B. Whitmore (STScI)—NASA/ESA

On a clear night, staring up at the stars induces a sense of simultaneous wonder and insignificance. Humanity time and time again finds itself lost amid the vastness of a universe that we are still struggling to understand. There are many questions we ask ourselves when looking toward the heavens, but one of them always feels just outside of our grasp: Across all the billions of light-years of starry sky above us, could we possibly be the only life?

Scientists have explored this question for years. In 1961 physicist Frank Drake developed a mathematical equation to help solve it:

N = R * f p n e f l f i f c L

The equation aimed to find the number (N) of intelligent civilizations within the boundaries held by the subsequent factors—in our case, the Milky Way Galaxy. R * is the rate of formation of stars that could potentially allow for the development of intelligent life on planets nearby; f p is the fraction of said stars that actually have planetary systems; n e is the number of planets in a solar system with an environment that could sustain life; f l is the fraction of said planets that do sustain life; f i is the fraction of life-sustaining planets on which there is intelligent life; f c is the fraction of intelligent civilizations that have survived long enough to develop communication technology to send signals of their existence into space; and L is the length of time that these civilizations emit these signals before ceasing to exist. The commonly cited numbers for these variables simplify the equation to N = 10 × 0.5 × 2 × 1 × 0.1 × 0.1 × L, which simplifies even further to N = L/10. We as a civilization have been broadcasting into space since 1974, so, according to this equation, even if we cease to exist as a species in 2074, there would be 10 intelligent civilizations in our galaxy alone.

To break these numbers down further, scientists use the Kardashev scale, which splits intelligent life into three categories. Type I civilizations are able to use all the energy available on their home planet (we are approaching this; most scientists agree that we are currently at a 0.7 on the Kardashev scale, with a full Type I being about a century off). Type II civilizations can control and channel all the energy of their host star, and Type III civilizations have access to power equivalent to that of their host galaxy.

Even before the Drake equation and the Kardashev scale, many scientists were convinced that there must be a plethora of intelligent civilizations sprinkled across the galaxy. It wasn’t until a lunchtime conversation between astrophysicists that doubt was cast upon the old theories, and the outcome of that conversation continues to challenge even contemporary mind-sets. The story goes that in 1950 Enrico Fermi and his colleagues were discussing the existence of alien life over lunch. The question that Fermi asked the table became infamous in its simplicity: “Where is everybody?” The room fell silent because, well, nobody had an answer. Originally, the question was meant to attack the idea of interstellar travel, the possibility of which Fermi wasn’t confident in. But the question remains: if there were civilizations scattered across the stars by the billions, why haven’t we heard from them? It is from these questions, the Drake equation, and the Kardashev scale that the true paradox was born. The Milky Way is about 10 billion years old and 100,000 light-years across. If aliens had spaceships that could travel at 1 percent of the speed of light, the galaxy could have already been colonized 1,000 times. Why haven’t we heard from any other life?

That very question is the Fermi paradox. It has sparked numerous explanations for the silence we’ve been experiencing. Some scientists think that the silence is the product of something they’ve coined the Great Filter, an evolutionary wall impermeable to most life. For these scientists, there are two basic possibilities regarding the Great Filter: it’s either behind us or in front of us. If it’s behind us, scientists have speculated that it may have occurred at the creation of life itself or at the jump from single-cell prokaryotes to multicell eukaryotes. Either way, it implies that we are a rare case and that communication isn’t happening because we’re one of very few, if any, survivors. If the Great Filter is ahead of us, on the other hand, then we’re not receiving communication because advanced civilizations have hit the wall and ceased to exist—implying that we too will hit that wall eventually. Other scientists have come up with other explanations for this literal radio silence. Perhaps most of the universe is colonized and communicating, but we’re stuck in a desolate area far from the action. Or maybe Type III civilizations simply don’t care about communicating with inferior life like us. If they have all the power of an entire galaxy, maybe they can’t be bothered by us and our handheld cell phones. Some scientists even think that the lack of communication might be due to the existence of a predator species of which intelligent civilizations are afraid, and thus they refrain from transmitting so as not to reveal their location. The general consensus, however, is that if there are others out there transmitting signals, we’re probably just listening wrong: we don’t have the appropriate technology or understanding of the universe to receive or decode any messages just yet.

However, there’s still a chance it’s just us. According to the Drake equation, if a civilization could live at least a century after developing transmission technology, there could be 10 civilizations in our galaxy alone. But what if they couldn’t live for 100 years after developing this technology? As we begin to develop our own transmission technology, we also develop nuclear power, advance the warming of the climate, and exhaust our food sources with overpopulation. Is it such a stretch to say that perhaps an intelligent civilization cannot live for 100 years after developing space-penetrating transmission technology? If so, we can rework the Drake equation, and the answer changes drastically. If civilizations can typically survive for only 10 years after developing this technology, then N = 1, meaning that we may be the only intelligent life in our galaxy—or even the entire universe.