Other methods of passively seeking-out aliens include looking for hints of advanced civilizations, but not necessarily the ones that are transmitting. We might be able to detect a civilization's accidental radio "leakage" -- like Earth has been leaking radio into space for over a century. Perhaps these civilizations are so advanced that they might build huge structures around their stars that we can detect from afar -- a field known as the Search for Extraterrestrial Technology (SETT) or the Search for Extraterrestrial Artifacts (SETA). Perhaps that bizarre transit signal detected by the Kepler Space Telescope from Tabby's Star (KIC 8462852) is a huge solar collector -- a.k.a. the wonderfully named "megastructure" -- built by super advanced aliens! (Though the signal is likely comets, I prefer to ponder aliens.)

We could also assume these hypothetical aliens face similar challenges as us and perhaps we will detect the presence of an industrialized alien planet by seeking out pollutants choking their atmosphere, such as an excess in carbon dioxide or chemicals that can only be generated via industrial processes, like chlorofluorocarbons, or CFCs.

As our technology becomes more sophisticated, we're able to seek out weaker signals originating from farther star systems, but passive searches will always be stymied by the odds. There are billions of stars in our galaxy, we can't monitor every single one for an artificial signal. And even if we could, there's no guarantee that the hypothetical aliens will be transmitting at the specific time we decide to look. There are some ingenious ideas for narrowing our search and decreasing the odds -- such as "directed SETI", where we identify "habitable" exoplanets to target with our radio antennae -- but for passive SETI to be successful, it needs more observatories, more time (hundreds or even thousands of years) and a lot more funding to stand even a slim chance.

Indeed, there are many that oppose the idea that we should alert the universe that there is a habitable planet populated with intelligent creatures. British theoretical physicist Stephen Hawking is an outspoken critic of METI, echoing the concerns of Gertz and pointing out that an encounter with alien beings would more likely resemble the Roman Conquest than a meeting with a peaceful entity like the one from Stephen Spielberg's movie "E.T. the Extra-Terrestrial."

"Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach," Hawking speculated in 2015.

The fear about a violent, conquering force of rampaging aliens isn't without merit; when our planet's very existence is at risk -- even if the danger is extremely unlikely -- why would we advertise our presence? Would it be like dropping a bucket of chum in shark-infested waters? Do we really want to find out?

So, yes, these arguments alone should be enough to make us want to switch off our radio transmitters. But METI advocates, like Seth Shostak, Senior Astronomer of the SETI Institute, points to the potential benefits to mankind if we were to establish a collective effort to send messages to the stars. Chiefly, Shostak points out that having a fear of these hypothetical intelligent aliens is just an assumption based on our history. Just because humanity has a violent past, it doesn't necessarily mean other life forms would have evolved in a similar way.

"The universe beckons, and we can do better than to declare that future generations should endlessly tremble at the sight of the stars," he concluded in a 2015 New York Times article.

Shostak argues that to not transmit signals could limit technological developments, though I don't share this concern; it could be just as well argued that the lack of development in the field of METI could be offset by huge developments in passively seeking out signals. In the process of boosting our sensitivity to artificial signals, we could make huge gains in new radio astronomy techniques that will have very real spin-offs for science.

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There's the argument that, assuming a sufficiently-advanced alien life form, we are already in the Galactic Rolodex. We've been leaking radio signals into space and blasting radar far and wide for decades; anything within 100 light-years should already be aware of our presence. Perhaps there's some kind of "Star Trek"-like Prime Directive that prevents advanced aliens from making contact until they deem us developed enough to cope with an alien spacecraft landing outside the United Nations Building. If we are actually part of an interstellar ecosystem, I think we can safely assume "they" know we're here. So the question isn't "are we alone?" it's "why are they leaving us alone?"

We can speculate forever about whether or not other life exists and, if it does, what will it be like. But while caution should be urged when advertising our presence to the universe (and we definitely need some kind of international agreement outlining our rules of interstellar communication), we also shouldn't fear the unknown. The vast majority of concerns surrounding alien invasions are based on our perspective and our experiences, naturally. We can't assume that extraterrestrial life will evolve on the same, violent path. The universe is full of wonder and infinite possibilities, that shouldn't be overshadowed by irrational fear of monsters in the dark.