We look for them, we hypothesize about them, we write books about them, we make movies and television shows about them, and in some cases we worship them.

For as long as human beings have been wondering about the universe, we've been wondering if anyone else is out there. From what we can tell, the lack of evidence for technologically advanced alien civilizations seems to suggest that nobody is there. On the other hand, alien civilizations could be plentiful in the universe and we might have a difficult time noticing.

Either way, here are four possible explanations as to why we haven't yet seen any evidence of intelligent aliens in the universe.

1. Technosignatures Are Not Easy to Detect

Just because someone's out there broadcasting doesn't mean anyone is listening.

The universe primarily consists of empty space, and the distance between stars is vast. When it comes to broadcasting our presence in the cosmos, any sort of radio signal we emit from Earth isn't going to get very far.

When a radio signal leaves Earth, it gradually becomes weaker the further it goes. The reason for this is physics: electromagnetic waves disperse outwards as they move outward from their source, losing signal strength as the further they go. Like ripples in a pond gradually becoming weaker as they disperse outwards from their source, radio waves broadcast into space also become weaker the further they travel.

As an electromagnetic signal becomes weaker the further it travels, it then requires an increasingly large radio telescope in order to pick up the signal. Ultimately, it's extremely unlikely that any technological civilization more than a few light years away from Earth would be able to pickup any of our standard broadcasts, and vice-versa.

Very Large ArrayA line of radio telescopes scans the observable universe for signs of not being alone. Image: CGP Grey.

One way we might have a better shot at picking up alien signals would be if, rather than dispersing a radio signal outwards into space where it rapidly weakens, they instead directed a more powerful radio signal directly at us. This would preserve signal strength over time and allow us to pick up signals over significantly longer distances.

The downside to this approach is that a technological civilization would have to purposefully aim their broadcasting signal directly at us for an extended period of time. This is naturally a difficult thing to do, because Earth is a moving target. Further, they may not even know we're here, and it's more than likely that our receiving radio telescopes wouldn't be directed towards the part of the sky that the signal is broadcast from, in which case we would miss it altogether.

A broadcast from a technologically advanced civilization could conceivably have been the origin of the Wow! signal detected in 1977 by the Big Ear Radio Observatory. If anyone just happened to be broadcasting a powerful radio signal in our direction at the same time that a radio observatory of ours happened to have been oriented towards the correct region of space, the result just may just have been the detection of a strong, short signal from an alien civilization.

If this civilization is undertaking a strategy of intermittently broadcasting powerful radio waves towards narrow regions of space in the hopes of being heard by someone, there's a small chance that we may have been the lucky recipients of that message. Rather than an intentional signal that we picked up because we were supposed to, detecting the Wow! signal at all may have been like winning the lottery. Naturally, the reason we haven't detected it again is because winning the lottery doesn't happen very often.

Electromagnetic broadcasts aren't the only way to detect technological civilizations in the cosmos. Another way to achieve this is by analyzing the visible light spectrum when we look at exoplanets and other stars. When it comes to distant planets around other stars, our telescopes aren't yet powerful enough to observe and analyze the spectra of the atmospheres of individual planets around other stars.

We will eventually gain this ability, and at that point we'll have the smoking gun: being able to directly image planets tens or hundreds of light years from Earth and determine the chemical composition of their atmospheres. This could enable us to determine whether or not a technologically advanced (ie. greenhouse-gas emitting) civilization exists on a planet in our galactic neighborhood.

In lieu of directly observing an exoplanet's atmosphere, the other option is to closely observe stars for signs of alien megastructures being constructed—this is an ability we've more recently acquired. In 2015, observations of KIC 8462852 (Tabby's Star) showed anomalies in the star's brightness observed over time, including large irregular dips in light output. While a natural explanation is more likely, it has been suggested that what we observed with Tabby's Star is similar to what we'd expect in the case that an alien civilization were in the midst of constructing massive structures in orbit around a distant star.

These types of objects are referred to as megastructures, and could include things like a Dyson Sphere or Dyson Swarm, or some sort of artificial Ringworld encircling a star. If an alien civilization were in the midst of constructing such an object, we would likely be able to detect such activity with our current technological capabilities. That we haven't found any yet doesn't mean they're not out there—there are hundreds of billions of stars in our galaxy alone, and we've only taken a close look at a small fraction of them.

Dyson SphereAn artistic rendering of what a Dyson Sphere might look like upon completion. Image: Wikimedia Commons.

2. They may Choose not to be Seen

Even if an advanced alien civilization did exist and was aware of our existence, they may not necessarily want to go out of their way to say hello.

