by Avi Loeb
Image by Mapping Specialists, Ltd.
Working with pencil and paper, my research team modeled the idea until we reached the point where only more sophisticated computer hardware could advance it further. Volker Bromm, a graduate student at Yale at the time, took on that task, and over the past two decades he and other theorists have established that, indeed, the process we outlined for the birth of stars could give rise to the early galaxies. Models and theories are invaluable, but data that proves both remains essential. I wanted to see the gas clouds our theory predicted, which meant trying to find evidence that was about thirteen billion years old.
When astrophysicist-detectives confront the challenges of scale that the universe presents, they can become overwhelmed. They do, however, have one asset without parallel in any other academic discipline. They have the ability to look back in time.
Because light travels at a finite speed, the farther out we look, the farther back in time we see. And since the universe had similar conditions everywhere, by peering deep into space, we can view our own past.
The deeper into space we look, moreover, the older the objects we uncover. To look at a star four light-years away, such as Proxima Centauri, is to look at a star as it was four years ago. But if we focus our telescopes on a galaxy that was thirteen billion light-years away when its light was emitted, we are glimpsing the universe as it was thirteen billion years ago. To peer that far back into the “dark ages” of the universe, the moment when the clouds of gas from which the first stars arose gathered, is a monumental scientific challenge. It also forces us to contemplate the incomprehensibly vast timescales of the universe. Humans today live, on average, nearly seventy-three years. To have seen the first lights in the universe come on thirteen billion years ago, we would have had to live almost one hundred and eighty million lifetimes—an idea that is especially absurd considering that the Earth is only about 4.5 billion years old and that we believe the planet has supported life for only 3.8 billion of those years.
Looking into the universe, astrophysicists also come face to face with the physical immensity of the cosmos. We can see light that was emitted earlier in cosmic history. The universe resembles an archaeological dig centered on us. The deeper we look, the more ancient are the layers we uncover. This exhibit of cosmic history continues all the way back to the edge of the visible sphere around us, located at the Big Bang, 13.8 billion years of light travel time. It takes light originating beyond this edge more than the age of the universe to reach us, and so more distant regions are not visible to us.
It is very presumptuous for us to assume that we are the only intelligence in this vast cosmos. Even though life as we know it and life as we do not know it may exist on numerous other planets, it is most likely that we will encounter relics of extraterrestrial technologies before establishing contact with any living civilization. This must be kept in mind as we contemplate explanations for the mysterious properties of interstellar objects like ‘Oumuamua.
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My research on the cosmic dawn contributed to the creation of a new field of study, what has come to be called “twenty-one-centimeter cosmology.” It is a branch of radio astronomy that maps the universe in three dimensions using radiation from hydrogen atoms that started out with a wavelength of twenty-one centimeters that was subsequently stretched by cosmic expansion.
You may recall that this is the same meter-wave radio spectrum that humans fill with the noise of their televisions, radios, cell phones, and computers—an insight that inspired Matias Zaldarriaga and me to wonder if other advanced civilizations would likewise emit such noise. But my initial interest in twenty-one-centimeter emissions was as a means to stare back to a time well before any civilization was possible. At this phase of my career, I wasn’t hunting aliens; I was hunting hydrogen.
After the Big Bang, hydrogen was the most abundant element in the universe by a wide margin; the early universe was about 92 percent hydrogen atoms and 8 percent helium atoms. But at this point, the hydrogen in the universe wasn’t emitting any radio signals that we can detect today. That is because in the immediate fiery aftermath of the Big Bang, the vast majority of the ordinary matter of the universe, hydrogen, was ionized.
Neutral hydrogen atoms consist of a single proton and a single electron. But at high temperatures and with intense ultraviolet radiation, they are broken (ionized); the hydrogen atom sheds its electron and exists as a single positively charged proton. This changes hydrogen’s behavior—or, more accurately, the type of radio signal it emits. The electron bound to a neutral hydrogen atom can transition between energy states, higher and lower, and in doing so emit a photon, or light particle, in the form of a radio wave twenty-one centimeters long. But ionized hydrogen cannot.
About three hundred and eighty thousand years after the Big Bang, the universe cooled down enough for electrons and protons to combine and form neutral hydrogen atoms, and we can start to seek the element’s telltale signature, twenty-one-centimeter radio waves. For hundreds of millions of years, hydrogen atoms remained neutral, passing between their higher and lower energy states and emitting waves right up until stars and then galaxies began to form—and the hydrogen in the universe was ionized all over again.
Stars emit more than visible light; they also emit ultraviolet radiation, which can split hydrogen atoms into their component parts, electrons and protons. When the first stars turned on, they re-ionized the universe’s neutral hydrogen atoms. This was less a moment than an epoch, a long period when ultraviolet light from the earliest stars and black holes split the universe’s dark fog of neutral hydrogen into protons and electrons. But the changing chemistry of the universe gave astrophysicists data to search for—namely, the absence of twenty-one-centimeter emissions. Ionized hydrogen atoms do not emit these radio signals, but neutral hydrogen atoms do.
