Xiao Yu’s thoughts momentarily froze.
This situation was simply far too unexpected. Xiao Yu had mentally prepared himself, no matter who this unknown entity claimed to be, he wouldn’t be surprised. Yet in the end, it still managed to surprise him.
Xiao Yu’s first reaction was disbelief. With a solid scientific foundation, he was keenly aware of just how unreliable the concept of time travel truly was. Never mind faster-than-light motion in the physical world, aside from curvature flight, which was essentially a cheat code, Xiao Yu honestly didn’t know of any way to achieve faster-than-light travel.
The limitations of relativity still tightly bound all matter within flat space, preventing them from breaking the light-speed barrier. Moreover, even if something did achieve faster-than-light speed in flat space without relying on curvature flight, Xiao Yu still wouldn’t believe it could truly lead to time reversal. The reason was simple, there were paradoxes involved.
Suppose faster-than-light movement really could cause time to flow backward. Would this reversal apply only to the object moving faster than light, or would it affect the entire universe? If it affected the entire universe, there was a difficult concept to imagine, how could a single minuscule object, merely by moving faster than light, influence the whole cosmos?
If it only affected that individual object, causing it to traverse into a past point in spacetime, then, since the main universe had not experienced any time reversal, what exactly was the place it had crossed into?
In such a scenario, the only plausible explanation seemed to lie in the multiverse theory, an infinite array of timelines. This theory, which was quite popular during Earth’s era, posited that the number of universes was infinite and that even the smallest deviation could give birth to a new universe. For example, when a person got up in the morning to go to work, whether they stepped out with their left foot first or their right foot first could create a new universe. In one universe, they stepped out with their left foot; in another, they stepped out with their right.
As is well known, the position of an electron is uncertain. According to this theory, the motion of a single electron could constantly give rise to countless parallel universes. In one universe, a certain electron appears at point A; in another universe, the same electron appears at point B. These two universes differ by that one tiny detail.The parallel universe theory can very elegantly resolve time paradoxes. However, it still contains a fatal flaw, one that can be exposed with a simple logical deduction.
According to the theory of parallel universes, the true main universe contains an infinite number of parallel universes. ‘Infinite’ is a crucial concept here. It means that no matter how minuscule the probability of an event occurring, there must be at least one universe where it actually happens.
For instance, suppose someone, while eating, ends up with a coin in their food. Let’s say the odds of that happening are one in ten thousand. Now, imagine that coin happens to be a priceless artifact passed down from ancient times, an event with odds of one in a billion. Combined, the probability of someone eating a meal and ending up with a priceless ancient coin is one in ten trillion. That’s already an extremely small probability. But because the number of parallel universes is infinite, it’s guaranteed that in at least one universe, this exact thing happens. In other words, in that universe, someone really did eat a meal and find a priceless coin in it.
From this, one can draw a rather interesting conclusion, if the probability of a civilization developing to the highest level of technological advancement is one in ten trillion, and the probability of such a top-level civilization mastering the technology to traverse different universes is one in one hundred trillion, and the probability that the exit point of such a universe-traversing civilization just happens to be right in front of your computer, and that their arrival ends up breaking your computer, is one in one thousand trillion, then, when calculated together, the probability of your computer being broken by a civilization that crossed over is one in one million trillion trillion trillion. But don’t forget, the number of parallel universes is infinite. So even if the probability of this event occurring is only one in one million trillion trillion trillion, even at such an infinitesimal chance, it is still bound to happen, because parallel universes are infinite. It’s that simple.
Yet here we are, still sitting safely in front of our computers, and no sudden idiotic civilization has appeared and smashed it. From this, we can counter-argue that an infinite number of parallel universes does not exist. Perhaps parallel universes really do exist, but they are definitely not infinite in number. Their quantity must be limited.
