Excerpt from Alien Earths by Lisa Kaltenegger, plus links to reviews, author biography & more

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?

The Great Silence

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.

The premise that anyone who could call us would do so immediately, seems flawed, making the Great Silence less eerie.

Talking to a Jellyfish

If we ever found another civilization that we could communicate with using radio signals or visible light—both of which travel much faster than spacecraft—I often wonder what we would say. What questions would we ask? And how would we ask them? It seems unlikely that they would understand English, Chinese, Spanish, or any of the other thousands of languages spoken on our beautiful planet. The experience might end up being like a human trying to talk to a jellyfish. I've attempted that; the results were less than promising. And in that case, the jellyfish was right there in front of me. I could see it and could have touched it (but I refrained), and I listened for any sounds it might make in an attempt to learn its language (with no success). Note that I am not an expert at interspecies information exchange, though there are other scientists worldwide studying the communication of dolphins, whales, chimpanzees, and dogs, among others—they might fare better. To interpret and understand other species, it is critical to observe actions and other visual cues and combine these with your interpretation of sounds. It is a daunting task. Imagine how much harder it would be with a civilization you couldn't even see? An advanced interstellar civilization trying to converse with a less advanced one would be a bit like humans trying to interpret the movement of a school of fish, which is dynamic, purposeful, and even beautiful but, ultimately, puzzling in its intentions.

Excerpted from Alien Earths by Lisa Kaltenegger. Copyright © 2024 by Lisa Kaltenegger. Excerpted by permission of St. Martin's Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.