Extraterrestrial life refers to intelligent life from outside our planet; we usually call them aliens. People are quite keen on the subject of alien invasion. Many science fiction movies use this as a selling point and promote productions with this theme. However, actual alien invasion would be devastating to mankind, as humans would be a completely different species in the eyes of aliens. Aliens might treat humans without any sympathy or humanity, just as we have treated other animals in the past. They could carry out a massacre against us that would surpass Hitler’s massacre of the Jews. Of course, it is also possible that extraterrestrial life would be friendly and visit Earth in the spirit of tourism, friendship, or curiosity instead of invasion. In any case, it is much more important to safeguard against alien invasion and attack than to hope for friendliness.

So far we have not found any evidence of the existence of extraterrestrial life. Archaeological excavations and geological surveys of Earth have not turned up evidence of aliens visiting our planet in the past either. The idea of aliens is still a product of people’s subjective imaginations.

Even so, from the long-term perspective of human survival, we have sufficient reason to consider the threat of extraterrestrial life seriously.

Humans are the only intelligent life we currently know of. Apart from traces of life factors found in very few fallen asteroids, no other clues of extraterrestrial life have been discovered. We can only infer the possibility of alien existence through the study of Earth’s humans and deduce their possible location and threat to us accordingly.

One: Life on Earth

After close distance observation and in-depth scientific analysis of the solar system planets and their satellites, it can be confirmed that within the solar system, only Earth has the conditions to produce complex life. What gave Earth the power to produce life?

Celestial collisions were an inevitable process of Earth’s formation. Many asteroids, meteorites, and comets contain water molecules. When they collide with each other to form larger celestial bodies, these water molecules are transferred accordingly. Volcanoes and magma flow are formed through collisions, and their high temperature evaporates water molecules into carbon dioxide gas. Earth’s early atmosphere was mainly composed of carbon dioxide gas.

Carbon dioxide cannot nurture complex life; it produces a greenhouse effect instead. Shortly after the formation of the ocean, microbes formed in the water. The earliest microbes were born 4.28 billion years ago, only a few hundred million years after Earth formed. Large numbers of algae creatures appeared shortly after and absorbed carbon dioxide and released oxygen.

This process of carbon dioxide to oxygen conversion took more than three billion years because Earth had a high demand for oxygen. Oxidation reactions would occur when oxygen encountered most metals—iron oxide, copper oxide, and alumina are all conjugates of oxygen and elements. The oxygen released by algae had to meet the demands of these oxygen consumers first.

The geological era 530 million years ago was known as the Cambrian Era. During this time, a number of complex organisms seemed to appear simultaneously in the vast ocean; this phenomenon was known as the Cambrian explosion. We know that animals need oxygen to survive, so the Cambrian Era must have been an era when oxygen production exceeded oxygen consumption. The surplus of oxygen was a prerequisite for the Cambrian explosion. Organisms only existed in the ocean at this time, so the land was barren of life. One hundred million years passed—about four hundred million years ago—before life came to land. Plants and animals arrived on land at the same time.

While animals must absorb oxygen to survive, plants absorb carbon dioxide and produce oxygen; therefore, plants should have existed on land shortly after marine life came into existence. Why then did plants and animals arrive on land simultaneously after such a long interval? Scientific research shows that the ozone layer above Earth’s atmosphere formed four hundred million years ago, identical to the timeline of biological life arriving on land. Is this a coincidence?

The glory of the sun is often described as the number one element for life. Without the sun’s light and warmth, life could not be. The sun’s energy is nuclear energy, so its glory includes not only the warmth we need but also extremely strong lethal rays, like ultraviolet rays, X-rays, and gamma rays. If astronauts did not have protective devices, they would be killed by sunrays in space.

The atmosphere is a protective layer that absorbs harmful rays, but it is far from enough. About four hundred million years ago, the ozone layer formed due to a surplus of oxygen, and it proved to be particularly capable of absorbing harmful rays. Although the ozone layer is relatively small compared to the atmosphere, it was enough to protect life from harmful rays and allow them to emerge from the ocean’s protection and onto the land.

