Let’s assume, for a moment, that the universe is teeming with life. In that case, the obvious question is: Where is everyone? In my introductory astronomy class, “From Black Holes to Undiscovered Worlds,” I ask my students to suggest possible explanations for why we have had no credible record of alien visitors to date. I am going to skip all discussion of supposed UFO sightings here, since the subject is full of poor observations and would require a book-length reply like Carl Sagan’s thought-provoking The Demon-Haunted World, one of my favorite reads.
Among many other insightful points, Sagan asks why alien species that have surpassed us so monumentally in technology that they can travel from star to star would need to abduct a whole person to study. Even a comparatively less advanced species like us humans has developed the technology to take DNA samples from hair or saliva. Wouldn’t collecting these samples from unsuspecting humans be a much more effective way to study them than beaming people up into their spaceships one by one?
For the record, most of my students’ theories involve either doomsday scenarios—alien civilizations have destroyed themselves before they can reach out to find others—or the endless void—we haven’t seen anyone else because we are the only life the cosmos ever produced.
This puzzle of absent aliens is not new. Enrico Fermi, the Italian-American physicist and Nobel Prize winner, famously posed the question “Where is everybody?” in a conversation on the possibility of extraterrestrial life in 1950. If technological civilizations are common in the universe, surely some would have developed sufficiently to visit or at least contact us by now?
This mystery is known as the Fermi Paradox; the discrepancy between the absence of evidence for advanced extraterrestrial life and the high likelihood that it should exist. Casting a dark shadow over the conversation at the time was the fact that scientists were in the midst of developing nuclear weapons that could wipe out civilization on Earth.
Carl Sagan asks why alien species that have surpassed us so monumentally in technology that they can travel from star to star would need to abduct a whole person to study.
How abundant might intelligent civilizations be within our vast universe? One way of thinking about this was proposed by the American astronomer Frank Drake, a pioneer in the search for extraterrestrial intelligence (SETI), who in the 1960s developed a systematic process to assess SETI’s prospects. Searching for what he called “a whisper we can’t quite hear,” he wove together a number of factors in a framework that is called the Drake Equation.
The seven interlinked factors began with well-constrained estimates of the rate of star formation, educated guesses on the likelihood of which ones might have planets circling around them, and the fraction of these capable of nurturing life, before continuing on to wild speculations about the probability of life actually evolving, to the fraction of life-forms that might develop intelligence, and the even smaller percentage that might be capable of interstellar communication.
The very last factor of the Drake Equation poses a question that can evoke either boundless enthusiasm or chilling pessimism about our chances to connect with any alien civilization: How long can technological civilizations survive?
The vastness of space is only occasionally dotted with stars, with enormous distances between them. For me, these become much easier to imagine when I shrink them in my mind to the scale of everyday objects. Let’s reduce our solar system—from the Sun to the outermost planet, Neptune—to the size of a cookie with a diameter of about two inches. How far away do you think the Sun’s closest neighbor is? Two cookies away? Five? One hundred?
It is much farther than that—nearly nine thousand cookies. Or, on the same cookie scale, about four football fields away. To chart the distances between stars in the cosmos, you need larger units than miles or kilometers, or cookies. Using a light-year as our cosmic yardstick makes it easier to comprehend these unimaginable expanses.
Light travels at an incredible speed: about 190,000 miles (~300,000 km) per second, or an astonishing six trillion miles (~nine trillion km) per year. It takes light only about one second to travel between the Earth and the Moon, about 240,000 miles (~380,000 km), and a mere eight minutes to cross the distance from the Earth to the Sun. In those eight minutes, light travels a relatively tiny cosmic distance of ninety-three million miles (~150 million km). The closest neighboring star to our Sun is Proxima Centauri, at an enormous distance of twenty-five trillion miles (~forty trillion km) away.
Even light takes about four years to travel that vast distance. So as well as conveying distance, a light-year scale tells us how long it takes for light to make the trip. We humans are starting to venture into our solar system, but these distances are small compared to the spaces separating the stars.
Our galaxy is about one hundred thousand light-years across. If a civilization had the means to navigate at even ten percent of the speed of light, it could, in principle, cross the galaxy in about a million years. In principle.
Most of the travel time would be spent voyaging through empty space: even a trip between our Sun and its closest stellar neighbor would take decades. Most of the trip would be endlessly boring because the distances between stars are so vast. And moving at such breakneck tempo would be exceedingly dangerous, since, running into even a small grain of interstellar material at that speed could result in disaster for the spacecraft and everyone on it.
