"I'll go with you. After all these years, how could I face it if I left you behind? We might as well make this one big gamble together. We've always been busy with our academic work, and gambling has always seemed so far removed from us. This time, let's find out what it feels like to be gamblers," Academician Xiao said.
"Academicians, look at the situation we're in, and you're still arguing over this? Forget it, I know there's no use trying to persuade you. Academician Zeng, just tell us what to do. Only five enemy warships are pursuing us right now, and we can still afford the losses. If those losses can buy the Deep Blue's safety, it's well worth it. Just tell us what to do. The enemy fleet has already reached a region of space a hundred light-years away. At five hundred times the speed of light, they can get here in less than half a year, even while searching along the way."
"Here's what I have in mind." Academician Xiao began his explanation.
The insects were three-dimensional beings. If they needed to travel through a higher dimension, there had to be a three-dimensional spatial structure to hold the swarm in place. Otherwise, everyone knew what would happen to three-dimensional beings in four-dimensional space, and that would seriously affect the outcome of the transfer. So the Insect Race's leader, or whoever was controlling them from behind the scenes, had created something called a four-dimensional space bubble. It was like humans needing to build roads to travel elsewhere. Without roads, they could hardly take a step, with mountains and rivers blocking their way, and they might also get lost in forests or grasslands. Four-dimensional transfer worked on the same principle. They surmised that the outside of the four-dimensional space bubble was the fourth dimension, but the space inside it had to be three-dimensional. It was like a Möbius strip: the space outside the band was three-dimensional, but the band itself was two-dimensional. This Möbius strip was a three-dimensional bubble for two-dimensional beings. A four-dimensional bubble for three-dimensional beings was much more complex, however. A four-dimensional space bubble was more like a human airplane or spaceship; unlike a road, it was not a fixed passage.
A four-dimensional space bubble in transit would not appear in three-dimensional space. Only after reaching its destination would it move toward the greater universe. Once the bubble's three dimensions intersected with the three dimensions of the greater universe, the swarm would arrive in the greater universe. At that point, the bubble would lose its fourth dimension as its energy gradually dissipated into the greater universe or as a result of damage caused by the swarm, and its three-dimensional structure would merge completely with the greater universe. But for some reason, a very small number of space bubbles might not have completely lost their connection to the fourth dimension. As long as they found the location of one such bubble, they could enter the fourth dimension and temporarily evade pursuit by that star system-level civilization.
Because this space bubble had a three-dimensional structure, or rather, because its three-dimensional portion had already merged with the greater universe, even a spaceship passing through the region would have trouble detecting it. It was, after all, normal space. They could observe the projection of a four-dimensional object in three-dimensional space, but they could not see the projection of four-dimensional space in three-dimensional space. It was like pouring a drop of water into a glass of water: they were the same thing to begin with, so they could merge perfectly.
Since the space bubble was connected to the fourth dimension, gravitational waves might be the only thing capable of detecting it with their current technology. Gravitational waves were ripples in space. Ordinary electromagnetic waves would undergo no change as they passed through the bubble, but gravitational waves were different. Gravitational waves existed on the basis of space, and when they passed through the bubble, the fourth dimension would absorb some of their energy. They still had three hundred thousand warships spread throughout this region of space. Now they needed those ships to use most of their energy to become gravitational sources, disturbing three-dimensional space and producing three hundred thousand gravitational waves. By observing how those waves propagated, they could locate the four-dimensional space bubble.
"Academician, how can the warships produce gravitational waves?" Hu Yu asked. Previously, all the gravitational waves they had observed had been detected after extreme celestial bodies, such as neutron stars and black holes, disturbed space. Hu Yu did not yet know whether it was feasible to use a warship's energy to produce gravitational waves.
"It's simple. In fact, we're already disturbing space when we travel at warp. Now all we need is for every warship to switch its warp engine on and off in place at a set frequency. That will also produce gravitational waves. Although these waves will be extremely faint, our space technology has advanced a great deal, so detecting these spatial fluctuations won't be a problem. Every warship will be both a gravitational source and a receiver. Once the surrounding warships receive data from these gravitational waves, they'll transmit it through the four-dimensional network to the science ship for analysis and comparison. As long as the received data for even one gravitational wave differs from the transmitted data, we can pinpoint the four-dimensional space bubble."
