Exploring the starry sky is the eternal dream of mankind. On a clear night, whenever we look up, we will see stars all over the sky. Since ancient times, the starry sky, with its unparalleled vastness, profundity, beauty and mystery, has aroused countless reverie of mankind. The famous American sci-fi TV series "Star Trek" has such a short but meaningful inscription: starry sky, the last frontier. The first time I watched this TV series, I was fascinated by this inscription read by magnetic voice.
In ancient times, humans explored the starry sky with naked eyes, and later began to use telescopes, but the first step towards the starry sky was in 1957. That year, the first spaceship launched by human finally flew out of the atmosphere of our blue planet. Twelve years later, man left his footprints on the moon. Three years later, man launched Pioneer 10 deep space probe into the outer solar system. 1983, Pioneer 10 flew out of Neptune orbit, becoming the first spacecraft launched by human beings to fly out of the solar system.
It has been just over twenty years since the launch of the first spaceship. tsiolkovsky's prediction that "man will first carefully cross the atmosphere and then conquer the whole space around the sun" has come true, and the pace of human exploration of the starry sky is not fast. However, compared with the endless starry sky, this step is still too slow. The pioneer 10, who was the first to fly out of the solar system, is now gliding in the cold and silent space. Among the stars in the sky, how many years will it take to fly to the next star? The answer is two million years! By then, it will fly to Taurus, which is 68 light years away from us. The distance of 68 light-years is extremely huge relative to any scale on the earth, but it is undoubtedly insignificant relative to the galactic center 30,000 light-years away, the Andromeda nebula 2.2 million light-years away, the Virgo galaxy cluster 60 million light-years away and other distant celestial bodies. Human curiosity has no boundaries, but even though the speed of human spacecraft is many times faster, even close to the upper limit of physical speed-the speed of light, measured by the distance of interstellar space, is still extremely slow.
So, is there any way to make the spacecraft break through the speed limit in some way, so that it can cross those almost infinite distances in a very short time? Science fiction writers took the lead in spreading the wings of imagination. 1985, carl sagan, a famous planetary astronomer at Cornell University, wrote a science fiction novel called Contact. Sagan has a strong interest in exploring intelligent life outside the earth. His first guest appearance as a science fiction writer was to raise money for SETI project and look for intelligent life outside the earth. His novels were later made into movies, which won him wide popularity.
Sagan told a touching story in his novel: a female scientist named Ellie received a series of radio signals from intelligent creatures from other planets. After research, she found that this series of signals contains a way to build a special device, which allows humans to meet the sender of the signal. After hard work, Ellie and her colleagues successfully built this device, and through this device, they crossed the distant interstellar space and made their first contact with intelligent creatures on other planets.
However, in what way did Ellie and her colleagues use the equipment built according to the methods provided by intelligent creatures on other planets to let travelers cross the distant interstellar space? This is where Sagan needs bold "fantasy". His original idea was to use black holes. But Sagan is not an ordinary science fiction writer after all, and his scientific background makes him hope that his science fiction will not contradict the known laws of physics as much as possible. So he called his old friend, Professor Kip S Thorne of California Institute of Technology. Thorne is an expert in gravity theory. Sagan asked him to make a technical evaluation of his ideas. After thinking and rough calculation, Thorne quickly told Sagan that black holes could not be used as a tool for interstellar travel. He suggested Sagan use the concept of wormholes. As far as I know, this is the first time that the term wormhole has entered science fiction. After that, all kinds of science fiction novels, movies and TV series have adopted this term, and wormholes have gradually become the standard term in science fiction stories. This is the result of a little communication between science fiction writers and physicists.
The communication between Sagan and Thorne not only brought a brand-new term to science fiction, but also opened up a new research field for physics. In physics, the concept of wormhole was first put forward by C.W. Miesner and J. A. Wheeler in 1957, which happened to be the same year that the first spacecraft was launched. So what exactly is a wormhole? Why is it used by science fiction writers as a tool for interstellar travel? Let's use a simple example to illustrate: as we all know, it is necessary to walk an arc from one point to another on the surface of an apple, but if a moth eats a wormhole between these two points, it can walk a straight line between these two points through the wormhole, which is obviously closer than the original arc. Extending this analogy from the two-dimensional apple surface to the three-dimensional physical space is what physicists call a wormhole, which can form a shortcut between two points, which is why science fiction writers like wormholes. As long as there is a suitable wormhole, no matter how far it is, it may become close at hand, and interstellar travelers will no longer be subject to the remoteness of space. In some sci-fi stories, the highly developed civilized world uses wormholes for interstellar travel, just as we use highways to travel between towns today. In the famous American sci-fi movie and TV series Stargate, a device called Stargate left on earth by alien civilization can be connected with stargates on many other distant planets, and people and equipment can be sent to those distant planets almost instantly. Wormholes have become a paradise for interstellar travelers in science fiction.