A million years is a long time compared to a human lifetime, or even to humanity’s evolution, but some stars and their planets are much older than ours. If older civilizations exist, our galaxy might already contain their outposts, relics, or signals indicating advanced technology. But we have not encountered any yet. (Nor have we traveled very far from home.) So, as my students often suggest, do we lack alien visitors because the distances between habitable worlds are just too vast to navigate?
Let’s leave reality and get inspired by solutions presented by science fiction. While I personally love the idea of moving at faster-than-light speed, like the fictional starship Enterprise in the sci-fi Star Trek franchise, warp speed is most likely impossible to achieve, even in the future, because our universe is bound by the laws of physics. Based on everything we know, faster-than-light travel is a barrier we cannot cross.
But why do we automatically assume that other civilizations would even want to visit or communicate with us? How intriguing would Earth appear as a destination?
In an alternative visually stunning vision imagined in Luc Besson’s 2017 film Valerian and the City of a Thousand Planets, through complex but possible marvels of technology, enormous space stations navigate the cosmos while their passengers experience the wonders of the universe. The fictional spaceship contains a vast metropolis that is home to species from a myriad of alien worlds.
For now these possibilities to cross the galaxy are beyond us. But perhaps there is another way aliens might reach us. Since light travels at an astonishing speed, that means messages encoded in radio signal can travel fast. Often, the word “light” is used only to describe the narrow range of electromagnetic radiation that our eyes have evolved to see.
Imagine that you are holding a prism, a wedge of glass, and you pass a beam of white sunlight through it. A brilliant cascade of colors emerges ranging from deep reds to vibrant violets: the spectrum of visible light. Yet, what you see is but a minute portion of the full range of electromagnetic radiation that extends far beyond our human sight, into the infrared and ultraviolet, radio waves and gamma rays, all different notes in this grand cosmic composition of light.
One way to find advanced, communicating civilizations would be to collect radio signals being beamed our way that are not naturally occurring. While astronomical objects like galaxies generate radio signals as well, scientists are looking for signals that stand out, maybe—a kind of cosmic greeting.
But these interstellar greetings would dissipate in the vastness of space. Every doubling in distance reduces the signal strength to one-quarter of its previous volume, so at a certain distance, even the loudest shout becomes an imperceptible whisper—and that is assuming anyone is listening. Astronomers are looking for these radio signals but they have not found any yet. Does that really mean that there is no other life in the cosmos?
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Giant traveling space stations are not yet available, and we can’t break the known laws of physics, so this Great Silence of the cosmos looms dauntingly. This has led scientists (and my students) to suggest the possibility that even if life had existed somewhere else in the past, some barrier like a cataclysmic event has destroyed it and prevented civilizations from venturing into our galaxy—a Great Filter, so to speak, that has so far prohibited alien intelligence spreading through the cosmos.
This Great Filter could lie in our past. For instance, maybe it is astonishingly complicated to start life on a planet. Or what if it’s easy for life to begin but almost impossible for it to get past the earliest microbe stage? If alien life did become intelligent and technologically savvy enough to build satellites and capable of sending spaceships traveling through a planetary system, that technology might also be powerful enough to destroy every corner of their planet.
Or the cataclysmic filter could lie in our future. How hard is it for a civilization to survive its own technological growth? Maybe other life-forms have destroyed themselves before they could travel to the stars. A very depressing thought. But on the bright side, in that scenario, they are a much bigger danger to themselves than to us. Nuclear bombs and climate change are just two of many possibilities that could lead to the destruction of a civilization.
But why do we automatically assume that other civilizations would even want to visit or communicate with us? Let’s set aside the issue of what atmosphere and environment potential alien visitors would need to survive; how intriguing would Earth appear as a destination?
Imagine that you could visit one of two planets: the first is five thousand years younger than Earth, and the second is five thousand years older. Both show signs of life and are at a similar distance. Which one would you pick? Whenever I ask this question, most people pick the older, more advanced planet. Let’s assume a fictional alien civilization was given the same choice. Using that reasoning, our spectacular planet becomes a bit less interesting.
Don’t get me wrong, Earth is my favorite planet, but in terms of technology, we are just getting started. True, twelve astronauts have visited the lunar surface, but so far, human beings have not even reached the nearest planet, let alone the nearest neighboring star. Given a choice, would Earth really be the planet to pick—yet? In the optimistic case of a cosmos teeming with friendly worlds, the Earth is not yet at the grown-ups’ table.
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From Alien Earths: The New Science of Planet Hunting in the Cosmos by Lisa Kaltenegger. Copyright © 2024 by the author and reprinted by permission of St. Martin’s Publishing Group.
