Alien technology signatures could be how humanity first discovers extraterrestrial life
Many human technologies, from cell towers to fluorescent light bulbs, could serve as beacons, indicating the presence of life if an alien were to look at Earth.
Radios and lasers for greeting
These early, foundational attempts to detect radio or laser signals from another civilization were all looking for focused, powerful signals that would have been sent to the solar system on purpose and meant to be found.
One of the most popular SETI strategies today is the search for intentional radio and laser signals. This approach, however, is based on the assumption that extraterrestrial civilizations want to communicate with other technologically advanced life. Humans rarely send targeted signals into space, and some researchers believe that intelligent species may avoid broadcasting their locations on purpose. The SETI Paradox refers to the search for signals that no one may be sending.
Radio waves are leaking
Though humans do not send many intentional signals into space, many of the technologies we use today produce a lot of radio transmissions that leak into space. If these signals came from a nearby star, they would be detectable.
The global network of television towers constantly emits signals in multiple directions, which leak into space and can accumulate into a detectable, albeit relatively weak, radio signal. Researchers are still looking into whether or not current radio telescopes can pick up radio waves from cell towers on Earth. However, the Square Kilometer Array radio telescope will be able to pick up even fainter radio signals with 50 times the sensitivity of current radio telescope arrays.
However, not all man-made signals are as unfocused. Astronomers and space agencies communicate with satellites and spacecraft in the solar system using radio wave beams. Some researchers study asteroids using radio waves for radar. The radio signals are more focused and pointed out into space in both of these cases. If an alien civilization happened to be in the path of these beams, it would almost certainly be able to pick up on these clearly man-made signals.
Aside from discovering an alien spacecraft, radio waves are the most common technological symbol depicted in science fiction films and books. However, they are not the only signals that could be present.
Since stars are by far the most powerful energy source in any planetary system, astronomer Freeman Dyson proposed in 1960 that a technologically advanced civilization could collect a significant portion of the star's light as energy with what would essentially be a massive solar panel. Many astronomers refer to these as megastructures, and there are several methods for detecting them.
After utilizing the energy in the captured light, advanced technology would re-emit some of the energy as heat. Astronomers have shown that this heat can be seen as extra infrared radiation that a star system gives off.
A megastructure could also be discovered by measuring its dimming effect on a star. Large artificial satellites orbiting a star, for example, would periodically block some of its light. This would manifest as dips in the apparent brightness of the star over time. Astronomers could find this effect in the same way that they find faraway planets today.
Pollution is another technological signature that astronomers have considered.
On Earth, chemical pollutants such as nitrogen dioxide and chlorofluorocarbons are almost entirely produced by human industry. These molecules can be detected in the atmospheres of exoplanets using the same method that the James Webb Space Telescope uses to search for signs of life on distant planets. It could be a sign of life if astronomers discover a planet with an atmosphere filled with chemicals that can only be produced by technology.
Finally, artificial light or heat from cities and industry, as well as a large number of satellites orbiting a planet, could be detected using large optical and infrared telescopes. With the technology we have now, a civilization would need to make a lot more heat, light, and satellites than Earth does in order to be seen across the vastness of space.
Which signal is the most effective?
Because no astronomer has ever discovered a confirmed technosignature, it's difficult to predict what will be the first sign of alien civilizations. Astronomers have thought about what a good signal might look like, but no one knows what technology from other planets might look like or what signals might be out there.
Some astronomers advocate a broad SETI approach that looks for anything in space that current scientific knowledge cannot explain naturally. Some people, like us, are still looking for both intentional and unintentional technological signs. The bottom line is that there are numerous ways to detect distant life. Because no one knows which approach will succeed first, there is still a lot of exciting work to be done.