Thus, the moment when the twenty-one-centimeter emission signal disappears is the moment the stars were born. Like the famous Sherlock Holmes story that turned on the dog that did not bark, this scientific mystery became the case of the hydrogen that no longer produced its twenty-one-centimeter emission.
As I write, the search is under way for data that will help us pinpoint when, exactly, the stars began to shine. In South Africa, a multi-antenna array called the Hydrogen Epoch of Reionization Array (HERA) is currently measuring twenty-one-centimeter emissions from the early universe. The Hubble Space Telescope recently identified a galaxy that flipped its lights on just three hundred and eighty million years after the Big Bang. And the James Webb Space Telescope—whose first science advisory group I served on, decades ago—is expected to launch in 2021 and be able to find galaxies at even earlier times. Under development are the twenty-four-and-a-half-meter Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope, which has an aperture diameter of thirty-nine meters.
Our contact with the data from these efforts has only begun and with it the winnowing of explanations for how the stars came to shine. And the answer, when we discover it, will bear immediate relevance to the question of whether intelligence other than our own is out there in the expanse of space. If ‘Oumuamua is alien technology, then it is a near certainty that its designers also looked into the dim past of our common universe and likewise teased out meaning from ionized and neutral hydrogen. To be curious enough to explore space in the neighborhood of one’s own solar system or out among the stars is by definition to be curious about the universe—what its properties are, what explains its past, and what predicts its future. It is not just that our own curiosity and behavior is our best guide to the curiosity and behavior of extraterrestrial life. It is also true that the insights of science will provide us with the common language we need to make sense of extraterrestrial intelligence, perhaps even communicate with it. Science also provides us with a means of making sense of what we discover, however fleeting, however partially. For if we can build it, the odds are great that another intelligence, if it is out the
re, has done the same.
9
Filters
If the lightsail hypothesis is true, there are two possible explanations. One is that ‘Oumuamua’s makers intentionally targeted our inner solar system; the other is that Oumuamua is a piece of space junk that happened upon us (or we upon it). Either of these interpretations could be accurate regardless of whether the civilization that created ‘Oumuamua still exists today. But given what we know about the universe and about civilizations, we can make some inferences about which interpretation is likely correct and what implications it holds for us and for whoever (or whatever) created ‘Oumuamua.
The space-junk idea is similar to the asteroid/comet hypothesis in an important way: it implies that ‘Oumuamua is part of an incredibly huge population of similar objects. Every star in the Milky Way would have to send, on average, a quadrillion of these things into interstellar space in order to make it conceivable that one would happen to zip past our telescopes just as we trained them on the sky. That translates to one launch every five minutes from every planetary system in the galaxy, and it assumes that all civilizations live as long as the thirteen-billion-year-old Milky Way, which is certainly not the case.
The idea that civilizations could manufacture their way to such a density of objects, critics argue, seems even more unreasonable than all the conjectures concerning planet formation and the release of material from outer clouds to produce a sufficient population of rocks. To fill the universe with space junk at such a density, a great many civilizations would have to spend a great deal of time ejecting a great deal of material. Of course, the moment we posit an intelligence behind some materials’ construction, we also do away with the need for a random distribution of materials. After all, we did not send our five interstellar rockets off on random trajectories. Scientists decided to send them toward certain stars, and we can anticipate that another intelligence would do the same.
We should also avoid the trap of imagining interstellar spacecraft as rare and precious, as our paltry five interstellar probes could suggest. Given the rarity with which humanity has sent material out into interstellar space, the hypothetical abundance I have postulated would seem unreasonable indeed.
This scenario seems a little less unreasonable if we think of this possibility against the potential projected rate of ejecting StarChips using the Starshot Initiative that my colleagues and I proposed to Yuri Milner. We estimate that once the investments have been made to build a suitably powerful laser and launch it into space, the relative costs of sending many thousands, indeed millions, of StarChips into interstellar space drops exponentially.
But the abundance of interstellar spacecraft in the scenario I’ve just described will seem most reasonable, perhaps, if we return to our plastic bottle.
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Right now, the United States Space Surveillance Network tracks more than thirteen thousand man-made objects orbiting Earth. These include everything from the International Space Station to defunct satellites, from orbital telescopes like Hubble to discarded rocket stages and even nuts and bolts left behind by astronauts. It also includes roughly twenty-five hundred satellites it has taken us fifty years to put into space.
Indeed, during that brief window of time, our efforts to send material up into our planet’s orbital plane have been sufficient to make space junk a looming problem. For example, in 2009, two satellites, Russia’s inactive Cosmos 2251 and America’s active Iridium 33, collided at approximately 22,300 miles per hour above Siberia. The result was an instantaneous cloud of debris, which increased the risk of more collisions. This was the first known collision between satellites and it underscores the danger of a rising amount of junk orbiting Earth.