The moon is another great “shield.” As a satellite of Earth, it has blocked the attack of many foreign objects. At the same time, a series of other bigger planets like Jupiter, Saturn, Uranus, and Neptune revolve around the sun outside of Earth. They possess greater mass and stronger gravity than Earth. It is their gravity that pulls asteroids away from the inner circle of the solar system, shielding Earth from impact with extraterrestrial objects. Some scientists have asserted that Earth would never have conceived intelligent life without its protectors in the solar system. The birth of intelligent life takes a very long time, so if a large celestial collision occurred whenever the birthing process was close to completion, all past efforts would come to naught.

In recent years, humans have focused space exploration efforts on our neighbors, Mars and Venus. Our detectors have landed on these two planets and much close proximity observation has been carried out as well. Observations show that although Mars’ distance from the sun is only one third greater than that of Earth, its surface temperature is lower than -60C. The diameter of Mars is only half of Earth’s diameter, and its mass is one- tenth of Earth’s, making it difficult to capture external air. Mars only has an extremely thin layer of air on its surface; its density is less than 1 percent of Earth’s atmosphere density and is mainly composed of carbon dioxide. Therefore, the environment on Mars is not conducive to the survival of life.

Venus has the most similar conditions to Earth. Its diameter is only slightly smaller than Earth, its weight is 80 percent that of Earth’s, and it is only a quarter distance closer to the sun. Unfortunately, if Earth were paradise, Venus would be hell. Its surface temperature can reach 500C, there is no water, a thick layer of carbon dioxide atmosphere with pressure nine times higher than the Earth’s atmosphere blankets its surface, and traces of volcanic magma erosion linger all around. No life could possibly survive in such an environment; however, we have discovered traces of shallow oceans on the surface of Venus. These shallow waters disappeared about three billion years ago.

Why does Venus lack the ability to produce life when its planetary conditions are so similar to Earth’s? Moreover, why do these two similar planets have such differences in their natural environments? Many explanations have been suggested, and among them, two are highly recognized. The first view holds that the answer lies in Venus’ slightly closer proximity to the sun. When the sun’s temperature rose three billion years ago, it evaporated the water on Venus. This means that the one-quarter disparity between Venus and Earth’s distance to the sun completely changed the fate of the two planets.

The other view holds that the different fate of the two planets is not due to their distances to the sun but to Venus’ slow rotation. The metal magma at Earth’s center produced a magnetic field due to Earth’s rapid rotation, whereas the slow rotation of Venus could not produce such a force field. This foreshadowed the fate of the two planets. Scientists believe that the magnetic field plays a decisive role in planet evolution and the birth of life, because it prevents solar winds and cosmic rays from harming life. Venus experienced asteroid collision just like Earth in its early stages. It formed oceans and a carbon dioxide atmosphere as well; however, due to the absence of its own magnetic field, solar winds and cosmic rays eliminated algae and even bacteria from Venus. With no system to absorb carbon dioxide and produce oxygen, the carbon dioxide atmosphere thickened increasingly, strengthening the greenhouse effect and raising surface temperatures until all water was evaporated and the surface temperature reached its current 500C high.

Mars does not have a magnetic field either, even though it spins faster than Earth. Its small mass does not allow for magma at its core, so Mars is also void of magnetic field protection. The air on Mars’ surface is also very thin and does not offer sufficient protection against solar particles and cosmic rays. If complex life-forms ever existed on Mars, they would have gone extinct some time ago.

When astronauts observed our planet from space, they found the earth to be a beautiful blue planet. It stands out like a small oasis in a vast desert. Over decades of research and exploration, scientists have marveled at the stringent conditions required to produce complex life. After summing up their results and carrying out a series of experiments, many returned to one simple starting point: Earth is just one occasional occurrence in space. On one occasional planet in one occasional star system in the vast universe, a series of occasional incidents occurred over a relatively short period of time to produce an environment suitable for life, thus producing a monumental miracle: the birth of mankind.