Since time reversal is highly unlikely, at least based on Xiao Yu’s current level of technological understanding, and since the odds of someone from a finite parallel universe claiming to be himself from five hundred thousand years in the future are also extremely low, then where did this guy in front of him come from?
After hearing what this entity had to say, Xiao Yu’s first reaction was, this guy is a liar.
Xiao Yu’s vigilance did not relax. On the contrary, it became even more intense. He said coldly, “I can’t believe you. Give me an explanation. Until you give me that explanation, I won’t believe a single word you say.”
“Oh? What kind of explanation do you want?” Xiao Yu soon received a message. The tone within it was full of curiosity.
“Simple. Explain how you came here from five hundred thousand years in the future. How did you do it? Within my scientific framework, space-time traversal is impossible. If I’m to accept space-time traversal as real, then my existing theoretical system would collapse. And if it’s based on a flawed theoretical system, I don’t believe I could develop to your level of advancement five hundred thousand years in the future. That’s a contradiction. Unless you can reasonably explain that contradiction, I cannot believe you.” Xiao Yu said.
“Your reasoning is flawed. In ancient times, Earth’s humans believed the Earth was flat and the sky was round. That was obviously a mistaken theory, but even under that incorrect belief system, humanity still developed a brilliant civilization. The progress of science lies in continuously extracting patterns from phenomena and then using those patterns to solve other problems. Science permits the existence of error. The value of scientific advancement lies in the constant correction of those errors.” the unknown entity replied.
“You still haven’t answered me. How exactly did you reverse time?” Xiao Yu remained guarded and questioned warily.
“Very well, since you want to know, then I’ll tell you.” the entity replied. “Why shouldn’t time be able to flow backward?”
“It’s simple. Entropy increases.” Xiao Yu answered.
“In an isolated system, entropy only increases and never decreases, right? But what if there’s external energy input, then could entropy be reduced? And if entropy decreases, wouldn’t time flow backward?”
Xiao Yu froze for a moment. In truth, had the entity proposed a faster-than-light time reversal theory or the infinite parallel universe theory, Xiao Yu would have refuted it without hesitation. But he hadn’t expected this particular line of reasoning.
This one, Xiao Yu couldn’t refute. The reason was simple, theoretically speaking, it was correct. As long as entropy could be reduced, time could indeed flow in reverse. But, but… how could entropy decrease? Could there really be an external universe?
Xiao Yu didn’t know. This was beyond the scope of his current understanding. Yet even though he was puzzled for a brief instant, he quickly regained his composure.
Xiao Yu sneered.
“So you’re saying you borrowed energy from an external universe to make time in this universe flow backward? Then I do have a question I’m very curious about. If you’re capable of traversing universes and reversing time on a cosmic scale, is there anything you can’t do? What would compel you to travel five hundred thousand years back to find me?”
That defies reason. Xiao Yu was confronting this unknown existence with the utmost caution. From both scientific and logical perspectives, he would scrutinize every single thing it said. If its explanations were flawless and entirely self-consistent, maybe, just maybe, he might believe ten percent of it.
“I don’t have such overwhelming power.” the unknown entity replied with a sigh. “In fact, I can’t even be certain whether there really are other universes outside our own, let alone travel to them or borrow their energy.”
“Then let me ask, how did you manage to reduce entropy?” Xiao Yu asked with great interest.
“Very simply, by using the energy of this grand universe to reduce the entropy of this grand universe.” the entity replied.
“Absurd.” Xiao Yu rejected the idea outright. “I’ve never heard of anyone being able to lift themselves up by their own hair.”
“Who said… I reversed time for the entire universe?” the unknown entity said slowly. “What if I treated the Milky Way as an isolated system, then drew energy from the other galaxies to reduce the entropy of the Milky Way alone, thereby reversing time for the entire Milky Way and returning it to the present from five hundred thousand years ago, how’s that explanation?”
“Hm?” Xiao Yu felt as though a massive hammer had struck his CPU, sending his mind into a momentary crash.