If alien life exists, what traces will it leave?

Author: Global Science

Are you curious about the search for extraterrestrial life? Speaking of this topic, science fiction fans will definitely think of a movie called "Contact", which can be said to be half adapted from Carl Sagan's "Contact" because the script was written by Carl Sagan himself.

Contact by Carl Sagan

Sagan began to conceive the prototype of this film in 1979. However, the film encountered some changes in the middle, and it was not officially filmed until 1997. At that time, Carl Sagan had already left us, and he might not have seen the final film.

Contact Movie Poster

But this still does not affect the fact that this movie is a very good work. Especially the female lead, I think she performed very well, showing the feeling of despair and expectation. So, later when I learned that this character had a prototype, I was quite shocked. I didn't expect that there really is such a scientist in the real world.

The actress is Jodie Foster, a very famous actress. Her prototype is also from an American organization for exploring extraterrestrial civilizations called SETI (Search for ExtraTerrestrialIntelligence), and she is the founding scientist of this organization, named Jill Tarter.

Jill Tate

Jill Tarter spent a long time exploring extraterrestrial life. There is a place called the Allen Radio Telescope Array in California, USA, which is a radio array that they funded. They can use this radio telescope array to conduct research on searching for extraterrestrial life signals.

Allen Telescope Array

The Arecibo radio telescope in Puerto Rico is also mentioned in this movie. It also plays a big role in SETI work. At that time, scientists also sent a message to the outer space through this telescope.

Scientists did a lot of crazy things in the 1970s and 1980s, such as the Arecibo message. In 1974, scientists sent a message through the Arecibo radio telescope. The Arecibo message mentioned numbers, human DNA, the position of the earth in the solar system, and even the radio telescope used to send the message, as well as its approximate size. If this message was really received by aliens, it is very likely that they would know a lot of information about the earth.

Arecibo radio telescope

The message was sent to a place with a very dense number of stars, called the globular cluster Messier 13. That area is very far away from us, and it is unlikely to be received by anyone in a short period of time. At that time, there was probably no consideration of whether it was risky or not. Now that "The Three-Body Problem" has become popular, everyone has started to discuss how risky such behavior is.

Arecibo Information

Looking back, we found that the exploration of extraterrestrial life actually involves two points - scientists generally use two methods to explore extraterrestrial civilizations. The first method may be as we mentioned before. Assuming that the extraterrestrial civilization is advanced enough to emit electromagnetic waves, we can use certain radio equipment or other methods on Earth to search for electromagnetic waves and see if there are any so-called structural things in the electromagnetic waves. Once there are structural things, it may represent information, perhaps transmitted by intelligent life.

Of course, there are other directions, such as actively looking for some characteristics of alien civilizations. The most classic one may be the Dyson sphere. The concept of the Dyson sphere was proposed by Freeman Dyson before 1960. At that time, everyone was actually caught up in the search for aliens. So people would think, if an alien civilization has reached a certain level, what kind of actions will it take when using energy?

Freeman Dyson

According to the classification of Type I, Type II, and Type III civilizations, if a civilization reaches Type II and wants to use the energy of its entire star, it may set up an electric panel that absorbs all solar energy and cover its star to absorb as much energy as possible from the sun. This creates a problem. In the signals we collect, the sun's light is not so constant and may show the characteristics of artificially changing its light. Once such a signal is discovered, we may think that someone is using solar energy, and a civilization that can reach this level to use solar energy may be a very advanced civilization.

This reminds me of something very interesting I saw before. People initially used the transit method to observe exoplanets. The transit method is because in most cases, you can see distant stars clearly, and the stars will shine, but it is difficult for you to see the planets directly. However, if the planet happens to move between you and the star, it will slightly block part of the star's light.

Through the change in brightness, we can analyze some data about the planet's possible motion period and size. Someone accidentally discovered that when a planet passed by its star or its sun, the brightness of the sun suddenly changed greatly, perhaps by 10% to 20%. So people were very excited at the time, saying that it was possible that the people on the planet built a structure that was a bit similar to the Dyson sphere, and then blocked the sun's light. But after a long discussion, everyone believed that it was some interstellar dust that blocked the star's light, rather than some civilization creations that we expected.

How to search for extraterrestrial life

Many of the above attempts are still based on the human civilization on Earth. I think there is a good saying before, it is difficult to imagine something you have never seen, something you never know what it is. There is also a saying before, if you lose something, you may like to look for it under the street light, which is a habitual action. But in fact, when exploring extraterrestrial civilizations, we may have many differences. Their living environment, their formation method, and even the process of their civilization may be completely different from our earth civilization.

But scientists don't have any other options when searching. For example, the conventional way of thinking is to first look for a habitable planet, which is what the Kepler program and many previous programs have been doing. After finding the planet, we will see if it is in the habitable zone. The habitable zone proves that its water can exist at around 0°C (or in liquid form at the local air pressure). After such a condition is met, we will consider what its atmospheric composition is like.

If the civilization above is not an intelligent civilization, scientists used to think that if it showed oxygen or methane, it would be considered a biological modification behavior, and it would be regarded as a good indicator of the existence of life. But in fact, subsequent scientific research has found that it may not be. Because many natural phenomena can actually generate oxygen and methane in the atmosphere. Although it is a highly active substance, it was previously believed that it might be consumed and could not exist for a long time. But in fact, it can also be continuously produced through some geological activities or some other behaviors, filling the atmosphere. So for now, scientists no longer explicitly use these two points as a typical feature of the existence of extraterrestrial life.