However, we are not satisfied with this explanation. One scientific conclusion is for certain: as long as there is sufficient time and conditions, material activity will produce life—even intelligent life like humans. Nonetheless, too much time and too precise conditions are required, so we cannot accurately predict whether miracles of life exist in other corners of the universe.

Two: Prerequisites for the Birth of Intelligent Life

1. Star Systems That May Produce Intelligent Life

The conditions required for the birth of intelligent life are extremely precise. First, there must be a stable and continuously burning star. At the same time, the planet in question must satisfy many requirements and encounter many chance opportunities over a very long period of time. Our current abilities are far from adequate to apply this series of conditions to all the planets outside the solar system; however, we can set out some of the prerequisites needed. It must be noted that these are merely the indispensable conditions required to produce life, not the conclusive conditions that will guarantee life. Intelligent life cannot exist without these conditions, but these conditions do not promise the birth of intelligent life, since there are still many factors we do not yet understand. Let us first analyze the stellar conditions required to conceive intelligent life.

The star that forms a basis for intelligent life must satisfy the following:

a. It must be a star in the stage of hydrogen fusion; only stars in this stage burn stably.

b. Life cannot be born in large star systems. The greater the mass of the star, the more intense its internal thermonuclear reaction is, and the shorter it stays in its main star sequence. For example, a star 1.2 times the mass of the sun only stays in its main star sequence for about three billion years; according to the birth time of humanity (i.e., five billion years), even stars slightly larger than the sun cannot suffice.

c. Smaller stars cannot produce intelligent life around them either. The smaller a star, the less light and heat it produces, the closer a planet must be to obtain necessary warmth and glory. However, when planets are too close to stars, the gravitational pull of the star will slow its rotation, causing one side of the planet to bask in light and warmth while the other side is swallowed by darkness and cold. Complex life cannot appear on planets like this.

In addition, once a star is small enough to be a red dwarf, its surface will be very unstable and produce large flares and lethal rays. This type of star system is not suitable for the survival of complex life; 70 percent of the Milky Way is composed of red dwarves.

d. A completely suitable star must have experienced billions of years of stable burning, since intelligent life itself takes billions of years to form.

e. Intelligent life can only exist on a solid planet that has carbon, nitrogen, oxygen, iron, silicon, and other heavy elements. Since heavy elements are produced by large stars, only second- or third-generation stars can produce life around them.

f. Many binary and triple stars exist in the universe, but these star systems cannot produce life. In a star system like this, the stars will interfere with the planets around them due to their gravitational pull, resulting in very unstable planets.

g. It cannot be too close to large stars. Large stars end their lives as supernovas; before intelligent life-forms are sufficiently evolved, they do not have the ability to prevent the thermal radiation and cosmic ray injury brought on by supernovas. It cannot be too close to medium-sized binary stars either, since they form supernovas as well.

2. Planets That May Produce Intelligent Life

A suitable stellar environment satisfies only one condition in the birth of intelligent life. A suitable planet is also necessary, and it is much harder to come by. To use the solar system as an example, Earth is the only planet able to produce intelligent life. Even though Venus and Mars are very similar to Earth, they do not meet the necessary requirements. What conditions does a planet need to satisfy in order to produce intelligent life?

a. The planet must be large enough to “capture” atmosphere.

b. The planet’s atmosphere must be composed of carbon dioxide, oxygen, and also nitrogen. Hydrogen and helium exist in abundance within space, but planets with carbon dioxide and oxygen are rare.

c. The planet must be a metal-element based planet. In astronomy, all heavy elements apart from hydrogen and helium are considered metal elements; only metal planets are solid rock planets that can provide a foothold for life. Such planets are exceedingly rare in the universe.

d. The planet must be a suitable distance away from stars; this distance is called the habitable zone. If a planet is too far away it will not receive sufficient warmth; if it is too close it will be overheated.

e. The planet’s orbit around the star cannot have too large an eccentricity. If the eccentricity is too large and the orbit is flat oval in shape, the near point will be too close to the star and the far point will be too distant, resulting in extreme heat and cold.

