Have humans captured signals from extraterrestrial civilizations? The search for the unknown

When we look up at the stars, we naturally wonder: Are we the only civilization in the vast space? In the days when astronomical observations were not well developed, people imagined that there was life on other planets in the solar system. However, when we uncovered the mysteries of these planets one by one, we found that if we want to find similar people in the universe, we have to look further into the distant space.

The vast starry sky

Conduct radio observations

When people began to use radio to transmit signals, they naturally thought that since radio signals can cross oceans and send information to distant places, aliens might also use radio signals to transmit information about civilization to other parts of the universe. Soon after radio communication technology was put into use, scientists began to try to explore alien signals.

In the late 19th and early 20th centuries, physicist Tesla, radio pioneer Marconi and others successively claimed to have discovered signals from Mars. At that time, even the best telescopes could barely distinguish the topography of the surface of Mars, and the mountains and rivers similar to those on Earth made people imagine the existence of Martians. Once the strange radio signal had a little connection with the appearance of Mars, people would naturally think whether it was a message from Martians. In 1924, when the distance between Mars and the Earth became relatively close, the United States launched the "National Radio Silence Day" event from August 21 to August 23. All radio stations were silent for 5 minutes every hour to provide a "quiet" environment for listening to the Martian signals. However, judging from our detection results today, there is no civilization on the surface of Mars, and these efforts are of course fruitless.

In various science fiction works, people have imagined the appearance of many extraterrestrial intelligent creatures. However, humans still know nothing about their true form.

When radio astronomy was born in the early 20th century, radio astronomers soon realized that these radio telescopes could not only capture natural signals from astrophysical processes, but also artificial signals emitted by extraterrestrial civilizations. In 1960, Cornell University astronomer Drake proposed the concept of "water hole". He believed that if other civilizations have the same life form as us, they would naturally have a special liking for the substance "water". Near 1.4GHz, there is an area between the radio spectrum lines composed of hydrogen and hydrogen and oxygen, which can be used to superimpose artificial signals. Although Drake himself found nothing in his searches near several stars at the time, the concept of "water hole" profoundly influenced the subsequent search for extraterrestrial civilizations and is still one of the preferred frequency bands.

Frequency band of water tunnel

In 1957, the National Science Foundation of the United States funded Ohio State University to build a radio antenna dedicated to the search for extraterrestrial civilizations. This may be the earliest funding project for extraterrestrial civilizations. In 1971, NASA also joined the ranks of funding for monitoring extraterrestrial civilizations.

Since 1980, relevant researchers have begun to receive relatively stable funding. In 1971, some researchers even proposed the "Cyclops" plan to build a huge radio telescope array consisting of 1,000 100-meter diameter telescopes to search for radio signals from extraterrestrial civilizations, with a total budget of up to 10 billion US dollars. However, such a grand idea is difficult to obtain financial support, and the search for extraterrestrial civilizations has not made any breakthrough progress. To put it bluntly, not even a group of aliens have been found.

In the early 1990s, NASA allocated more than $10 million per year for related research. But after seeing no results, the funding came to an abrupt end in 1994.

Since then, radio astronomy observations used to search for extraterrestrial civilizations have received almost no funding from the public sector. Fortunately, however, many wealthy people who have a strong interest in extraterrestrial civilizations have generously donated money to support many observation plans from the 1990s to the present. Microsoft co-founder Paul Allen and Intel founder Gordon Moore each supported the "Phoenix Project" using large radio telescopes including the Arecibo telescope.

Arecibo radio telescope

From 1994 to 2004, the program conducted narrowband observations of more than 800 stars within about 250 light years in the 1.2-3 GHz frequency band, but the millions of "signals" it detected were all certified as interference signals originating from the Earth. In 2015, Russian entrepreneur Yuri Milner, a physicist, launched the "Breakthrough Initiative" and pledged to invest $100 million to promote a number of research programs related to the exploration of extraterrestrial life, including the "Breakthrough Listen" program. In the implementation of these programs, astronomers have also "smartly" stopped seeking to build dedicated large radio telescopes, but instead obtained observation opportunities by sharing telescope observation time with other research.

Searching for exoplanets

In addition to using radio telescopes to listen to signals in space, people are also searching for the existence of extraterrestrial civilizations along another path. The intelligent creatures we know of at present cannot survive on stars, so discovering planets outside the solar system is the first step to finding their existence. Although people have long speculated about exoplanets, it was not until 1995 that the first exoplanet was discovered. In 2011, the launch of the Kepler Space Telescope ushered in a major explosion in the discovery of exoplanets.

