If an alien If we look at Earth, many human technologies – from cell towers to fluorescent lamps – could be a beacon indicating the presence of life.
We are two astronomers working on the search for extraterrestrial intelligence — or SETI. In our research, we try to characterize and detect signs of technology originating from beyond Earth. These are called technosignatures.
While scanning the skies for a telecast of some alien Olympics might sound simple, searching for signs of distant, advanced civilizations is a much more nuanced and difficult mission than it seems.
Saying “hello” with radios and lasers
The modern scientific search for extraterrestrial intelligence began in 1959 when astronomers Giuseppe Cocconi and Philip Morrison showed that radio emissions from Earth could be detected by radio telescopes at interstellar distances.
That same year, Frank Drake launched the first SETI survey, Project Ozma, pointing a large radio telescope at nearby Sun-like stars to see if he could detect any radio signals coming from them. After the invention of the laser in 1960, astronomers showed that visible light could also be detected from distant planets.
These early, seminal attempts to detect radio or laser signals from another civilization were looking for focused, strong signals that would have been intentionally sent into the Solar System and should be found.
Given the technological limitations of the 1960s, astronomers did not seriously consider looking for broadcast signals – such as television and radio broadcasts on Earth – that would leak into space. But a beam of a radio signal, with all its power, focused towards Earth, could be detected from much further away — just imagine the difference between a laser and a weak light bulb.
Searching for intentional radio and laser signals is still one of the most popular SETI strategies today. However, this approach assumes that alien civilizations want to communicate with other technologically advanced life.
Humans very rarely send targeted signals into space, and some scholars argue that intelligent species may deliberately avoid broadcasting their locations. This search for signals that no one can send is called the SETI Paradox.
Leakage of radio waves
Although humans do not transmit many intentional signals into the universe, many technologies that humans use today produce many radio emissions that leak into space. Some of these signals would be detectable if they came from a nearby star.
The global network of television towers constantly emits signals in many directions that leak into space and can accumulate into a detectable, if relatively faint, radio signal.
Research is ongoing into whether current RF cell tower emissions 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 greater sensitivity from current radio telescope arrays.
However, not all man-made signals are so misguided. 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 marked in space. Any extraterrestrial civilization that happened to be in line of sight of these rays would probably be able to detect these unmistakably artificial signals.
Aside from finding an actual alien spacecraft, radio waves are the most common technologies featured in science fiction movies and books. But those aren’t the only signals that could be out there.
In 1960, astronomer Freeman Dyson theorized that since stars are by far the most powerful source of energy in any planetary system, a technologically advanced civilization could harvest a significant portion of starlight as energy with what would essentially be a huge solar panel. Many astronomers call these megastructures, and there are a few ways to detect them.
After using the energy in the captured light, the technology of an advanced society will re-radiate some of the energy as heat. Astronomers have shown that this heat could be detected as extra infrared radiation coming from a star system.
Another possible way to find a megastructure would be to measure the effect of dimming on a star. Specifically, large artificial satellites orbiting a star would periodically block some of its light. This would show up as dips in the star’s apparent brightness over time. Astronomers could detect this phenomenon similar to how distant planets are discovered today.
A lot of pollution
Another technical signature that astronomers have thought about is pollution.
Chemical pollutants—such as nitrogen dioxide and chlorofluorocarbons on Earth—are produced almost exclusively by human industry. It is possible to detect these molecules in the atmospheres of exoplanets with the same method that the James Webb Space Telescope uses to search distant planets for signs of biology. If astronomers find a planet with an atmosphere full of chemicals that can only be produced by technology, it could be a sign of life.
Finally, artificial light or heat from cities and industry could also be detected by large optical and infrared telescopes, as could a large number of satellites orbiting a planet.
But a civilization would have to generate far more heat, light and satellites than Earth to be detectable across the vastness of space using the technology humans have today.
Which brand is better?
No astronomer has ever found a confirmed technosignature, so it’s hard to say what the first sign of extraterrestrial civilizations will be. While many astronomers have thought a lot about what a good signal might do, ultimately, no one knows what alien technology might look like and what signals are out there in the Universe.
Some astronomers advocate a generalized SETI approach that looks for anything in space that current scientific knowledge cannot physically explain. Some, like us, continue to look for both intentional and unintentional technologies.
The bottom line is that there are many paths to detecting distant life. Since no one knows which approach is likely to succeed first, there is still much exciting work to be done.
This article was originally published on The conversation with Macy Houston and Jason Wright at Penn State. Read the original article here.