However, the wormhole proposed by Miesner and Wheeler is extremely small and will disappear in a very short time, so it cannot be a channel for interstellar travel. After Sagan's novel was published, Thorne became interested in wormholes, and he and his student Mike Morris began to study wormholes in depth. Unlike Miesner and Wheeler, Thorne is interested in wormholes that can be used as interstellar travel channels, which are called traversable wormholes. So what kind of wormhole can be a traversable wormhole? One of the first conditions is that it must exist for a long time and cannot disappear without waiting for interstellar travelers to cross. So the wormhole that can be crossed must be stable enough first. How can wormholes exist stably? After research, Thorne and Morris found a bad result, that is, there must be some strange substance with negative energy in the wormhole! Why is there such a conclusion? That's because matter converges inward when it enters the wormhole, but flies outward when it leaves the wormhole. This process from convergence to scattering means that there is some repulsion in the depths of the wormhole. Because the gravity of ordinary matter can only produce convergence, only negative energy matter can produce this repulsion. Therefore, in order to make the wormhole a channel for interstellar travel, there must be negative energy substances. This result of Thorne and Morris is the starting point for people to study the wormhole.
Why didn't Thorne and Morris have a good result? Because people have never observed any substance with negative energy in the macro world. In fact, in physics, people usually set the energy of vacuum to zero. The so-called vacuum means nothing, and negative energy means "less" matter than a vacuum with nothing, which is almost contradictory in classical physics.
But many things that classical physics can't do have become possible with the development of quantum theory in the early 20th century. Fortunately, the existence of negative energy is an example. In quantum theory, vacuum is no longer nothingness, it has an extremely complex structure, and a large number of virtual particle pairs are produced and annihilated every moment. 1948, Dutch physicist Hendrik Casimir studied this virtual particle state between two parallel conductor plates in vacuum, and found that their energy is smaller than that in ordinary vacuum, which shows that there is 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. His discovery is called the Kashmir effect. Nearly half a century later, in 1997, physicists' experiments confirmed this weak interaction, which indirectly provided evidence for the existence of negative energy. In addition to Kashmir effect, physicists have also found the existence of negative energy in other research fields since 1970s and 1980s.
So all kinds of exciting studies show that there seems to be negative energy substances in the universe. Unfortunately, these known negative energy substances are all produced by quantum effect, so the number is very small. Take the Kashmir effect as an example. If the distance between parallel plates is one meter, then the density of negative energy it produces is equivalent to only one elementary particle (negative mass) in every billion cubic meters! Moreover, the larger the spacing, the smaller the negative energy density. The negative energy density produced by other quantum effects is similar. Therefore, on any macro scale, the negative energy produced by quantum effect can be ignored.
On the other hand, physicists have also estimated the amount of negative energy substances needed to maintain a wormhole, and found that the larger the radius of the wormhole, the more negative energy substances are needed. Specifically, in order to maintain a wormhole with a radius of one kilometer, the amount of negative energy substances needed is equivalent to the mass of the entire solar system.
If the existence of negative energy substances brings a glimmer of hope for interstellar travel in wormholes, then these more specific research results have poured cold water on this hope. Because on the one hand, all the effects of known negative energy substances are quantum effects, and the negative energy substances produced are extremely small even on a microscopic scale. On the other hand, the negative energy substances needed to maintain any wormhole in the macro sense are astronomical! The huge gap between them undoubtedly casts a heavy shadow on the prospect of building wormholes. Although the numbers look depressing, don't forget that when we talk about wormholes, we are talking about a science fiction theme. Since we are talking about science fiction, let's be optimistic. Even if we can't build wormholes ourselves, there may be other civilized creatures in the universe who can build wormholes, just like the story of the stargate. Even if no one can make a wormhole, there may be a natural wormhole somewhere in the vast universe. So let's assume that one day in the future, humans really built or discovered a wormhole with a radius of one kilometer.
Can we use it for interstellar travel?
At first glance, a wormhole with a radius of one kilometer seems to be enough to meet the requirements of interstellar travel, because such a radius is enough for a large-scale interstellar spacecraft to pass through. People who have seen science fiction movies may be deeply impressed by the special handling of interstellar spacecraft crossing wormholes. From the screen, the spacecraft is surrounded by an infinitely beautiful optical illusion composed of starlight and radiation from the distant sky. It seems that the spaceship is crossing a narrow passage of time and space.
