From the discoveries of Earth-like exoplanets to finding water in every corner of our solar system, from the ubiquitous presence of life in extreme environments to the reception of bizarre radio signals from space, it is harder and harder not to speculate that life could pop up somewhere other than just Earth. But where are they? The Nobel Prize-winning physicist Enrico Fermi once asked this question during a lunch with his colleagues. Besides the most well-known zoo hypothesis, which hypothesizes that we are placed inside some cosmic zoo, observed but left alone to develop without external interference, many other theories have been proposed, with some sounding more probable than others.
Life, after all, could be rare in the universe. One might think the evolution of life on Earth should be quite typical, but many believe the opposite is true: the conditions needed for the evolution of life (biologically complex life forms) are rare or even unique to Earth. Under this assumption, life requires a series of serendipities such as a galactic habitable zone, a central star and planetary system having the requisite character, a right-sized terrestrial planet that is located in the Goldilocks Zone, and many other prerequisites that favor the emergence of carbon-based life. Even if complex life forms are common, say one day we find bacteria on Mars, intelligence may not be.
Under the assumption that intelligent life forms exist, there are still many factors that limit the chances for us to find them. One of the popular hypotheses out there is that life has a tendency to self-destroy or destroy others. We do not need to look deep into the history of humankind to find the examples of both scenarios.
On the self-destruction front, the famed sci-fi writer Arthur C. Clarke once wrote that it has yet to be determined whether intelligence has any survival value. Despite the apparent advantages humans have through their high intelligence in comparison to other species on Earth, we also have the unfortunate combination of two qualities: we are very good at constructing tools and systems, but terrible at long-term planning and forecasting. The duality of civilizations like ours can lead to self-destruction before or shortly after developing radio or spaceflight technology, through nuclear war, resource depletion, climate change, or rogue artificial intelligence. According to a recent paper that attempted to unravel the secret behind the cosmic silence, human society could be at the brink of extinction. It is believed that the Earth should be habitable for animals at least a billion years into the future. Based on how long it took proto-primates to evolve into a technological species like us, it should leave enough time for the process to happen up to 23 more times. On that time scale, there could also have been others before us, but there's nothing in the geologic record to indicate that we are not the first.
Regarding aggression against others, intelligent beings have the tendency (and the means) to destroy other species, intelligent or not. As feared by the renowned astrophysicist Steven Hawking, alien civilizations could be rapacious marauders, “roaming the cosmos in search of resources to plunder and planets to conquer and colonize.” Human beings are just like ants from their perspective, easily crushable if standing between them and their desired resource. Because of such potential risk, he advocated for the maximum caution in human kind’s search for extraterrestrial life forms.
Some would argue that intelligent life forms are perhaps too far away from each other to communicate. Space is vast and communication (assuming the transmission is traveling at the speed of light) takes a lot of time and requires resources. With an incredibly sensitive radio telescope, Earth's television and radio broadcasts would only be detected up to 0.3 lightyears away (that is less than 1/10 the distance to our nearest neighbor star Alpha Centauri A). And the signal would take up to 4 months to be received. If two civilizations are separated by several thousand of lightyears, it is possible that one or both cultures may become extinct before meaningful dialogue could be established.
According to a newly emerging theory, as unorthodox as it might sound, scientists believe that the aliens have deactivated themselves, which makes them impossible to detect. These extraterrestrial intelligent beings in question, who have taken a post-biological form of existence, could have been sleeping all along while they wait for the universe to cool down. The assumption is that the consciousness of the entire civilization has been uploaded to a digitalized mechanism, which resembles a computer. To maximize its computational capacity, this artificial life would have chosen to be in sleep mode and would not be waking up again until the universe gets colder.
Many alien hunters have also questioned if we have been listening properly. The SETI (acronym for Search for Extraterrestrial Intelligence) programs operate with limitations regarding the type of transmission and the origin of the signals. A message from aliens is much easier to detect if the signal energy is limited to either a narrow range of frequencies or directed at a specific part of the sky. That is why telescopes could have missed signals of unconventional frequencies or signals resembling background noise. If the radio transmissions come from the star systems that are in a lower priority on the list, we could have easily missed them.
The Arecibo Message in graphic format. Credit: Wikipedia
The Arecibo message is a prime example from the alien’s perspective. In 1974 an interstellar radio message carrying essential information about humanity and Earth was sent to globular star cluster M13 from the Arecibo Observatory in Puerto Rico, in the hope that extraterrestrial intelligence might receive and decipher it. But aliens in cluster M13 would have to aim their telescopes toward the region of space to which our message was sent, and set them to an appropriate range of frequencies.
Some have long suspected fast radio burst (FRB), a high-energy astrophysical phenomenon of unknown origin, to be the aliens’ preferred method of communication to reach other sentient beings in the universe. It is so transient that it lasts as short as a few milliseconds. This September, Breakthrough Listen – a global astronomical initiative to find signs of intelligent life in the universe, has detected 15 more brief but powerful radio pulses from FRB 121102, a mysterious source from a distant galaxy. Since the discovery of the first FRB in 2007, many FRBs have been detected. But FRB 121102 is different - it is the only one known to repeat. As exciting as this might seem, SETI researchers are still unclear about what kind of message the FRB transmissions carry. Is it simply a signal telling other intelligent beings that life exists elsewhere in the universe? Does it encode blueprints to build a vessel that can travel through space?
Sometimes promising leads in the search for aliens can turn out to be a false positive. Back in 1977, a team of astronomers studying radio transmissions from an observatory at Ohio State called the "Big Ear" recorded an unusual 72-second signal - it was so strong that team member Jerry Ehman scrawled "Wow!" next to the readout. But this year through the close-up observation of 266/P Christensen, a comet returning to the night sky nearly 40 years since the “Wow!” signal, the Center of Planetary Science has pinpointed the source of the mysterious transmission to the hydrogen cloud accompanying the comet.
Maybe one day we will hear a real “hello” from intelligent beings from galaxies far far away, putting a definitive end to Fermi’s famous question. But then humankind will need to prepare for what comes next. Just like the old saying, be careful what you wish for.
The Fermi Paradox II — Solutions and Ideas – Where Are All The Aliens.
Credit: Kurzgesagt – In a Nutshell