f. The planet’s rotation axis cannot tilt too much. If the axis tilts too much, the planet will rotate on its side. Like Uranus, the planet will experience day for half a year and night for the other half. It will also experience half a year of summer and half a year of winter. During summer, light will bake the planet until temperatures reach hundreds of degrees, but in winter, there will be no light and temperatures will reach -200°C. This environment would not be conducive to life.

g. The planet must satisfy a variety of other requirements to produce complex life; some may be absolutely necessary while others may not. For example, it is generally believed that water is indispensable to survival of life, but there are differing views regarding this point. Life on Earth metabolizes on an oxygen-water system, and it is speculated that life on other planets may metabolize on a nitrogen-liquid ammonia system, or some other material system. A suitable magnetic field may be necessary, as it prevents cosmic rays and stellar radiation from harming life. The existence of a series of larger surrounding planets, like Jupiter, may also be necessary to block frequent impact from asteroids. And so on.

Three: The Location of Extraterrestrial Life

1. Possible Locations for Extraterrestrial Life

Let us take a look at the universe surrounding the solar system based on the above stellar and planetary requirements (though they are only necessary conditions and not sufficient conditions). We will use fifteen light-years as a boundary, since any distance beyond that would take hundreds of thousands of years of travel to on even the fastest non-manned spacecrafts.

There are thirty stellar system in this range, ten of which are binary stars and one triple star, so there is a total of forty-two stars. The only star that meets the stellar requirements is the Tau Ceti, 11.68 light-years away. Tau Ceti is a single G-star like the sun; it is yellow in color. The sun’s spectral type is G2, while Tau Ceti’s spectral type is G8, so it is slightly cooler than the sun (its temperature is 45 percent the sun’s temperature), and it stays in its main star sequence for about the same time, which is four billion to six billion years.

These conditions have made Tau Ceti an observation target for scientists. In recent years, astronomers have deduced that Tau Ceti may have five planets with masses two to seven times that of Earth. Among them, the fourth planet, called “planet e,” is in the habitable zone; its mass is 4.2 times the mass of Earth. At present, global scientists within the field all regard this speculation with great interest, as the existence of planet e and the possibility of it producing intelligent life all remain to be studied.

In conclusion, stars that can facilitate intelligent life are very rare, and it is even more rare for there to be suitable planets around them. Even if planet e did exist by Tau Ceti in a habitable zone, the probability of it meeting the other precise conditions for producing life are overwhelmingly negative. After all, the existence of a solid rock planet within the habitable zone of a suitable star is only one of the many basic conditions necessary to produce intelligent life.

Today, we can be certain that more planets exist outside of the solar system; only thousands have been discovered since 1995, and most of them are larger hydrogen and helium planets like Jupiter. We have not yet reached an in-depth level of observation and research regarding these planets; even the existence of planets around Tau Ceti are mere speculation.

Clearly, we cannot determine the location of extraterrestrial life according to planetary indicators; we can only rely on stellar indicators. Only one thing is for certain: planetary indicators will only be more stringent than stellar indicators.

2. The Search for Extraterrestrial Life

On March 2, 1972, the Pioneer 10 space probe was successfully launched into space by NASA. Its mission was to fly out of the solar system, after surveying Jupiter, in search of extraterrestrial life. On June 13, 1983, after crossing Neptune, it became the first man-made spacecraft to fly out of the solar system. An alternative definition of the solar systems stipulates that any area adhering to the solar system’s gravitational constraints belongs to the solar system. By this definition, Pioneer 10 still has to travel twenty thousand years before it leaves the solar system. Pioneer 10 carried a gold-anodized aluminum plaque marked with the cosmic positions of Earth and the solar system through multiple neutron stars, human figures of a male and female, and two circles indicating that the simplest material molecules were composed of two hydrogen atoms. This is a calling card of Earth’s humans that will last one billion years. It flies aboard Pioneer 10 towards Taurus; two million years later, it will reach the star Aldebaran.

