I don't know when Hua Feng, Yun Meng, and Bai Feng started to spend the most time every day listening to the professors' almost endless conversations and students' Q&A.

Regarding wormholes, time travel, and interstellar exploration, many times although they had to endure one lesson after another, but fortunately, their cultivation speed did not fall because of this, and after exploring the cultivation techniques in the Yan Emperor's decision, many things that were previously confused, Hua Feng was able to solve them by himself.

His strength has also made great progress, in addition to the theory, most of the time they will conduct actual combat training, under their mutual promotion, Hua Feng and Yun Meng have almost reached the cultivation of the 65th level of the Yuan Emperor Initial Peak successively, and Bai Feng has also successfully broken through the Yuan Emperor Realm to reach the 62nd level of the Yuan Emperor Realm, and the vampire bloodline that was once difficult for her to suppress has also become a lot better, and her desire for blood has decreased a lot.

Hua Feng couldn't help but sigh at the magic of the Snow Master's family inheritance in the face of Yunmeng's rapid entry. But in this era, the strength of strength is not only based on the level of cultivation, but also about the flexible use of each cultivator's own vitality or energy.

These days, there has always been a knot trapped in Hua Feng's heart. In addition to those teachers with different endowments, the true identity of Mr. Sun, who was the first to tell them about the Journey to the West culture, has always been something he wants to understand. Although, at this time, he had advanced by two levels, but he still couldn't see the depth of his strength every time they met on campus.

But fortunately, no matter what, he could feel that Teacher Sun Xing had no ill will towards him and his students, and his occasional words were like keys to the gates of a new world, allowing everyone's cultivation state of mind to improve in different realms. But these still can't wipe out Hua Feng's suspicion and interest in him.

On the podium, the teacher's voice was still the same, seemingly tireless, as if there was no end.

"So what kind of wormhole can be traversable? One of the first conditions is that it must exist long enough that it can't disappear before the interstellar traveler can cross it.

Therefore, traversable wormholes must first be stable enough. How can a wormhole be stable? Thorne and Morris have found a not-so-good result, which is that there must be some kind of strange substance with negative energy in the wormhole! Why is this the conclusion possible?

That's because matter converges inward when it enters the wormhole, and scatters outward when it leaves the wormhole, and this process of convergence to scattering means that there is some kind of repulsion in the depths of the wormhole. Since the gravitational pull of ordinary matter can only produce the convergence effect, only the negative energy matter can produce this repulsive effect. Therefore, in order for wormholes to become a passage for interstellar travel, there must be matter with negative energy. Thorne and Morris's results were the starting point for research into traversable wormholes. ”

Why didn't Thorne and Morris turn out well? This is because people have never observed any negative energy in the macrocosm. In fact, in physics, the energy of a vacuum is usually set to zero. The idea that a vacuum is nothing, and negative energy means that there is "less" matter than a vacuum that has nothing, is an almost paradoxical statement in classical physics.

But many things that classical physics could not do became possible in the early twentieth century with the development of quantum theory. The existence of negative energy is fortunate enough to be one of the examples. In quantum theory, the vacuum is no longer nothing, it has an extremely complex structure, and a large number of virtual particle pairs are created and annihilated at any given moment.

In 1948, the Dutch physicist Hendrik Casimir studied this virtual particle state between two parallel conductor plates in a vacuum, and found that they had less energy than an ordinary vacuum, which showed that there was a negative energy density between the two parallel conductor plates! On this basis, he found that there was a weak interaction between such a pair of parallel conductor plates. This discovery of his became known as the Kashmir effect.

Nearly half a century later, in 1997, physicists experimentally confirmed this weak interaction, thus indirectly providing evidence for the existence of negative energy. In addition to the Kashmir effect, physicists have also discovered the existence of negative energy in some other research fields since the seventies and eighties of the twentieth century.

So, all the exciting research shows that there does seem to be negative energy matter in the universe. Unfortunately, all of the negative-energy matter known to date is produced by quantum effects, and is therefore produced in extremely small quantities. In the case of the Casimir effect, if the parallel plates are spaced one meter apart, the density of the negative energy produced is equivalent to one (negative mass) elementary particle per billion cubic meter of volume!

