“Eyes of the Sky”: Mailboxes for “Replying Letters” to Alien Civilizations

Science fiction films mention that when people on Earth discover alien civilizations, there will always be a classic “prop”: a large building with a “cauldron”-like disc (a long antenna sticking out from the center of the disc). From the thriller-style “Heavenly Strike” in 1996, to the 1997 classic “Time and Time Contact”, to the low-cost film “Macro World” in 2019, we can find the majestic appearance of “Cauldron”.

The standard name of “Cauldron” is “Radio Telescope”. Astronomers use radio telescopes to harvest a lot of valuable astronomical discoveries, but the literature and art have nothing to do with aliens. Finding traces of extraterrestrial civilization is also the task of radio telescopes.

The cosmic background glow supports the “big bang theory”
Astronomy originated from stargazing. The stars we see when we look up are actually light from celestial bodies that spreads to the earth. Considering that light itself is an electromagnetic wave, since celestial bodies can emit light, they can theoretically emit electromagnetic waves of different wavelengths. As early as the early 20th century, the famous Edison and Planck both advocated that solar energy emits radio waves.

Based on this theory, scientists believe that in addition to observing the celestial body through visible light, radio waves are also a means. In 1931, Carl Yansky, an engineer at Bell Telephone Company in the United States, accidentally discovered radio waves from space. His basic job is to use huge directional antennas to study the interference of transoceanic wireless telephones.

There is a kind of noise, which peaks every 23 hours and 56 minutes. This time interval is exactly the length of the earth’s sidereal day—that is, the rotation time of the earth relative to the universe. So Yansky suspected that the signal came from the center of the galaxy.

Because Bell did not have astronomical research requirements, Yansky was unable to further observe signals from outer space with higher configuration equipment. However, this discovery determined a new direction for astronomical observation. In 1937, radio engineer Grote Raiber erected a parabolic dish-shaped radio telescope with a diameter of 9 meters. The first radio telescope for astronomical observation was born, and the branch of astronomy, radio astronomy, also ushered in its starting point.

Radio astronomy pushes the development of astronomy to new heights. From the perspective of the electromagnetic spectrum, the wavelength range of visible light is very narrow, about 360 to 860 nanometers. The radio wave wavelength covers a range from a few centimeters to hundreds of meters. The larger the range, the more content. Observing stars through visible light, it is obvious that the observation range is very limited. The astronomical secrets hidden in radio waves are much richer.

In the 1960s, astronomy made four extremely important discoveries, namely pulsars, quasars, cosmic microwave background radiation and interstellar organic molecules. These are all discoveries with the support of radio telescopes. Among them, the cosmic microwave background radiation is the most familiar to the public. In popular science introductions, it is generally presented in the form of “Big Bang Theory”.

The astronomy community believes that the universe originated from the rapid expansion of a singularity. During the expansion process, the universe continued to dilute and cool down from a uniform, high-heat state. In 1948, the American physicist Gamow proposed the “hot big bang” hypothesis and made a more detailed conjecture about the big bang process. One of the conjectures is that the current universe is bathed in residual radiation from the early high-temperature universe.

After Arecibo retired, FAST became a “one-eyed” and its significance became even more significant.

From an optical telescope, the space between the celestial bodies is completely dark, and there is no trace of residual radiation. But in 1964, Arnold Penzias and Robert Wilson used radio telescopes to discover a faint background glow in pitch darkness. The true face of Lushan cosmic microwave background radiation was revealed for the first time, and Gamow’s “hot big bang” hypothesis was also confirmed. The “Big Bang Theory” has evolved from a possible explanation of the origin of the universe to the only theory that supports current observations and data.

From “bigger” to “arrangement”
The contribution of radio telescopes is huge, but the long wavelength of radio waves hindered the development of radio astronomy in the early stage.

The observation capability of an astronomical telescope is determined by the aperture of the telescope and the wavelength of observation. The larger the aperture and the shorter the wavelength, the higher the resolution of observation. The wavelength of visible light is at the nanometer level, so optical telescopes can get higher resolution content with a smaller aperture. The wavelength of radio waves is at least tens of thousands of times that of visible light, which means that even if the aperture of a radio telescope is hundreds of times that of an optical telescope, the clarity of the observation content of the former is far from that of the latter.

In 1962, British physicist Martin Ryell used the principle of interference to invent a synthetic aperture radio telescope. By setting up a radio telescope array, the resolution of the radio telescope was greatly improved.

From the perspective of the development of radio astronomy, making a single-aperture radio telescope larger and building a radio telescope array are two parallel research paths. The advantage of a single-aperture telescope is that it is single-pole direct entry, reducing the possibility of signal interference by noise, and later data processing is more convenient. However, the observation resolution of a single aperture is limited, and the larger the aperture, the higher the cost.

The advantage of the array is that it has high observation resolution, the cost is lower than that of a single-aperture telescope under the same aperture, and the observation resolution and observation frequency can be customized by controlling the interference mode. However, the probability of the array receiving noise is higher than that of a single-aperture telescope, and the post-processing of the data takes more effort.

Single-aperture telescopes and arrays have different focus points. For some weak signals, the single-aperture telescope captures better and can prevent the signal from being overwhelmed by noise. And if you want to make fine observations of stronger signals, arrays are a better choice.