"Good. In that case, Academician Zeng, I'm handing command of the fleet over to you. Just issue whatever orders are needed. The three hundred thousand warships guarding the Deep Blue will spread out as well. I know that if they're too close together, the gravitational waves will interfere with one another and affect the accuracy of the data, so they'll become gravitational-wave sources a little later."
These gravitational waves were like a drop of ink injected into the middle of a tank of clear water. As time passed, the ink would keep spreading outward through the water in a sphere. The outermost regions of space were probed first. Half a month passed without any news, leaving everyone with mixed feelings. They worried because nothing had been found, and they wondered whether Academician Zeng's method could really locate the supposed four-dimensional space bubble. The enemy fleet had already engaged the Hunter-Killers several times. The enemy attacked as soon as they appeared, and the Hunter-Killers were like fish on a chopping block before the warships of a star system-level civilization. This also supported the conjecture that the other side wanted to eliminate any possibility of the secret leaking. On the other hand, this might mean the bubble was not in the outer regions. If it were, the Deep Blue would still have to run great risks and travel for a long time to reach it.
Six hundred thousand Hunter-Killers, six hundred thousand frequencies, six hundred thousand gravitational waves propagating through space. The Hunter-Killers roamed the starry expanse, evading pursuit by the Star System Level civilization while waiting for the gravitational waves to travel, and praying that the Star System Level civilization would show no sign of moving toward the Deep Blue. For the time being, the Deep Blue was lucky. The five Star System Level warships had remained in the outer regions, hunting the Hunter-Killers, and had failed to notice the Deep Blue sitting motionless in the void.
Time slipped by, and every passing second meant a greater chance that the Deep Blue would be discovered. The space physicists on the science ship had not left their posts to rest for many days. Their hair was disheveled, their faces grimy and unshaven, and their eyes bloodshot, but every one of them worked intensely as though on stimulants.
Even Academician Xiao, who did not understand physics, had never left the science ship. Like Hu Yu, he stared at the strings of numbers flashing across the big screen. Hidden among those numbers was the fleet's hope.
"Academician, we've found anomalous data!!!" a scientist shouted.
"Let me see." The academician immediately examined the data on the big screen. Sure enough, among the data received from more than two hundred thousand gravitational waves, he found an anomaly. The amplitude and wavelength of this wave at the receiving end were completely different from those transmitted.
"Cross-check it immediately. Confirm the transmitter and receiver."
"Yes, Academician. I've already sent another Hunter-Killer, the one closest to the receiving ship, over there. It can cross-check the data in five minutes, to rule out errors caused by an instrument malfunction." Although the chance of a spaceship's instruments malfunctioning was very small, science demanded rigor, especially now. The data had to be two hundred percent accurate. Everyone waited anxiously, but time seemed to pass exceptionally slowly.
"Academician, the cross-check is complete. There's nothing wrong with the data. More than fifty thousand warships received this gravitational wave, and only the two warships in this direction received anomalous data. The transmitter was our science ship," the assistant replied.
"What?" Academician Zeng could not believe it.
"That's right, Academician. Because we're at the center of the fleet, the fifty thousand Hunter-Killers closest to us received the data from this gravitational wave one after another."
"Where are the two warships that received the anomalous data? How far are they from us?"
"Academician, this one is the closest. It's approximately... two light-days away."
"Ah, I should have thought of it sooner. I should have thought of it sooner," Academician Zeng said, blaming himself.
"Academician, what's wrong? We've found the space bubble now, at most two light-days away. We can go examine it in person right away. If nothing goes wrong, the fleet will be able to get through this crisis safely," Hu Yu said comfortingly.
"General, we underestimated Mr. Daozi. I'm beginning to suspect that he brought this space bubble here himself, or even built it. We have only our own ignorance to blame for not discovering it until now."
"Academician, there's no need to blame yourself. Let's hurry and verify it. That star system-level civilization has already discovered our presence. The Hunter-Killers on the perimeter are being hunted down now, and we've lost more than a hundred of them."
"Let's go."
