Most scientists believe in aliens. In the 1960s, Frank Drake derived a formula calculating the likelihood of extra-terrestrial civilizations, and with current estimates at the formula’s parameters, that likelihood is very high. There are simply so many planets out there that some of them are bound to have intelligent life, right?

Well, as physicist Enrico Fermi asked over half a century ago, where are they? The fact that there’s such a high probability of lots of other civilizations existing but we haven’t found any — popularly called the Fermi Paradox — is frustrating. It questions not only our knowledge of the universe but also the fundamental properties of humanity itself. Are we really alone?

The simplest explanation is: yes. We are the only civilization to ever have existed, because perhaps having life as intelligent as us (I mean, have you seen my SAT scores?) really is unlikely even in the entire universe. Or perhaps we’re not the only ones, but civilizations are so spread out that in terms of communication we might as well be.

A similar but slightly more depressing explanation, supported by Carl Sagan, is that civilizations inherently destroy themselves. If this seems unlikely, consider how close we came during the Cold War. And according to MIT researchers, if humans keep depleting Earth’s resources at unsustainable rates, we will experience a global economic and population collapse around the year 2030 — perhaps the beginning of humanity’s downfall. If civilizations never last long, we may not be able to catch messages from other beings within the right time frame. Electromagnetic waves, after all, can take millions of years to travel between planets.

But scientists have more interesting solutions to the Fermi Paradox as well. Perhaps, some say, other aliens have thought processes that are so much faster or slower than ours that a simple “hello” could be only a nanosecond, or could take several years. To us, such communications might be indistinguishable from background noise. Or perhaps every other alien civilization is doing what we’re doing: a whole lot of listening, but barely any sending.

Others theorize that perhaps civilizations tend to become so advanced that they achieve a “post-biological” state. Species at this level might destroy their physical selves and live entirely in virtual computer networks (like “The Matrix” but without all the goopy human organ farms), or they might turn themselves into microorganisms all working together at the cellular level for increased efficiency. Such a civilization might be prevented from communicating with us due to this structure, or they may be so hyperintelligent that even saying hello to us is like trying to teach calculus to mosquitoes.

Some of the most interesting proposed answers are those in which alien civilizations know of our existence and have the capability to communicate with us, but choose not to. Perhaps aliens don’t want to interfere with our natural evolution and technological advancement until we’ve reached a certain point, like in the movie “Contact.” They watch us make discoveries about subatomic particles and say, “Aww, look at those cute little humans! Growing up so fast!” And eventually, when we’re ready, they’ll send us a note: “Our galactic society has been watching you, and we would like to meet you in (ha ha) person. Email if you are interested, and await further instructions. Tell no one.”

A more disheartening slant on this involves a dictatorial alien regime. These oppressive beings fear an uprising amongst their subjects, who are spread across many worlds, and thus impose a strict radio silence to prevent any sort of mass organization. Because of this, we hear nothing. (With explanations like this one, it’s important to remember these are scientific theories, not arbitrarily made up stories. Those would be totally, totally different.)

So why haven’t we heard from all of the aliens that surely are out there? (Or perhaps we have: a radio telescope in the ‘70s picked up a strong 72-second signal that matched expected alien message properties. Unfortunately, it was never detected again.) Ultimately, we don’t really know. But it’s fun to guess.