How do you make contact with other life forms in space?

The method for searching for intelligence in space is based on the assumption that intelligent beings arrive at some stage in their development to use radio technology. We are broadcasting, but is anyone receiving? Second article in the series

Omri Vandel, "Galileo"

If, in a future project, signs of biological activity are discovered on the surface of an extrasolar planet, how can we find out if there is developed life there, or even intelligent life? With the telescopes that exist today, we are not able to directly distinguish extrasolar planets, and even the planned telescopes, such as the SIM and the TPF, will not be able to distinguish the details on their faces. Sending a robotic spacecraft to orbit the planet and photograph it up close, like the missions to Mars and other planets in the solar system, is not practical because is there anyone there?
The enormous distances even to the nearest stars.
A different method for searching for intelligence is based on the assumption that intelligent beings reach at some point in their development the use of radio technology - human culture reached this about a century ago. Some of the radio waves transmitted on the surface of the earth (mainly in the short wavelengths used in FM channels, television broadcasts and radar devices) leak into outer space and can be detected using suitable receivers even at a great distance from the earth.
Similarly, radio waves from a technological civilization on the surface of an extrasolar planet are in principle detectable by radio telescopes on Earth. These telescopes look like huge plates, and are mainly intended for astronomical research using natural radio signals, which are emitted from stars, galaxies and other celestial bodies. The largest radio telescope today is located near the city of Arecibo in Puerto Rico, and the diameter of its "dish" is 300 meters.
From what distance can you expect to receive intelligent broadcasts, if they do take place? The intensity of the radio transmissions decreases according to the square of the distance, so the radio transmissions of the Earth (or of a distant planet) that scatter in all directions are greatly weakened at distances where other stars are.
The signal strength can be increased by sending a deliberate beam, if the location of the potential listeners is known. To do this, you can reverse the direction of operation of the radio telescope, and use it to transmit (instead of receive) a beam in a certain direction.
Even with the technology that exists today, the use of directed transmissions can allow reception at very large distances. For example, the Arecibo telescope can detect directional signals from an alien civilization transmitting through a similar radio telescope from up to 1,000 light-years away. It is understood that if there are civilizations more developed than ours, it is likely that they can transmit at a much higher power, and therefore it is possible to discover their transmissions from greater distances.

What frequency are you looking for?
When we come to search for intelligent signals, the question arises as to which frequency, or frequency range, should be searched for. It turns out that nature itself helps us answer this question. As we know, in communication in general the transmitted signal must be stronger than the background noise. At relatively low frequencies, the possibility that there is life, and perhaps even civilizations, outside the earth has always fascinated the human imagination. In the article we will try to evaluate the possibilities for existence of life and culture in other worlds. First article in the series
(below 1 GHz), the natural background noise in our galaxy is high. On the other hand, at high frequencies, above about 10 GHz, absorption in the atmosphere and cosmic background radiation noise increase. Therefore, the most suitable frequency band is in the 1-10 GHz region, as can be seen in the diagram.
It can be said that searching in this frequency range is similar to searching for a coin under a lamp - but it is not exactly like that. A presumption of an advanced intelligent culture whose scientists will know the "frequency window", and if they broadcast radio broadcasts aimed at communication with other cultures, they will use frequencies in the area most suitable for reception. Nature even "marked" one particular frequency in the frequency range in the "window".
It turns out that the hydrogen atom - the most common element in the universe - emits radio radiation with a characteristic wavelength of 21 cm, which is 1,428 MHz. An advanced culture wishing to transmit to other cultures may choose this frequency as an "agreed frequency" (of course it will be necessary to "filter" the intelligent signals from the radiation that originates from natural processes, and see below).

An important question is how to differentiate between natural radio signals and artificial-intelligent signals. It turns out that there are several criteria for this: content, structure and bandwidth.
A. Natural signals sound like random noise, while intelligent signals are likely to contain ordered content such as Morse code.
B. Natural signals are generally unstructured, while intelligent signals are likely to have a defined structure - for example, frequency modulation, periodicity, or discrete groups of signals.
third. Natural signals are usually spread over a wide range of frequencies, while signals transmitted by an intelligent culture are likely to be concentrated in one wavelength or in a narrow range of wavelengths, such as radio station transmissions, and this for reasons of saving energy and transmission channels.

No little green people, yet
These criteria are not always a sufficient guarantee that the signal is not natural. For example, in 1967 a radio signal from outer space was discovered that varied cyclically, repeating itself 33 times per second with vigorous precision. The excited astronomers called it by the identification name LGM - acronym (in English) for "little green people". In a short time, other similar sources were discovered, but it turned out to be a physical phenomenon - neutron stars that rotate rapidly around their axis and send a strong radio beam into space - objects that are now called pulsars.
In 1960, a young astronomer named Frank Drake, who worked at the radio telescope at the Green Bank Observatory in Virginia, suggested looking for intelligent radio signals from space - he aimed the telescope at two nearby stars similar to the Sun, and repeated the observations for several months.
The project, called OZMA (the name of the client from the book "The Wizard of Oz"), opened the field of SETI - the search for extraterrestrial intelligence (Search of Extra-Terrestrial Intelligence). Since then, dozens of similar projects have been conducted, but much more extensive and sophisticated. In recent years, a joint project of millions of personal computers has been established in the analysis of SETI observations in search of an intelligent signal (a project known as SETI@home), but so far without a positive result.
One of the most advanced means, currently being built as part of the SETI project, is the Allen telescope array, which when completed will include 350 radio antennas with a diameter of 6 meters each, and will be one of the most powerful and sophisticated radio telescopes in the world. Among the planned tasks (in addition to normal radio astronomical observations) - an examination of a million stars at a distance of up to 1,000 light years in the 1-10 GHz frequency range, with sufficient sensitivity to detect a signal at the transmission power of the Arecibo telescope.

