METI: The Search For Extraterrestrial Intelligence Has A Highly Controversial Twin

For over a century, humanity has attempted to search for signals from alien civilizations out there in the universe. Early efforts in the Search for Extraterrestrial Intelligence (SETI) focused on listening for messages from within the Solar System , before turning our eyes and ears to the galaxy and universe beyond.

So far, these searches have yielded no alien signals. One signal continues to defy explanation , but in all probability has a natural or human cause. But is it unfair to assume that alien civilizations would broadcast, given that we (apart from our stray signals ) do not regularly broadcast ourselves?

Enter METI, or Messaging to Extraterrestrial Intelligence. It is not fair to say that humanity has not sent out any messages to the universe, but they have certainly been limited. NASA's Voyager I and Voyager II probes contain the Golden Records , containing sounds and images "selected to portray the diversity of life and culture on Earth". These will likely never be found by aliens. But we have sent other signals, such as the Arecibo Message sent to the globular star cluster M13 roughly 21,000 light-years from Earth. The message was tiny at 210 bytes of data, but contained information including that there were 4 billion humans on Earth at the time, a stick figure of a human being, a representation of a double helix, and a drawing of the Arecibo radio telescope that sent the message.

In 2015, the group METI International emerged, with the ultimate goal of sending out messages that will reach alien civilizations. In October 2017 , they sent such a message to potentially habitable super-Earth exoplanet GJ273b , just 19 light-years from Earth.

Here's where METI gets a little controversial. The Arecibo message also contained a map of the Solar System, and there are (highly speculative) reasons why we might not want to contact aliens at all, let alone include a return address, or contact aliens in our cosmic backyard.

One reason why METI is controversial is what has become broadly known as the "Dark Forest Hypothesis" explanation of the Fermi Paradox.

In short, the universe appears to be teeming with potentially habitable planets, and yet we have never detected a signal that has come from an alien civilization.

The particularly dark Dark Forest Hypothesis – outlined in Liu Cixin's Remembrance of Earth's Past trilogy – proposes that alien civilizations keep their existences hidden, as you can never know the intentions of other civilizations. They could be hostile or benevolent, and you have no way of knowing which – or what their intentions are.

Say you are a friendly civilization, and you discover an alien civilization that unbeknownst to you is also friendly, and roughly on par with yours in terms of development. While you know you are friendly, you do not know if they too are peaceful. What's worse, given the expanse of space and the time it takes to receive light and signals from such civilizations, you cannot know how their society will progress in the intervening time between when the signal from that civilization reached yours. In the interim, they could have progressed incredible amounts technologically, meaning that even if the planet you are viewing looked peaceful and not far in advance of your own, by the time the light reaches you that all could have changed.

There is also the added pressure of resources. You also know from your own planet (according to the books) that life spreads and uses as many resources as are available to it, but the resources in the universe are finite.

Say that you conclude that they are peaceful, you are still left with the challenge that you don't know that they know that you are peaceful. If they think, or even suspect, that you are malicious, it would make sense that they would attempt to destroy you before you could do the same to them. You could communicate with them – like here on Earth – to further reassure them of your intentions. But in space terms, your new message could take hundreds or thousands of years to arrive, and in the meantime they could have launched an attack to destroy you. With such a chain of suspicion , Liu concludes, the only logical step for a civilization that values survival is to stay quiet in the forest, and destroy those civilizations that cry out, before they do the same to you.

Though popularized by fiction, hostile aliens are an idea taken seriously by scientists. In 2015, while launching a project to listen out for alien civilizations, Stephen Hawking explained why it's probably best not to say hello back.

"We don't know much about aliens, but we know about humans," Hawking said at the event, per Space.com . "If you look at history, contact between humans and less intelligent organisms have often been disastrous from their point of view, and encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced. A civilization reading one of our messages could be billions of years ahead of us. If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria."

The Dark Forest hypothesis is so very far from confirmed, but hostile aliens are still something to bear in mind when deciding whether or not to contact other species, and who should hold that decision. American author and scientist David Brin, for example, questioned "whether small groups of zealots should bypass all institutions, peer critique, risk appraisal or public opinion, to shout 'yoohoo' into a potentially hazardous cosmos."

In a way, the existence of METI could be used to argue against the Dark Forest Hypothesis. Say there are highly advanced civilizations out there. It's a reasonable assumption to say that some portion of the populations of these advanced civilizations could have access to advanced broadcast technology. Another reasonable assumption is that if they had become that advanced, they would have achieved this civilization using science rather than magic.

Unless for some reason the civilizations are just not interested in life outside their planet, they'd likely compile information on other star systems just as we do. So assuming that access to this information and broadcast technology isn't limited civilization-wide somehow, surely there would be a certain foolhardy proportion of the population who would attempt to contact other civilizations. While obviously speculating wildly, you could imagine scenarios where an especially risk-taking alien METI could attempt to warn other younger civilizations about the Dark Forest nature of the universe. We haven't heard from these actors either, which you could say implies there is a different explanation for the great silence than the Dark Forest Hypothesis.

Or perhaps the METI problem only happens in young civilizations who broadcast intermittently, before we have learned the dark forest nature of the universe, and do everything we can to stay quiet.