The negative energy densities produced by other quantum effects are similar. Therefore, at any macroscopic scale, the negative energy generated by quantum effects is negligible.

On the other hand, physicists have also estimated the amount of negative energy matter needed to maintain a traversable wormhole, and found that the larger the radius of the wormhole, the more negative energy matter is needed. Specifically, the amount of negative energy matter required to maintain a wormhole with a radius of one kilometer is equivalent to the mass of the entire solar system.

If the existence of negative energy matter offers a glimmer of hope for interstellar travel using wormholes, then these more concrete findings pour cold water on this hope.

On the one hand, all the effects known so far that produce negative energy matter are quantum effects, and the negative energy matter produced is extremely small even when measured on a microscopic scale. On the other hand, the amount of negative energy required to sustain any macroscopic wormhole is an astronomical amount! The huge gap between the two undoubtedly casts a heavy shadow over the prospect of building wormholes.

While the numbers may seem depressing, don't forget that when we talk about wormholes, we're talking about a science fiction topic. Since we are discussing the topic of science fiction, let's take a more optimistic view. Even if we don't have the ability to build wormholes ourselves, there may be other civilizations in the universe that have the ability to build wormholes, as in the Stargate story. Even, even if no one is capable of building wormholes, there may be natural wormholes in some corner of the vast universe. So let's assume that one day in the future, humans actually build or discover a wormhole with a radius of one kilometer.

Can we use it for interstellar travel?

At first glance, a one-kilometer radius of a wormhole seems sufficient for interstellar travel, as such a radius is geometrically large enough for a spacecraft of considerable size. Those who have watched a sci-fi movie may be impressed by the special handling of the starship through the wormhole. From the screen, the spaceship is surrounded by an infinite visual illusion composed of starlight and radiation from the distant sky, and it seems that the spaceship is passing through a narrow passage in time and space.

But the reality is far more complicated than this fantasy. In fact, in order for the ship and crew to safely traverse the wormhole, the size of the geometric radius is not the main problem faced by interstellar travelers. According to the general theory of relativity, matter encounters a very difficult problem in a region that is highly curved through a spatial structure such as a wormhole, and that is tension.

This is due to the uneven distribution of the gravitational field throughout space, and a familiar manifestation of it is the tides in the ocean. As a result of this tension, when the spacecraft approaches the wormhole, the spacecraft crew will gradually feel that their bodies are stretched in the direction of the wormhole, and in the direction perpendicular to it. This feeling is caused by the unevenness of the gravitational field of the wormhole. At first, this tension is only slightly uncomfortable, but as the ship approaches the wormhole, the tension increases rapidly, about a thousandfold for every tenth of the distance decreased. When the spacecraft is still 1,000 kilometers away from the wormhole, this tension is already beyond the limit of what the human body can bear, and if the spacecraft does not turn back quickly by this time, all the crew members will die under the deadly tension.

Fly some further distance and the ship itself will disintegrate under the terrible force of tension, and eventually, the insanely increased tension will tear the ship and its crew into a long string of subatomic particles. Flying out of the other end of the wormhole is this long list of subatomic particles whose source has long been indistinguishable!

This is how interstellar explorers will end up trying to traverse a wormhole with a radius of one kilometer. A wormhole with a radius of one kilometer is not a traveler's paradise, but an explorer's hell.

Therefore, for a wormhole to be traversable, an obvious further requirement is that the tension on the spacecraft and crew as it passes through the wormhole must be very small, and calculations have shown that this requirement can only be met if the radius of the wormhole is extremely large. So how big is a wormhole to be used as a passage for interstellar travel?

Calculations have shown that a wormhole with a radius of less than a light year exerts enough tension on the ship and its crew to destroy the atomic structure of matter, which no sturdy spacecraft can withstand, let alone a vulnerable crew.

Therefore, for a wormhole to be traversable, its radius must be much larger than one light-year.