Are there extraterrestrial civilizations in the universe? For this question, I believe everyone will tend to give a positive answer. In fact, the scientific community does not deny the possibility of the existence of extraterrestrial civilizations, and in the process of exploring the universe, scientists have also been trying to find possible extraterrestrial civilizations in the universe.
As far as the current situation is concerned, there are not many ways to find extraterrestrial civilizations, and one of the common methods is to collect and analyze a large number of electromagnetic waves from the deep space of the universe, and try to find clues of extraterrestrial civilizations from them, and when using this method, electromagnetic waves with a wavelength of 21 centimeters will be of special attention to scientists.
So, what's so special about 21 cm? Why do scientists believe that extraterrestrial civilizations use 21 centimeters of electromagnetic waves? Let's talk about it in detail.
Scientists speculate that if there are really extraterrestrial civilizations in the universe, then when their scientific and technological level develops to a certain extent, they are likely to look for other intelligent civilizations in the universe like we humans, and in the process, they are likely to consciously send information to the universe in anticipation of the response of other civilizations, and electromagnetic waves, as a good information carrier, will of course be a good choice.
Due to the limitation of the level of science and technology, we cannot monitor the electromagnetic waves from the deep space of the universe in all frequency bands, in this case, we need to choose a frequency band that is most likely to be used by extraterrestrial civilizations. How should you choose? The first thing scientists think of is a specific frequency of electromagnetic waves emitted by the ultrafine transition of neutral hydrogen atoms.
According to quantum theory, in the energy level of an atom, due to the influence of factors such as electron spin, nuclear spin and nuclear charge distribution, the energy level of an atom usually does not have a simple single energy value, but there will be a small split, and this small energy difference is called ultrafine structure.
Neutral hydrogen atoms refer to electrically neutral hydrogen atoms, although their structure is very simple, only composed of a proton and an electron, but they may also have a superfine structure, because protons and electrons have spins, which will have two possible states for them: 1. The spin direction of protons and electrons is the same; 2. The spin directions of protons and electrons are opposite. For the sake of description, we can refer to the former as a "symmetric state" and the latter as an "antisymmetric state".
That is, even when the energy level of a neutral hydrogen atom is in the ground state (the lowest energy level), there will be a slight energy difference due to the different states of the spin directions of the protons and electrons. Scientists have found that since the energy of the "symmetric state" is slightly higher than that of the "antisymmetric state", when the neutral hydrogen atoms are in the "symmetrical state", they will spontaneously transition to the "antisymmetric state", and this phenomenon is called the "superfine transition of neutral hydrogen atoms".
The "ultrafine transition of neutral hydrogen atoms" emits an electromagnetic wave with a frequency of about 1420 MHz, which scientists call the "celestial 21 cm-hydrogen line" because of its corresponding wavelength of about 21 centimeters.
It is important to know that most of the universe is a region of low temperature, low pressure and low density, and the energy levels of atoms and molecules in these regions are basically in the ground state, and it is almost impossible to radiate visible light, so these regions have become "dark regions", and it is difficult for us to detect them by optical means.
However, because hydrogen is the most abundant element in the universe, they are widely present in space, and even in these "dark regions", there are a large number of neutral hydrogen atoms, so the "neutral hydrogen 21 cm line" released by them has become a powerful tool for human beings to explore space, and has therefore been widely used in radio astronomy and other fields.
So it's reasonable to speculate that when an alien civilization reaches a certain level of technology, it's easy for them to notice the "neutral hydrogen 21 cm line" and keep an eye on it.
And if this is the case, then when they want to send information to other intelligent civilizations in the universe through electromagnetic waves, they are likely to consider using 21 cm electromagnetic waves in the same frequency band as the "neutral hydrogen 21 cm spectrum line". The reason is simple: since they will be concerned about this particular electromagnetic wave, other intelligent civilizations are likely to be concerned about it because of similar scientific knowledge.
In addition, there are other advantages to using 21 cm electromagnetic waves, which have two main points: 1. This electromagnetic wave is located in a specific position in the radio spectrum and is not easily "drowned" by background noise in the universe; 2. This electromagnetic wave has a longer wavelength and can easily pass through the gas and dust in the universe and then travel farther, in addition, they can also easily pass through the atmosphere of the planet and then reach the surface of the planet.
It can be seen that these benefits undoubtedly further increase the possibility of extraterrestrial civilizations using such electromagnetic waves, and it is precisely because of this that scientists believe that extraterrestrial civilizations are likely to use 21 centimeters of electromagnetic waves to send us messages.
It is worth mentioning that although we have not found any confirmed signals from extraterrestrial civilizations until now, there are still suspected signals, the most famous of which is the "Wow signal" detected by the "Big Ear" observatory of the SETI project on August 16, 1977 in the 21 cm band.
(↑ The "Wow signal" comes from the direction of Sagittarius, and its emission source is about 1800 light-years away)
Unfortunately, due to the Earth's rotation, the Big Ears observatory only listened to it for 72 seconds, and after that, the signal never appeared again......