Recently, a study published by the National Aeronautics and Space Administration (NASA) shows that if there are advanced extraterrestrial civilizations in our nearby stellar systems, we may be able to detect extraterrestrial civilizations by observing the air pollution in their systems. The study evaluated nitrogen dioxide, an air pollutant.

"Most of the nitrogen dioxide on earth comes from human activities - such as car exhaust and fossil fuel power plants." "Most of the nitrogen dioxide in the lower atmosphere (about 10 to 15 kilometers above the ground) comes from human activities," said Ravi kopalap of NASA's Goddard Space Flight Center, the lead author of the study. Therefore, the observation of nitrogen dioxide on a livable planet may indicate the existence of industrial civilization on the planet. " It is understood that the research paper has been accepted by the Journal of astrophysics. But is it really feasible to explore extraterrestrial civilization by evaluating air pollutants?

Is it reliable to monitor nitrogen dioxide and search for alien civilization

Ping Jinsong, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, told Science and Technology Daily that, taking the earth as an example, if a small optical telescope is set up on the moon or Mars, the pollution gases from industrial sources in the earth's atmosphere can be detected.

According to Ping Jinsong, if it is found that the total amount of gas emissions from non industrial sources exceeds the maximum amount of gas emissions from non industrial sources, it can be inferred that the excess gas is likely to be generated by industrial activities.

Ping Jinsong stressed that although this method of detecting nitrogen dioxide to find extraterrestrial civilization may be feasible, in principle, the detection sensitivity required to determine the concentration of nitrogen dioxide by spectral detection has reached the limit of current human technical ability.

Looking forward to the birth of more "landmarks" of civilization

In addition to nitrogen dioxide, other gases that are difficult to produce directly in nature can also be used as markers. For example, freon, a product of highly developed human science and technology, can be monitored by the same spectral measurement method because it has endangered the biosphere due to its pollution and damage to the earth and is being gradually replaced by other products.

Similarly, industrial gas pollutants, especially those associated with energy conversion and storage, can be used as markers for detection.

"It is estimated that within a year or two, scientists will give a very detailed list of marker gases." According to Ping Jinsong, according to the rules of star formation and evolution, planets are generally surrounded by stars. By observing the stellar spectrum with astronomical telescope, we can calculate the species and relative total amount of the planet's pollutant gases, and infer whether there is an extraterrestrial civilization on the planet and the development of the civilization.

"In order to refine the judgment basis, we can set up mini observation equipment on the lunar surface, monitor the total amount of these components in the earth's atmosphere, and build relevant models, which can be used to reasonably determine the degree of development of extrasolar civilization that can be monitored in the future." Ping added.

Human beings have been working hard to explore the extraterrestrial civilization

A potential and default starting point of looking for extraterrestrial civilization is to look for extraterrestrial intelligent life similar to the civilization level of human beings on earth or with higher civilization level. In addition to searching for extrasolar planets with abnormal atmospheric pollution, Ping also listed other detection methods currently being carried out by the scientific community.

After the realization of high-power radar technology, scientists and engineers believe that there should be a certain number of advanced civilizations outside the earth, with the ability to use high-power radio transmitters to carry out communication. To this end, engineers and radio astronomers use the world's most sensitive radio antennas, including the collapsed Arecibo antenna and the running fast sky eye antenna, to search for unnatural strong radio signals that may come from outside the solar system.

From the perspective of quantum mechanics, scientists also believe that since the emergence of the universe until now, there have been many quantum states existing at the same time, so there are multiple parallel universes, and correspondingly there are "extraterrestrial" civilizations in parallel universes similar to the earth civilization. Looking for the evidence of the existence of parallel universes is a way and method to find out the more generalized extrasolar civilization.

In addition, National Aerospace agencies have launched various kinds of probes to the moon and the major planets of the solar system to directly carry out in-situ search and exploration, which has ruled out the existence of advanced intelligent life on the moon and the planets of the solar system. At present, they are trying to find the possibility of the existence of the most primitive life, micro organisms.