What is the probability that extraterrestrial intelligence exists?

 Alien Life: Where Is It? The Top Six Theories

In our (very) brief time here on Earth, we have done a lot. We've launched rovers to Mars and probes to the limit of our solar system, sent people to the Moon and into space, and built enormous, complex telescopes to glimpse the farthest reaches of the cosmos.

But other groups have gone a step further in advancing humanity's exploration of the last frontier. To definitively determine whether we are alone in the universe, NASA, the European Space Agency, and the research team behind SETI have been working nonstop.

Numerous initiatives already exist to look for evidence of intelligent life in the stars. And although many of them have been scanning the sky for decades, we have not yet established communication. And therein lies a small issue.

The Fundamental Contradiction

Our solar system is, to put it politely, incredibly ancient. Although the exact age is still unknown, meteorite evidence suggests it is close to 5 billion years old, and the nearby star systems are probably billions of years older. 

Even while interstellar travel still seems like a pipe dream, a new technology that enables us to search the skies for signals from civilizations in the furthest reaches of space is being developed every year. 

These technologies have resulted in the discovery of an increasing number of known alien worlds and star systems, but our inventive ways of listening to space have not yet turned up anything that resembles extraterrestrial communications or civilizations.

It would appear that we should have established contact given the vastness and antiquity of our universe. Of course, we haven't.

Enrico Fermi, a physicist, pondered the now-famous question: "Given the size of our universe, why haven't we discovered intelligent alien life yet (or why haven't they found us)?" in the early 20th century. 

The Fermi Paradox and the Great Silence are other names for this phenomenon. Since Fermi initially posed this topic a century ago, scientists have put forth a wide range of potential solutions. Here are some of the most likely explanations for why he hasn't initiated contact.

According to basic probability, extraterrestrial life must exist. One idea holds that since we haven't yet established contact, there must be something preventing life from traveling across the galaxy, or at the very least, preventing it from interacting with other extraterrestrial species. 

The "Great Filter" is a force or an event that prevents civilization from reaching the previously specified level of interplanetary travel or communication.

If the hypothesis is correct, there are primarily two reasons why we haven't communicated: Either nations exterminate themselves before developing to a level at which they may explore the stars, or interplanetary travel is just not feasible on a technical level. Neither choice is really appealing.

The work's researchers also claim that the likelihood of the filter event is on par with or higher than that of the presence of extraterrestrial life. In his consideration of the subject, Robin Hanson, a research associate at Oxford University's Future of Humanity Institute, makes this argument.

Our solar system or adjacent systems have not been significantly colonized by extraterrestrial civilizations. Since none of the billion trillion stars in our former universe have done so, we may soon surpass them in terms of development and technology. This one piece of evidence suggests that a Great Filter is what separates advanced bursting enduring life from commonplace lifeless stuff.

How far are we from being involved in an event that would prevent us from ever discovering extraterrestrial life given that we have not been able to detect alien life (or leave the solar system much, for that matter)? The simpler it was for life to get to where it is now, the less likely it is that we will survive, according to Hanson. In other words, the more life there is in the cosmos, the more likely it is that we will either encounter a catastrophic catastrophe that would destroy all life as we know it or hit the cosmic limitations of technological development.

Avoid disturbing the aliens.

Another theory contends that although alien civilizations are undoubtedly real, they are dormant. That is the "aestivation hypothesis," which was advanced by researchers from Oxford's Future of Humanity Institute and the Astronomical Observatory of Belgrade. Aestivation is the term for an organism's prolonged inactivity, similar to a bear hibernating or a frog burrowing itself in the sand during hot weather.

According to the hypothesis, aliens may be "hibernating" until the climatic circumstances are ideal for them to become active and establish their super society. This theory was first presented in a study published in the Journal of the British Interplanetary Society in 2017. 

The researchers contend that because computing systems require cooling to operate, the laws of thermodynamics directly constrain computation. Because it is so difficult to keep modern technologies cool at scale, it is extremely challenging to develop them. In other words, the aliens are going dormant until the cosmos cools.

However, it could be reductive to reduce civilization growth to the types of circumstances that our present, flawed models can foresee. What if intelligent alien life has discovered a technique to circumvent the thermodynamic restrictions on computation? 