A year later, Pioneer 11 followed its predecessor on April 6, 1973, journeying towards Aquila. It will take about four million years to reach this constellation. On August 20 and September 5 of 2007, NASA launched the two detectors, Traveler 2 and Traveler 1, towards Sirius and Ophiuchus. These two detectors both brought greetings from Earth.

Sending detectors out of the solar system is only one way we deliver information to extraterrestrial life; the more common way is to use radio waves to send messages. In July 2003, with the support of NASA and other authorities, more than ninety thousand “electronic greetings” from more than fifty- two countries around the world flew towards five stars similar to the sun. The former Soviet Union’s Yevpatoria Observatory was the first to send a greeting message to aliens as early as November 1962. They used ordinary telegram codes to issue the simple words—“Peace, Soviet Union, Lenin”—into the depths of space.

In November 1974, the world’s largest radio telescope was built in Puerto Rico; in celebration, the telescope sent a three-minute telegraphic greeting to the Messier 13 globular cluster. Later, the Yevpatoria Observatory sent greetings to extraterrestrial life two more times in 1999 and 2001. They also sent a music recording of an electric organ concert into space.

The United States’ Messaging Extraterrestrial Intelligence (METI) initiative plans to use radio or laser signals to send some “topic factors” to neigh- boring planets by the end of 2018. Then they plan to send signals to planets hundreds or thousands of light-years away; this will be the first time information will be sent clearly and continuously to extraterrestrial planets. The idea is to send continuous greetings towards one target planet for months or years in order to connect with aliens.

People have always had a strong interest in aliens. Not only are we keen to send information to aliens, but we also do everything possible to learn about them. Despite the many efforts to contact extraterrestrial life, so far no reliable evidence of extraterrestrial life has been received.

The UFO phenomenon has always been an unsolved mystery. We have had records of UFOs since ancient times. Many consider these UFOs to be evidence of aliens visiting Earth, but no real proof of alien encounters has been verified. There have been reports of alien sightings and even alien abductions. Logically speaking, such claims probably have very little truth to them. In 2009, the British government announced the closure of their official UFO investigation team due to it being “a huge waste of time and money.”

Four: Reconsidering Extraterrestrial Life

Though we have no concrete evidence of alien life and we understand that the conditions required to produce intelligent life are extremely stringent, we are certain that extraterrestrial life exists in the universe. Because the universe is extremely vast, even miracles of the smallest probability can happen.

According to modern astronomical observation and speculation, there are about two hundred billion stars in the Milky Way and three hundred billion galaxies in the universe, putting the number of stars in the universe at billions of trillions. Since the birth of the universe 13.8 billion years ago, stars have evolved through at least four or five generations (first generation star systems cannot produce life). Earth cannot be the only planet with intelligent life during all this time.

Even so, the possibility of alien life visiting Earth and threatening humanity is very small. Due to the exacting conditions required to produce intelligent life, it is unlikely that such life exists anywhere near us, and it would be even more unlikely that they could travel to us.

The power source for interstellar travel alone would be a big problem. The Apollo moon landing spacecraft was only forty tons, but its launch rocket was more than two thousand tons, most of which was propellant fuel. The moon is only 380,000 kilometers away; our nearest neighboring star is more than one hundred million times further than that.

The millions of years interstellar travel requires is another great consideration. A series of technical, physical, and psychological problems must be resolved for such a journey. Moreover, any intelligent person would lack sufficient motivation to carry out such a trip. Millions of generations of evolution and would pass by during this journey, and the end would not justify the means. If intelligent life really did exist, they would most likely choose to send probes or electronic messages towards Earth to satisfy their curiosity, just like us.

Of course, someone might bring up the theory of time and speed from the general relativity theory. If Einstein’s formula was completely accurate, it further proves that interstellar travel is impossible, since it shows that time, speed, and weight are interrelated. When an object travels at a speed close to light speed, time will become very slow, while the object’s weight will increase. Once the object reaches the speed of light, its weight will reach infinity. In other words, before humans could reach the speed of light, we would be crushed by the weight of our own bones. There is also no force that could propel such a heavy spacecraft; this has been proven by high-energy particle accelerator experiments.

