Exploring Technosignatures: The Search for Extraterrestrial Intelligence
If an outsider were to observe the Earth, they may be able to detect various human-made technologies such as cell towers and fluorescent light bulbs, which could be seen as indicators of life on the planet.
The authors are astronomers who are involved in the search for evidence of intelligent life beyond Earth, known as SETI. In their research, they attempt to identify and detect technological signs of civilizations that exist beyond our planet, known as technosignatures. Although it may appear simple to search for a TV broadcast from an extraterrestrial source, it is actually a complex and challenging process to identify technosignatures from advanced civilizations that are far away.
The modern search for evidence of intelligent life beyond Earth began in 1959 when astronomers Giuseppe Cocconi and Philip Morrison demonstrated that radio transmissions from Earth could be detected by radio telescopes at interstellar distances. In the same year, Frank Drake initiated the first SETI search, Project Ozma, by using a large radio telescope to scan two nearby stars similar to the Sun to see if any radio signals could be detected coming from them. After the invention of the laser in 1960, astronomers discovered that visible light could also be detected from distant planets.
These initial efforts to detect radio or laser signals from other civilizations were all focused on searching for powerful, directed signals that were intentionally transmitted to the solar system and meant to be detected.
Given the technological limitations of the 1960s, astronomers did not consider the possibility of searching for broadcast signals such as television and radio broadcasts that would leak into space. However, a beam of a radio signal, with all of its power focused towards Earth, could be detectable from much farther away – think of the difference between a laser and a weak light bulb.
The search for intentional radio and laser signals is still a popular SETI strategy today. However, this approach assumes that extraterrestrial civilizations want to communicate with other technologically advanced life forms. Humans rarely transmit targeted signals into space, and some experts argue that intelligent species may actively avoid broadcasting their location. This search for signals that may not be being sent is referred to as the SETI Paradox.
Even though humans do not transmit many intentional signals into space, many technologies that we use today produce a large number of radio transmissions that leak into space. Some of these signals would be detectable if they came from a nearby star.
The network of television towers around the world constantly emits signals in many directions that leak into space and can accumulate into a detectable, though relatively faint, radio signal. Research is ongoing to determine whether the current emissions from cell towers in the radio frequency on Earth would be detectable using today’s telescopes, but the upcoming Square Kilometer Array radio telescope will be able to detect even fainter radio signals with 50 times the sensitivity of current radio telescope arrays.
Not all human-made signals are so diffuse, however. Astronomers and space agencies use beams of radio waves to communicate with satellites and spacecraft in the solar system. Some researchers also use radio waves for radar to study asteroids. In both of these cases, the radio signals are more focused and pointed out into space. Any extraterrestrial civilization that happened to be in the line of sight of these beams could likely detect these unambiguously artificial signals.
In addition to finding actual alien spacecraft, radio waves are the most common technosignatures featured in science fiction movies and books. However, they are not the only signals that could be out there.
In 1960, astronomer Freeman Dyson proposed that, since stars are by far the most powerful energy source in any planetary system, a technologically advanced civilization might collect a significant portion of the star’s light as energy using a massive solar panel-like structure. Many astronomers refer to these structures as megastructures, and there are several ways to detect them.
After using the energy in the captured light, an advanced society’s technology may re-emit some of the energy as heat. Astronomers have shown that this heat could be detectable as extra infrared radiation coming from a star system.
Another possible way to detect a megastructure would be to measure its dimming effect on a star. In particular, large artificial satellites orbiting a star would periodically block some of its light, which would appear as dips in the star’s apparent brightness over time. Astronomers could detect this effect in a similar way to how distant planets are discovered today.
Another technosignature that astronomers have considered is pollution. Chemical pollutants such as nitrogen dioxide and chlorofluorocarbons, which are almost exclusively produced by human industry on Earth, could be detected in the atmospheres of exoplanets using the same method that the James Webb Space Telescope is using to search for signs of biology on distant planets. If astronomers find a planet with an atmosphere filled with chemicals that can only be produced by technology, it could be a sign of life.
Artificial light or heat from cities and industry could also be detectable with large optical and infrared telescopes, as would a large number of satellites orbiting a planet. However, a civilization would need to produce far more heat, light, and satellites than Earth does in order to be detectable across the vastness of space using the technology that humans currently possess.
It is difficult to determine which signal would be the most effective at detecting extraterrestrial civilizations, as no astronomer has ever found a confirmed technosignature. While many astronomers have speculated about what might make a good signal, ultimately, it is unknown what extraterrestrial technology might look like and what signals are present in the universe.
Some astronomers support a generalized SETI approach which searches for anything in space that current scientific knowledge cannot naturally explain. Others, including the authors, continue to search for both intentional and unintentional technosignatures. The main point is that there are many ways to detect distant life, and since it is uncertain which approach will be successful first, there is still a lot of exciting work to be done.
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