The study's authors pose the question, "What if there are alternative types of value that may be generated? The aestivation theory would be irrelevant if they are incorrect about the connection between thermodynamics and technology. One of the other concepts presented here may apply in this situation.

An image of a "Gaian"-Esque bottleneck courtesy of Creative Commons

The "Gaian Bottleneck" theory contends that life requires specific environmental circumstances, which are uncommon, to evolve. Australian National University astrobiologists published their solution to the Fermi paradox in 2016.

According to the researchers, extinction is "the cosmic default for most life that has ever arisen on the surfaces of wet rocky planets in the Universe." This is because life alters the atmospheric concentration of greenhouse gases, making a planet livable only if it is truly inhabited. No life without habitability, no habitability without life—a Catch-22.

According to the researchers, extraterrestrial life must hold on to endure: "similar to attempting to ride a violent bull. Most of life declines." Only in the presence of an implausible feedback loop is life possible. Earth is an exception to the norm in this situation.

Held Hostage in Deep Oceans

After over ten years in transit, NASA's New Horizons spacecraft made history in 2015 when it completed the first-ever close flyby of Pluto. It gave humans their first glimpse of its frozen surface and sparked speculation about the existence of underground seas of water as well as a significant amount of methane and nitrogen. 

These questions add Pluto to a small but increasing list of worlds that have seas hidden behind a thick covering of ice and rock, including the moons of Jupiter, Saturn, and its moons Europa, Callisto, and Ganymede.

The lead scientist on New Horizons, Alan Stern, touches on a different hypothesis of where life could be hiding that heavily features those oceans. Changes like changing tides and dissipation take happen over a longer period in underground seas because they have a much more stable ecology than surface streams that are moving. 

The harsh environment and a deadly combination of gases on the surface are shielded from possible life in the waters by a strong outer shell. According to Stern, "Impacts and solar flares, neighboring supernovae, your orbit, whether you have a magnetosphere, and if you have a toxic atmosphere - none of those things matter."

Drilling through that dense, protective crust would be a significant challenge for any sophisticated extraterrestrial species that emerges in these deep waters. They wouldn't even reach the surface after all that effort, making communication with other planets much less likely.

Unnoticed Signals Array of Telescopes at Allen, National Science Foundation, source of the image

We've been using radio technology to listen for evidence of alien life for around the past eighty years. One of the largest is the Allen Telescope Array, which is located 470 kilometers (290 miles) northeast of San Francisco. Since 2007, 42 dishes have been ready to scan the heavens daily with the aim of picking up radio signals from extraterrestrial life.

What if alien life doesn't use those frequencies, though? Because we don't understand the proper wavelengths, attempts at communication may just be missing us.

Duncan Forgan of the University of St. Andrews in Scotland advises developing a galactic communications network rather than employing telescope arrays and scanning the heavens for radio signals. 

We may communicate with other universe residents by using the shadow that Earth casts when it passes in front of the Sun, much like how we flash our high lights to notify other vehicles. In order to transmit those encrypted messages as we pass in front of the Sun, Forgan proposes that we develop potent lasers.

As Forgan explains to New Scientist, "There are lots of things in the way if you want to speak with someone on the opposite side of the galactic center - dust, stars, a gigantic black hole - so you can take a long way around using the network." Civilizations in other galaxies may agree to utilize this "galactic communications network" to ensure their messages reach their intended receivers rather than letting them get lost in the void of space, creating a single system to cut through the noise.

We're being antsy

A minor blip in the vast history of the solar system and the cosmos as a whole, we have only been actively searching for extraterrestrial life for approximately a century. The Cornell University researcher and undergraduate in astrophysics and mathematics, Evan Solomonides, predicts that humans may not hear from alien civilizations for 1,500 years.

The likelihood of discovering life is examined by Solomonides in a study that was submitted to the American Astronomical Society. We estimate that just 1% of the galaxy has been reached so far, and we don't expect to get there until almost half the stars and planets have been discovered. 

Since we have barely investigated our own galactic neighborhood, Solomonides estimates that we will have to examine around half of the Milky Way galaxy before we learn anything. This will take some time.

The 1,500 years are not a deadline, Solomonides is cautious to point out. This is not to imply that if no one contacts us by then, we will in reality remain alone. We only assert that it is fairly improbable that we won't hear anything prior to that.