The threat of collision has been steadily increasing for years, in part due to ever more nations viewing space as a new frontier for conflict. Over a decade ago, China demonstrated the success of its anti-satellite missile technology by destroying its own Fengyun-1C weather satellite. India accomplished a similar feat in 2019, creating another four hundred pieces, give or take, of space debris. A consequence? The risk of impact to the International Space Station was estimated to have gone up 44 percent over ten days. No wonder the station is designed to maneuver out of the way of danger—assuming it has enough warning.
What humans do helps us predict what other civilizations are likely to do. We continue to be our own best data set for imagining the behavior of other civilizations and the consequences of that behavior. With that in mind, consider that a computer simulation looking out two hundred years predicts we will act in ways that will multiply the amount of space-junk objects that are larger than about eight inches by a factor of 1.5. And smaller junk will increase even more. The simulation predicts that the number of objects less than four inches will increase by a factor of somewhere between 13 and 20.
This junking-up of space is, sadly, in keeping with humanity’s treatment of its terrestrial habitat. In 2018, the World Bank issued a report entitled “What a Waste 2.0” in which it estimated that the world generated 2.01 billion tons of solid waste a year. The World Bank also projected that by 2050 that number could go as high as 3.4 billion tons. In 2017, the U.S. Environmental Protection Agency estimated that the average American generates 4.51 pounds of solid waste a day, and the United States is far from the greatest producer. While the United States and China produce the most greenhouse-gas emissions, it is the lower-income countries that produce the most solid waste due to their inability, driven by economics, to properly dispose of it.
Of course, from the vantage point of Earth itself, the origins of the world’s solid waste doesn’t matter. Much of it ends up in the oceans regardless.
One of the fastest-growing areas of waste is what is termed e-waste—discarded laptops, mobile phones, and home appliances that have been displaced by newer models. In 2017, the United Nations’ Global E-Waste Monitor estimated that in the prior year, the world had generated 44.7 million metric tons of electronic waste. And it estimated that by 2021, that would rise to 52.2 million metric tons.
Here our own civilization’s behavior offers, once again, another piece of evidentiary data we can consider when we wonder about ‘Oumuamua’s origins. If we suppose ‘Oumuamua was not a functioning probe or an inert buoy but rather another civilization’s defunct or even discarded technology, that suggests that another civilization acted in ways we can immediately identify with—that they were, like us, profligate in their production of materials, technological and otherwise, and that, like us, they were comfortable abandoning them when obsolete. Just because we have not yet reached the maturity to discard materials into interstellar space should not blind us to the possibility that our interstellar neighbors might have or, more likely, did.
Trash, both in its solid-waste form and its greenhouse-gas-emission form, is a useful analogy for a different reason: it suggests an answer to the question of how ‘Oumuamua might have ended up roaming the universe as space junk. Because one of the insights granted by pioneering physicists in this field—men such as Frank Drake, whose famous equation quantifies our chances of detecting a light signal from an advanced civilization in space—is that most of the technological civilizations that ever existed might now be dead.
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Enrico Fermi was one of the giants of twentieth-century physics. Among his accomplishments is the development of the first nuclear reactor and, as he was instrumental in the Manhattan Project and the production of the first nuclear bomb, he can claim some credit for the prompt ending of hostilities with Japan at the conclusion of World War II.
Toward the end of his storied career, during a lunch with his colleagues, Fermi raised a simple, provocative question: How do we explain the paradox that, given the vastness of the universe, the probability of extraterrestrial life seems high, yet there is no certain evidence for anything but terrestrial life? If life is common in the universe, he asked, “Where is everybody?”
Over the years, many answers have been formulated. O
ne is especially arresting and especially pertinent to the unfolding mystery of ‘Oumuamua and its implications for us.
In 1998, the economist Robin Hanson published an essay titled “The Great Filter—Are We Almost Past It?” Perhaps the answer to Fermi’s paradox was, Hanson argued, that throughout the universe a civilization’s own technological advancement overwhelmingly predicts its destruction. The very moment when a civilization reaches our stage of technological advancement—the window where it can signal its existence to the rest of the universe and begin to send craft to other stars—is also the moment when its technological maturity becomes sufficient for its own destruction, whether through climate change or nuclear, biological, or chemical wars.
Hanson’s thought exercise has sufficient plausibility that humanity would do well to consider the question in his article’s title: Is human civilization nearing its own filter?
It would be no small irony if Fermi is the solution to his own paradox, for, with Fermi’s help, we developed nuclear weapons seven decades ago. But even without nuclear weapons, we are moving to destroy ourselves by permanently changing the climate. The rise of antibiotic resistance, due to many factors but certainly including the largely indiscriminate use of antibiotics in industrial agriculture and livestock, also poses a threat. So do pandemics, accelerated and exacerbated by our industrialized assault on our planet’s ecosystem.
It is quite conceivable that if we are not careful, our civilization’s next few centuries will be its last. If that’s the case, the emissions we’ve been sending out into the universe from our radios and televisions—that outward expanding bubble of noise humankind started to generate only a century ago—and the five interstellar craft we have already launched could well end up the equivalent of dinosaur bones here on Earth, evidence of something once formidable and extraordinary that is now only material for other civilizations’ archaeologists.