A small spaceship carrying Hu Yu, Academician Zeng Zhaoyuan, and several space physicists detached from the Deep Blue Fleet. The small ship accelerated to half the speed of light in a very short time, then coasted on inertia. Building on gravitational-bomb technology, the fleet had long since solved the problem of balancing forces within a ship, allowing spaceships to undergo continuous high-g acceleration with conventional engines without harming the people aboard. Otherwise, when unable to perform warp maneuvers in battle, ships forced to rely on the pitifully small acceleration of a mere dozen-odd g's would become sitting ducks. The two Hunter-Killers that had received the anomalous data also coasted toward the small ship at half the speed of light along the line connecting them to the science ship, continuously sending gravitational-wave data to the small ship along the way.
If the small ship received anomalous data from the science ship's gravitational wave, or if the Hunter-Killers received normal data, either would establish the space bubble's location. Both sides moving at once would let them find it faster. To receive the gravitational-wave signal, a ship had to drop out of warp; otherwise, its warp engine would cause severe interference with the faint gravitational wave.
"Academician, we've received anomalous data," a space physicist shouted excitedly.
Receiving anomalous gravitational-wave data meant that the small ship was now inside the four-dimensional bubble. As the gravitational wave passed through the four-dimensional space bubble, interference from the fourth dimension altered it, producing a huge discrepancy between the transmitted and received data.
"Academician, nothing looks out of the ordinary here," Hu Yu said.
"That's right, nothing looks out of the ordinary, because the bubble's three-dimensional portion has completely merged with the three-dimensional space of our greater universe. But its fourth dimension is still very closely connected to three-dimensional space. In other words, the three-dimensional spatial wall is very thin here, making this the easiest place to enter the fourth dimension. Imagine an ant on an enormous sheet of white paper. Somewhere on the paper, water has soaked through it. If the ant were a two-dimensional being, the place where the water soaked through would be the easiest spot for it to break through from two dimensions into three."
"I understand that."
"One more thing. General, have you ever considered why it's called a four-dimensional space bubble?"
"Is there a problem with that, Academician? Doesn't it just have an extra fourth dimension?"
"No, I don't think it's as simple as adding a fourth dimension. Since it's called a four-dimensional space bubble rather than a four-dimensional passage, that suggests it also exists independently in the fourth dimension. It's like blowing bubbles. Although a bubble exists within three-dimensional space, it's also separated from it. If the bubble lands on a sheet of paper, an ant can enter the bubble, but it can't break through the bubble's wall into the wider three-dimensional space. The four-dimensional space bubble we've encountered may be something of the same kind. But now this bubble has burst, leaving only a puddle of water on the paper."
"Academician, I'm already satisfied if it can give the fleet somewhere to hide. Although it's damaged, both its practical and research value to us are immeasurable. Mr. Daozi really did leave us a great gift. By the way, Academician, now that we've found it, how do we get in?"
"That's easy. If I'm not mistaken, we just need to start the warp engine here. In fact, with a bit of luck, a Hunter-Killer passing through here should have discovered it long ago. Unfortunately, in the end we still had to rely on our wits to find it. Six hundred thousand Hunter-Killers, and not one of them passed through here," Academician Zeng said.
"Academician, when the Hunter-Killers left the Deep Blue Fleet, they moved in groups. As they got farther away, they gradually dispersed into legion-level formations, then brigade-level formations, and only finally operated individually. Besides, we're already eighty astronomical units from the fleet here, twice the distance from the Sun to Pluto's orbit. This space bubble is only the size of Earth. Think how tiny Earth is in the vast Solar System. Even if six hundred thousand Hunter-Killers charged around at random within that range, they wouldn't necessarily run into it. We're already lucky to have found it."
"General, would you like to see it for yourself?" Academician Zeng asked with a confident, mysterious smile.
"Of course."
"All right, we'll enter the fourth dimension immediately. But before we do, everyone, pay attention. After we enter the fourth dimension, whatever we find, don't act rashly. Although we've made countless conjectures about the fourth dimension in theory and in our imaginations, they're only conjectures. So, for safety's sake, everyone stay where you are and don't move. Whatever the issue, you must wait for my instructions before acting. Is that clear?"
"Understood," everyone replied.
"Start the warp engine."