Interstellar communication
Even if there are thousands of communication cultures in the galaxy at the same time, this does not guarantee that we will be able to discover their radio transmissions, due to the enormous distances. For example, if there are a thousand civilizations in the galaxy, the nearest civilization is about 3,000 light years away, far beyond detection range if the transmitted signal is not directed, and borderline even in the case of a directed signal with radio technology similar to ours.
The chance of two-way communication (that is, sending a message and receiving a reply) is even lower, since the message transit time must be taken into account. In the example we chose, let's assume that a culture 3,000 light-years away transmitted a directed signal that was received on Earth; Our answer will reach back to senders no earlier than 6,000 years after the original transmission. It is possible that the transmitting culture will become extinct before the answer arrives... Even if the transmitting culture still exists, it is likely that the transmitters of the original signal died long ago, so it is necessary to preserve the communication over many generations.

Fermi paradox: where are the aliens?
It is said that the famous physicist Enrico Fermi expressed his opinion on the possibility of the existence of extraterrestrial civilizations with the question "where are they all?" - Where are they all? A theorem that was named "Ferrami's Paradox".
The paradox stems from the assumption that an advanced culture will spread through the galaxy and populate all the worlds suitable for life, or at least visit them while leaving clear signs. If such civilizations existed, we should have seen their representatives or the signs of their visits on Earth. Since scientifically there is no evidence of such visits, the conclusion is that we are the only civilization in the galaxy.
It is possible, of course, that we really are the only or almost the only civilization in the galaxy, despite the vast number of worlds with potential for the development of life. The reasons for this may be related to the Drake equation:

1. The rare earth hypothesis (Rare Earth) - life on Earth was created due to a rare combination of astronomical and geological factors, and despite the potential for a large number of life-bearing planets, in fact they are extremely rare.
2. The hypothesis of rare intelligence - even if the number of planets on which living beings have developed is large, only in a few planets did life develop to the level of technological civilization (fc<<<1). Even if there are some civilizations in the galaxy today, they are tens of thousands of light years away, beyond the range of planet discovery, radio communication and space flights. However, many researchers are not convinced that the Fermi paradox is a real paradox, since the fact that we have not yet been visited by other civilizations (that may exist in the galaxy) can be explained in other ways.
3. Human culture is still "invisible" to most parts of the galaxy - other cultures have not yet discovered technological culture on Earth, since information about it, which has been spreading at the speed of light since the first radio transmissions (on short waves), has so far only reached a distance of Less than a hundred light years from Earth.
4. The unattractiveness hypothesis - although the information about the existence of biological life (not yet intelligent) on Earth has long since spread to all ends of the galaxy and reached all cultures capable of testing it (as our culture will probably be able to in a few decades using TPF and similar telescopes), however, Due to the multitude of life systems that are not intelligent, there is nothing unusual about the existence of the system on Earth that justifies the enormous expenditure of means and energy involved in interplanetary flight, even if it were possible for an advanced civilization. It is understood that there is no point in directing radio transmissions to a planet that lacks a technological culture capable of receiving them, and therefore no radio signals were sent towards Earth either.
5. Difficulties in long space flights - even if advanced civilizations have an interest in visiting planets with signs of pre-civilized life (as the Earth appears today to observers from distances exceeding a hundred light years), it is possible that the difficulties involved in space flight to interstellar distances are greater than that we imagine and therefore prevented the spread of civilizations according to the Fermi Paradox scenario.
6. The zoo hypothesis - according to this hypothesis, advanced civilizations discovered us a long time ago, but they avoid revealing themselves in order to preserve the original course of development of the earth, like in a huge zoo. Another version of this hypothesis is that in the concepts of the advanced cultures that await us, we are not yet ripe for contact (violent, unstable, even dangerous), and they are waiting for us to go through the "strings of puberty" before they reveal themselves to us.
In conclusion, despite the Fermi Paradox and the unsuccessful search (so far) by SETI, we cannot rule out the possibility that beyond the borders of the solar system there are many planets with life and possibly even civilizations. In other words, we don't know if there is life outside the earth, but if we don't look for it, we certainly won't find it.

Dr. Omri Vandel is an astrophysicist at the Rakah Institute for Physics at the Hebrew University, Jerusalem. Creator and lecturer of the course "Astrophysics and Life in the Universe", which has been studied by over a thousand students since 1999. Visiting professor at the University of Los Angeles (UCLA), where he taught, among other things, the above-mentioned course. His main research topics are black holes, quasars and active galaxies. In his spare time he is engaged in sports navigation, photography and the Esperanto language. Originally published in "Galileo" magazine

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