Kepler Space Telescope discovers potentially habitable exoplanets

Kepler Space Telescope

When a planet happens to be between the star it orbits and the Earth, the brightness of the star observed from the Earth will decrease slightly due to the obstruction of the planet. Once the planet leaves the position between the star and the Earth due to its revolution, the brightness of the star we observe will return to its normal level. By observing such brightness changes of stars, it is possible to determine whether there are potential planets around the star. This is the basic principle of the transit method for detecting exoplanets. The Kepler Space Telescope has a huge CCD array consisting of 42 CCD photosensitive elements. During the entire mission, the Kepler Space Telescope was aimed at the area of ​​the sky where the Cygnus constellation is located, and continuously recorded the brightness of 150,000 stars near the center of the Milky Way for scientists to search for clues to the existence of exoplanets.

The Kepler Space Telescope has confirmed the existence of a total of 2,300 exoplanets and has classified 4,500 celestial bodies as potential exoplanets. As astronomers analyze and acquire data from the Kepler Space Telescope, they are surprised to find that planets exist almost everywhere in space. In the solar system, there are no planets between the size of Earth and Neptune, but there are many such planets around other stars. This shows that the universe is actually a treasure trove of planets. Among these planets, the most likely to nurture life are those in the habitable zone. The distance between these planets and their stars can allow the existence of liquid water, providing the basic conditions for the existence of life similar to ours.

On December 5, 2011, astronomers used the Kepler Space Telescope to discover for the first time an exoplanet in the habitable zone, Kepler-22b, about 600 light-years from Earth. The surface temperature of this planet is about 21-22 degrees Celsius, and its orbital period is about 290 days, which is very similar to the Earth.

In recent years, we often see news about the discovery of "super-Earths". In fact, these news reports only report that scientists have discovered another exoplanet that is similar to the Earth in some aspects, which does not mean that the conditions there are completely similar to the Earth. In fact, the existence of liquid water is only one of the many conditions necessary for the birth of life. We have not yet fully confirmed that a certain exoplanet is completely suitable for the existence of life.

Active emission detector

Another way to search for extraterrestrial civilizations is to actively launch probes to bring back information about the Earth.

In the 1970s, the United States launched four probes, Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2. After completing their respective explorations of extrasolar planets, these four probes continued to fly beyond the solar system. Although the chance of meeting an extraterrestrial civilization in the vast space is very small, the astronomers and engineers who designed them have prepared for such contact. On Pioneer 10 and Pioneer 11, an aluminum alloy plate was attached to the probe to remind the extraterrestrial civilization that captured the probe where the probe came from.

Human information board on the Pioneer

In the universe, there is a celestial body named pulsar. The biggest feature of this celestial body is that it can emit very stable periodic signals. On the aluminum alloy plates carried by Pioneer 10 and 11, 14 pulsars and the relative positions of the sun and them are marked. The radiation frequencies of these pulsars are unique. If the civilization that captured this probe has the same technological level as us, they can determine which pulsars these pulsars are from the data of the pulsars that have been observed, and then determine the position of the solar system. The schematic diagram at the bottom of the aluminum alloy plate further illustrates that this probe comes from the third planet near the sun, which is our earth. The golden discs of Voyager 1 and Voyager 2 contain more abundant information, but the same method is used to mark the position of the earth.

The golden record player on board Voyager

At present, the universal time unit "second" and length unit "meter" on Earth are determined by standards jointly formulated and followed by the whole world. Obviously, it is impossible to describe to aliens using the units set by humans themselves, so scientists once again thought of using natural phenomena to convey unit information. They used a simple diagram to mark the image of spin transition in hydrogen atoms on the disk. According to our current understanding of the universe, hydrogen is the most widely existing substance in the universe and its atomic structure is also the simplest. Below the image, there is a short straight line to represent "1" in binary. The period of this transition is 0.704 nanoseconds, and the wavelength of the electromagnetic wave emitted is 21.106 cm. When marking the period of the pulsar and the distance from the earth, these two characteristic quantities of hydrogen atom spin transition are used as units of time and length. Based on our current understanding of the material world, the hydrogen atom spin transition phenomenon observed by aliens should have the same characteristics, and binary is an easy-to-understand mathematical expression. If their technological level is comparable to ours, they should be able to understand the information we want to convey.