This could explain why we don't detect any artificial radio signals from outer space, or why we haven't seen any of of those alien megastructures being built. If we were able to detect a signal or observe the construction of a large structure in orbit around a distant star, then that means other civilizations would also be capable of similar observations. And you never know who might be listening in or watching.

The cosmos is so vast that, if any technologically advanced alien civilization does arise and gain the capability to construct megastructures around stars, there's a good chance they may not be the first or only ones to do so. If there are multiple concurrent technologically advanced civilizations in one galaxy, there's always the possibility that a large-scale conflict could ensue. Other alien civilizations may even have observed such conflicts in the past, and then decided against revealing themselves in the hopes of avoiding a similar fate.

Of course, with sufficiently powerful telescopes, even the most introverted aliens wouldn't be able to keep their existence a secret forever. If there are any technological civilizations currently existing in our galaxy, it's only a matter of time until we're able to detect their presence.

3. The Rare Earth Hypothesis: We're All There Is

Despite the abundance of life here on Earth, we don't yet know whether or not life exists elsewhere in the universe. It might not.

The existence of any kind of technological civilization—such as that of our own—could be so rare that it's never happened anywhere else before. This could be for a variety of reasons: the emergence of life could be an almost impossible event, requiring very specific starting conditions and a lot of luck. Or, it could be that simple life arises fairly regularly but almost never evolves into complex multi-cellular life. Yet another possibility is that complex life forms are abundant in the universe, but the emergence of an intelligent species capable of developing advanced technology and pondering its own existential dread is quite unheard of.

It could also be all of the above: basic life is super rare, complex life is significantly rarer, and a technological civilization is... A once in 13.8 billion year event?

The problem with this hypothesis is that we just don't have enough data. We haven't found any extraterrestrial life yet, but then again we haven't tried that hard. Future missions to Mars, Europa, Enceladus, or Titan may yield the hard evidence we've been looking for—they may even find evidence of complex life forms existing in either of these places.

Another possibility is that our increasingly powerful telescopes will eventually detect signs of life in the atmospheres of distant exoplanets. Or, these same telescopes could discover signs of technologically advanced civilizations, finally putting our cosmic loneliness to an end. Until we have more data, we just don't know, and the universe as we presently observe and understand it seems to be eerily devoid of life.

4. The Great Filter Dilemma

In the case that the rare earth hypothesis doesn't hold any water and technologically advanced civilizations such as our own are common in the universe, our time could still be limited.

One suggestion is that the path towards achieving increasing levels of cultural and technological complexity is filled with existential threats that almost no civilization is able to overcome. Even today, we face catastrophic risks from climate change, global pandemics, environmental degradation, nuclear war, total economic collapse, hostile artificial intelligence, bio-engineering, and natural disasters, any of which could catastrophically cripple our global civilization.

Nuclear BombThe United States' Castle Bravo nuclear test in 1953 was 1,000 times more powerful than the bombs dropped on Japan in WW2. Image: U.S. Department of Energy.

It could be that life is common in the universe, that technological civilizations inevitably arise given enough time, but that the most common result for any advanced civilization is that it eventually self destructs. For us, this might mean we face extinction or catastrophic collapse in our near future.

This could be why we don't detect any alien signals or observe any alien megastructures being built: others did come before us, but they didn't last very long. The problem in this case is that we don't know how far along in our own development we are. It could be that we're further along than where a typical technological civilization would peak before declining. Or we could still be lagging behind or approximately average.

One problem with this hypothesis is that we have no way of knowing whether a Great Filter lays ahead of us or behind us. There could be numerous bottlenecks through which every advanced civilization must navigate in order to continue their existence.

If such Great Filters exist, a recent example may have been the Cold War and the threat of nuclear apocalypse—we somehow managed to squeeze through that one, despite a few close calls. Next up on the list of Great Filters could be climate change and environmental destruction. As things currently stand, our prospects for long-term survival are yet to be determined.

However we look at it, our presence in the cosmos and our seeming lack of neighbors, whether single-celled or civilized, puts us in an odd predicament.

We could be surrounded by technological civilizations and not even be aware of it. We could also be on a common trajectory for any civilization, with near impossible-to-survive catastrophes ahead of us that we may not even see coming. Or, we may have already passed through all of these obstacles and bottlenecks and we now find ourselves in a vast, welcoming cosmos that's ours to explore.

Even more profoundly, we could be the first technological civilization in the history of the universe to survive or to make it as far as we have so far. From a scientific perspective, this seems unlikely, but it does match the evidence (or lack thereof) that we've accumulated thus far. Only time will tell us where our place in the universe truly is.

Cover image credit: NASA/JPL-Caltech/ESA/CXC/STScI