We also often see “wormhole” travel methods in sci-fi movies. Would that be possible?

The wormhole concept is an inference based on the theory of general relativity. It is also known as the Einstein-Rosen bridge. According to the wormhole concept, time and space can be warped to form a shortcut between distant points, or it could form a shortcut between now and the future or now and the past. Based on this concept, people have proposed ideas of time travel and time machines that can change time and distance at will.

Realistically speaking, the wormhole is still a very immature inference, and it brings up many questions that cannot be answered. For example, some have asked what would happen if a person traveled back in time to kill their mother before they were born, or if wormholes would allow astronauts to return to Earth before they even left. This is clearly illogical. Many aspects of the wormhole theory cannot be solved with current scientific cognition.

Even Hawking, who was always at the forefront of such academic discussion, admitted in his book, A Brief History of Time, that the possibility of time travel still awaits conclusion.

Over the past centuries, we have been committed to finding traces of extraterrestrial visitation on Earth, but no such evidence has been discovered. If Earth could produce simple life-forms 4.28 billion years ago, it would have had basic survival conditions back then. If aliens wanted to occupy, colonize, or vacation on Earth, they should have arrived a long time ago. Since no extraterrestrial life has visited Earth in the past billions of years, we have reason to believe they will not arrive in the billions of years to come. Of course, this is just one type of logical reasoning.

The universe formed 13.8 billion years ago. First-generation star systems cannot produce life, but second-generation star systems should have existed ten billion years ago. It takes about five billion years for intelligent life to form, which means the earliest intelligent life should have formed five billion years ago.

Five billion years is an immensely long time for an intelligent species, so it should have been sufficient time to develop all manners of technology. So why have aliens not visited Earth? I think there are only two possible reasons.

First, the natural laws of the universe make interstellar travel impossible to overcome, even for the most intelligent beings. Since intelligent life can only be produced under very specific circumstances, the instances of such life would be few and far between. Even the alien life closest to Earth would be distant enough to negate the possibility of visitation.

Second, whenever intelligent life is formed, nature will also endue it with inherent flaws. Such flaws will cause the intelligent species to self-destruct before they have time to figure out the logistics of interstellar travel. Also, these life-forms may stop the development of interstellar travel technology for some reason. This will be examined further in this book.

Although the possibility of alien invasion is extremely small, it should not be completely ignored. Two considerations will be proposed here as reference: first, the occurrence of any major event will have warning signs in advance, including alien invasion. We will have time to take precautions accordingly to defend our home planet; perhaps that is all we can do. Second, it is very foolish to devote much research and preparation against alien invasions today. If extraterrestrial life really did invade Earth, they would possess much more advanced technology compared to our hundred thousand years of human history. They would be at least billions of years more advanced than humans; we could not hope to catch up to that in a few thousands or millions of years. Moreover, Earth has possessed suitable living conditions for 4.28 billion years without any sign of aliens, so it would be unnecessary to devote efforts to prepare for something of such minimal possibility within such vast time constraints.

The committed observation of extraterrestrial life is not much of a problem, but the continuous attempts to communicate with and send information to aliens is irrational. According to previous analysis, the possibility of alien visitation is very small, so this conclusion means two things. On one hand, if we cannot connect with aliens, all our efforts will have been wasted. This is too high a price to pay just for the satisfaction of curiosity. On the other hand, if aliens really could receive our information and travel to Earth with unimaginably advanced technology, humanity could be facing overall extinction.

It has been said that it is immoral for us to only detect alien information and not share our information as well. This is a very naive view. We observe aliens out of curiosity and goodwill, but we cannot assume that aliens will share our attitudes. The laws of survival always favor that the strong, highly civilized creatures capable of visiting Earth may not regard us as “people” (we are not the same species, after all) and kill us at will. That could be the end of humanity. Today’s decision-makers and scientists should stop such potentially devastating behavior just for the satisfaction of curiosity or some other purpose.



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