But the reality is far more complicated than this fantasy. In fact, in order to let the spacecraft and crew pass through the wormhole safely, the size of the geometric radius is not the main problem faced by interstellar travelers. According to the general theory of relativity, when a substance passes through a highly curved area such as a wormhole, it will encounter a very difficult problem, that is, tension. This is caused by the uneven distribution of gravitational field in all parts of space, and its common manifestation is the tide in the ocean. Because of this tension, when the interstellar spacecraft approaches the wormhole, the crew on the spacecraft will gradually feel that their bodies are stretched in the direction of the wormhole and squeezed in the direction perpendicular to the wormhole. This feeling is caused by the uneven gravitational field of the wormhole. At first, this kind of tension will only make people feel a little uncomfortable, but as the spacecraft approaches the wormhole, this kind of tension will increase rapidly. For every tenth of the distance, this kind of tension will increase by about 1000 times. When the spacecraft is 0/000 km away from the wormhole/kloc-,this tension has exceeded the limit that the human body can bear. If the ship doesn't return quickly by this time, all the crew will die under deadly tension. A little farther away, the spacecraft itself will disintegrate under terrible tension, and eventually, the crazy increase of tension will tear the spacecraft and its crew into a long string of subatomic particles. From the other end of the wormhole, it is this long string of subatomic particles whose sources have long been confused!
This is what happens when an interstellar explorer tries to cross a wormhole with a radius of one kilometer. A wormhole with a radius of one kilometer is not a paradise for travelers, but a hell for explorers.
Therefore, an obvious further requirement for the wormhole to become a traversable wormhole is that the tension on the spacecraft and its crew when crossing the wormhole must be very small. The calculation shows that this requirement can only be met when the radius of the wormhole is extremely large. So how big is the wormhole to be as a passage for interstellar travel? The calculation shows that a wormhole with a radius of less than one light-year can exert enough tension on the spacecraft and its crew to destroy the atomic structure of matter, which is beyond the bearing capacity of any solid spacecraft, let alone the fragile spacecraft crew. Therefore, in order to make a wormhole a traversable wormhole, its radius must be much larger than one light year.
What is the concept of a light year? It is equivalent to more than 1500 times the radius of the entire solar system (bounded by Pluto's orbit). If measured by the linearity of the earth, it is about 700 million times the diameter of the earth. Therefore, it is absolutely impossible for the sci-fi movie Stargate to build the entrance and exit of the wormhole on the earth and other planets, because the wormhole with such a narrow entrance can not only let people pass safely, but also tear everything around it into subatomic particles in an instant. In Sagan's story, some people objected that Ellie and her colleagues put the blueprints provided by intelligent creatures on other planets into practice, because they were worried that it might be a device used to destroy the earth. Their fears are actually quite reasonable. On the other hand, although a light-year is a huge linearity measured by daily distance, it is not surprising measured by interstellar distance. The linearity of our galaxy is about100000 times that of it. If there is a wormhole between the Milky Way and the Andromeda Nebula 2.2 million light years away, it is only a very small channel in terms of linearity. So there really will be such a passage in the interstellar space around us, but haven't we found it yet? The answer is no, because the real magic of a wormhole with a radius of one light year is not its linearity, but the number of negative energy substances needed to maintain it. The calculation shows that the amount of negative energy matter needed to maintain such a wormhole is equivalent to 100 times of the total mass of all luminous stars in the whole galaxy! The gravitational effect of this wormhole will be far greater than that of the whole galaxy. If such wormholes exist in the interstellar space near us, the motion of matter around millions of light years will be significantly affected, and we have found their traces in its gravitational field.
Therefore, it is not only impossible to build a traversable wormhole on the earth, but also almost impossible to have a traversable wormhole undetected in the whole interstellar space near us.
In this way, we can only discuss one possibility, that is, is it possible to have wormholes in other distant corners of the universe? We may never know the exact result of this problem, because the universe is too big. However, it is almost impossible to maintain the amount of negative energy substances needed for observing wormholes, which almost provides us with the answer. So far, human beings have never found negative energy substances on any macro scale, and all experimental methods to produce negative energy substances are to use weak quantum effects. In order to maintain a wormhole, there must be some mechanism to collect weak negative energy substances produced by quantum effect and reach a sufficient number. But can negative energy matter gather together? In recent years, physicists have done some theoretical research in this field, and the results show that the negative energy matter produced by quantum effect cannot converge indefinitely. The more negative energy matter accumulates, the shorter it can exist. So a wormhole is unstable without negative energy substances, and it will be unstable if there are too many negative energy substances! So what kind of wormhole can be stable? Preliminary calculations show that only wormholes whose linearity is twenty orders of magnitude smaller than that of atoms are stable!
This series of results is undoubtedly very cold. If these results hold, the possibility of crossing the wormhole will be basically ruled out, and those beautiful science fiction stories will become a mirage. Fortunately (or unfortunately), however, many of the above results are based on relatively advanced-and therefore relatively immature-physical theories. Whether the future research will fundamentally shake these theories and completely overturn many of the results we introduced above is still unknown. To take a step back, even if those physical theories are basically established, many of the results described above are only approximate results or special cases derived from those theories. For example, many results assume that wormholes are spherically symmetric, but in fact wormholes can be other shapes. The amount of negative energy substances needed and the tension generated by wormholes with different shapes are different. All these show that even if those physical theories are true, the conclusion we mentioned above is not necessarily that the way to completely open it is to use the principle of mutual attraction between substances to attract the positive and negative energy of two wormholes in time and space to open it.