But the search for extraterrestrial life has to continue, so I feel that scientists are really like looking for a needle in a haystack. They may look for various things that may reflect some laws and traces of life activities.

A new theory of assembly

In fact, there are many research institutes in the world doing similar things. For example, the Unknown Life Signal Laboratory mentioned in an article is doing this. Its abbreviation is LAB (Laboratory for Agnostic Biosignatures). They may think of using a more fundamental way to distinguish between the so-called natural behavior and life behavior.

Some of the concepts mentioned in it left a deep impression on me, one of which was proposed last year. Some scientists proposed the assembly theory, which is mainly used to judge the complexity of molecules. For example, taking the name Oumuamua as an example, the assembly theory believes that if you want to spell out Oumuamua as a string of characters, you must first add a u after o - this is the first step, and then add an m, a u, and an a, and then mua, these three letters can be regarded as a combination, and now you are equivalent to adding another mua combination to the back. So in total, you have added 5 steps.

The assembly complexity of the Oumuamua system is 5. One of its core ideas is that it believes that if it is a molecule related to life, then its complexity should be higher, especially after it tested many molecules and concluded that if its complexity is more than 15, then it is very likely to be a molecule related to life.

They also did some model calculations and believed that if its complexity exceeds 15, then the possibility of directly generating such a molecule under natural conditions may be only 1 in 10^23, which is a very small probability.

NASA specifically tested this scientist. They took many kinds of samples, but did not tell the scientist what they were, and asked him to distinguish what they were. There are many test samples that will make you feel quite difficult. For example, there is a fossil sample from 14 million years ago. You think it has become a fossil. From a geological point of view, it has been petrified. In theory, we may think that it is actually no different from the composition of rocks. But in fact, it can still be distinguished. Another sample comes from a meteorite. That is the Murchison meteorite. It has a characteristic that there are actually many organic substances on it. But it is not it does not represent the characteristics of life. And it is also famous for its complex composition. In this test, the assembly theory also clearly pointed out that this component is not a component of life. So overall, the assembly theory can better distinguish which ones are formed by life and which ones are not in the current test link.

Murchison meteorite

When you first saw this theory, you might think it's a little too theoretical, but when you see that the tests he did later were actually successful, you seem to be interested in it again. Because essentially, the baseline of his theory is that he believes that matter that reaches a certain complexity line must be caused by the activities of life. So whether this thing is universal in the universe environment, I think it's actually a question mark, but when he summed up the assembly number, he actually used some real life and non-life to summarize such a standard. So from an empirical point of view, this may be effective, but whether it will continue to be effective in the future, we may need further verification.

From another perspective, if the target is very close, such as the moon or Mars, we can actually send a probe to the place to verify whether there is water, complex molecules, and traces of life in the environment. But if it is too far, we may do it through some remote means.

In fact, many in-situ experiments have been done on Mars. Scientists conducted a radioactive carbon 14 experiment on Mars in the 1970s. They mixed radioactive carbon 14 into the Martian soil as an experimental group, and then sterilized the soil at 160°C as a control group. Let's assume that 160°C is enough to kill the microorganisms on Mars.

But the results are really amazing. They found that this experimental group, that is, the group that was not subjected to high temperature, really released carbon dioxide containing radioactive carbon 14, while the other group did not. But the problem is that other detections during the same period, especially some efforts to find organic carbon in situ, have not produced any results, so this experiment is equivalent to an isolated evidence, and it is difficult for us to say that it may be effective based on this experiment.

Lock and key structure

There is another problem, which is that all of these are actually still limited to the life system of our earth, or our current detection technology may still be limited to the life forms on earth.

This is a difficult dilemma to get out of at present. Although we have been emphasizing that we should understand the difference between life and non-life in a more fundamental way, and then make some identification in a more fundamental way that is not based on the earth. We just mentioned, for example, the assembly theory mentioned above is also making some attempts, but there may still be a long way to go. In addition to the assembly theory, there are actually 1 or 2 other ways to do this work.

For example, the "lock-key structure" theory is very interesting. It believes that in nature, although complex minerals can form some relatively regular arrangements, their regular arrangements are also relatively mechanical. However, if it is a life structure, it may need some more flexible structures, such as locks and keys that can be buckled together, or can be cleverly combined and targeted. Such structures may have an essential relationship with life activities. If such structures exist in a substance in large quantities, they may be related to life activities. This theory does not care whether these things are silicon-based, carbon-based, or what. It may hope to use some structured things to determine the difference between this thing and non-biological things, and then make a systematic judgment based on this.

The lock-key structure we mentioned earlier actually means that in the general non-living material level, there may be certain binding points, but this fine structure has not reached a certain threshold. However, if you are a life behavior, an active life behavior, due to complex biochemical needs, there may be a large number of such lock-key structures, so that it can be completely distinguished from the surrounding non-living matter.

But of course this is also a discussion on a theoretical level, which has already left aside the life we ​​know. It is just to find the common points from some phenomena of the particular essence of a life, and then put forward such a theory.

All in all, although the earth is so small, there are already so many lives. The universe is so big, if there were only us, wouldn’t it be such a waste!

This article is a work supported by Science Popularization China Starry Sky Project

Team/Author Name: Global Science

Reviewer: Zhou Xiaoliang

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.