The search for extraterrestrial signals at a wavelength of 21 cm is a promising scientific research field that has the potential to unveil mysteries about life and intelligence beyond Earth.
Is there extraterrestrial civilization in the universe? For this question, I believe people tend to lean towards a positive answer. In fact, the scientific community does not deny the possibility of extraterrestrial civilizations existing. During the exploration of the universe, scientists have made efforts to search for potential extraterrestrial civilizations.
Given the current situation, there are not many ways for us to search for extraterrestrial civilizations. One commonly used method is to gather and analyze a vast amount of electromagnetic waves from deep space and attempt to find clues about extraterrestrial beings. In using this method, the electromagnetic waves with a wavelength of 21 cm are of particular interest to scientists.
So, what makes the 21 cm electromagnetic waves special? Why do scientists believe that extraterrestrial civilizations would use 21 cm electromagnetic waves?
Scientists’ interest in 21 cm electromagnetic waves. (Illustrative image).
Scientists speculate that if there are indeed extraterrestrial civilizations in the universe, once their technological level develops to a certain point, they might seek other intelligent civilizations in the universe, much like we do. In that process, it is likely that they would consciously send information into space to anticipate responses from other civilizations, and electromagnetic waves, being good carriers of information, would certainly be a good choice.
Due to technological limitations, we cannot monitor the entire spectrum of electromagnetic waves from deep space, in which case, we need to select the frequencies that extraterrestrial civilizations are most likely to use. How should we choose? The first thing scientists think of is: an electromagnetic wave with a specific frequency released by the hyperfine transition of neutral hydrogen atoms.
According to quantum theory, at the atomic energy level, due to influences from factors such as electron spin, nuclear spin, and the distribution of nuclear charge, the energy levels of atoms are generally not a simple energy value but have many small subdivisions known as hyperfine structure.
We need to select frequencies that extraterrestrial civilizations are most likely to use. (Illustrative image).
Neutral hydrogen atoms refer to electrically neutral hydrogen atoms. Although their structure is very simple, consisting of one proton and one electron, they can also exhibit hyperfine structure because both proton and electron have spin – existing in two states: the spin direction of the proton and electron is the same; the spin direction of the proton and electron is opposite. For convenience of description, we can call the former state “symmetric state” and the latter state “antisymmetric state.”
This means that even when the energy level of the neutral hydrogen atom is in the ground state (the lowest energy level), there will be a small energy difference due to the different spin directions of the proton and electron. Scientists have discovered that because the “symmetric state” has slightly higher energy than the “antisymmetric state”, when neutral hydrogen atoms are in the “symmetric state”, they will spontaneously transition to the “antisymmetric state”, and this phenomenon is called “the hyperfine transition of neutral hydrogen atoms.”
“The hyperfine transition of neutral hydrogen atoms” will emit an electromagnetic wave with a frequency of about 1420 MHz. Since its corresponding wavelength is approximately 21 cm, scientists refer to it as the “21 cm neutral hydrogen spectral line” (21 cm hydrogen line).
You should be aware that most of the universe is a region of low temperature, low pressure, and low density. The energy levels of atoms and molecules in these regions are essentially in the ground state and can hardly emit visible light, making these areas become “dark regions” that are difficult to detect by optical means.
However, hydrogen is the most abundant element in the universe, so it exists widely throughout. Even in these “dark regions,” there is a large number of neutral hydrogen atoms, making the “21 cm neutral hydrogen spectral line” a powerful tool for humanity to explore the universe and thus is widely used in fields such as radio astronomy.
Therefore, a reasonable hypothesis is that when the technological level of an extraterrestrial civilization reaches a certain point, they will easily notice the “21 cm neutral hydrogen spectral line” and pay attention to it.
The 21 cm electromagnetic wave is not easily “drowned” by cosmic background noise. (Illustrative image).
If that is the case, when they want to send information to other intelligent civilizations in the universe through electromagnetic waves, they are very likely to consider using the 21 cm electromagnetic waves within the same frequency range as the “21 cm neutral hydrogen spectral line.”
Moreover, the use of 21 cm electromagnetic waves has other benefits: This type of electromagnetic wave occupies a specific position in the radio spectrum and is not easily “drowned” by cosmic background noise; This type of electromagnetic wave has a longer wavelength and can easily penetrate gas and dust in space and travel farther. Additionally, they can easily pass through a planet’s atmosphere and reach the planet’s surface.
It can be seen that these advantages will certainly enhance the possibility of extraterrestrial civilizations using such electromagnetic waves. For this reason, scientists believe that extraterrestrial civilizations are capable of using 21 cm electromagnetic waves to send information to us.