The eye-catching radio telescopes on the cover of “Time and Space Contact” are the famous “Very Large Antenna Array” (VLA). VLA is located in the desert of New Mexico, USA. Twenty-seven antennas with a diameter of 25 meters are arranged in a “Y” shape and are erected on the rails. The longest base line of the combination is 36 kilometers. It is currently the world’s largest integrated-aperture radio telescope.

If there is civilization in M13, it will take 50,000 years for their “replies” to return to Earth at the earliest.

Globular cluster M13

A more ambitious array plan is the “square kilometer array” (SKA). SKA is jointly constructed by more than ten countries around the world (China is one of the initiators), and consists of two parts: a low frequency array located in Western Australia and an intermediate frequency array located in South Africa. The name “square kilometer” comes from the receiving area of ​​all the mirrors facing the cosmic signal, which amounts to 1 square kilometer. In this way, SKA will have the highest image resolution of a radio telescope in history, and the sensitivity will be more than 50 times that of any existing radio telescope.

Due to the huge scale of the plan and multinational cooperation, the SKA, which was originally scheduled to be built in 2010 and completed in 2020, is currently scheduled to start construction in 2021.

In terms of single-aperture telescopes, the United States was also a leader. The Arecibo Telescope on the island of Puerto Rico is located in a volcanic crater. It was built in 1934 with a diameter of 305 meters and was later expanded to 350 meters. Regrettably, Arecibo encountered accidents such as the break of the auxiliary steel cable and the break of the reflector, and the break of the main load-bearing steel cable in 2020. The National Science Foundation announced its retirement and plans to dismantle it.

The 500-meter spherical radio telescope (FAST) built in Guizhou, China replaced Arecibo as the largest single-aperture telescope in 2016. FAST passed the national inspection and acceptance in January 2020 and was officially put into operation. Together with Arecibo, FAST called the earth’s two “eyes of the sky”. After Arecibo retired, FAST became a “one-eyed” and its significance became even more significant.

Reply after 50,000 years
“Eyes of the Sky” asked the sky and achieved remarkable results. Since its launch, FAST has discovered more than 240 pulsars alone. In addition, it is also expected to realize the detection of gravitational waves in the nanohertz within a short period of time, and capture the original gravitational waves during the Big Bang period.

Arecibo was retired, and the astronomy community lost an important observation tool. With the vision of exploring the mysteries of the universe together and building a community with a shared future for mankind, FAST will open access to scientists around the world. This helps to ensure that even if the “eyes of the sky” in the United States are closed, the exploration of the universe will not be delayed for all mankind.

In the process of human exploration of the universe, “Are we the only one?” is always a romantic and serious question. Radio telescopes have always been a puzzle-solving tool with high hopes.

American astronomer Frank Drake pioneered the use of serious methods to search for alien civilizations. When he was studying, he believed that, just as the earth’s radar, radio, television and other technologies would transmit radio waves into space, if other places in the universe had a certain degree of civilization, they would also leave traces of the use of radio.

Drake first tried to find signals of alien civilization in 1960, and his main tool was the first radio telescope of the National Radio Astronomy Observatory: Tuttle Telescope.

The 500-meter spherical radio telescope (FAST) built in Guizhou, China, replaced Arecibo as the largest single-aperture telescope in 2016

The Tuttle telescope aimed at the Epsilon star (Tianyuan 4) and the star Tau Ceti (the 5th Tiancang). Although there was no gain, this did not affect the full spread of the search plan. In the future, this similar effort in the astronomy community was called “SETI”-the acronym for the English phrase “search for alien life”.

Drake later became the leader of Arecibo’s upgrading. In 1974, after Arecibo completed the first major upgrade, Drake’s SETI work took another important step: He compiled a radio message with 1679 binary numbers, arranged according to specific circumstances, it will display A picture is hidden in the decimal notation, the structure of human DNA, the situation of the solar system, and so on.

This “Arecibo message” points to the globular cluster M13, which is 25,000 light-years away from Earth. If there is civilization in M13, it will take 50,000 years for their “replies” to return to Earth at the earliest. But if there are other civilizations along the way, maybe their reply will come sooner.

Even if the alien civilization sends a “reply” in 2021, the “mailbox” of Arecibo will no longer be there. Fortunately, FAST can replace the mission of receiving feedback. The FAST telescope is equipped with back-end equipment dedicated to the search of extraterrestrial civilizations, which can eliminate the interference of celestial signals and artificial signals from the earth, and collect narrow-band candidate signals in a targeted manner. Those faint clues to alien civilization may be hidden in it.

These serious scientific experiments are presented in a more dramatic form in literary works. In “Sky Strike”, the protagonist uses a small antenna used by neighbors to receive TV signals and sets up a set of receiving arrays, which roughly explains the principle of a comprehensive aperture radio telescope; in “Time and Space Contact”, the protagonist receives the transmission in a prime number. This is a tribute to the “Arecibo Information” that must be arranged with two prime numbers 73 and 23 to display the information correctly; “Macro World” simply interprets the aliens and sends the same binary expression and the same content arrangement. , To reply “Arecibo Information”.

As FAST becomes the only “eye of the sky” in the world, perhaps there will be more Chinese elements in the stories of future exchanges with alien civilizations. But perhaps even more amazing is that in the future, FAST “eyes of the sky” will allow mankind to see the real existence of extraterrestrial intelligence for the first time.

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