In a recent New Yorker article, the nuclear historian Alex Wellerstein collected testimony from several people who saw, firsthand, the flash from the first successful detonation of the atomic bomb, at the infamous Trinity Test, on July 16, 1945.
Wellerstein has a writer’s feel for quotes and anecdotes. According to one general, the flash was a “golden, purple, violet, gray, and blue light” that illuminated “every peak, crevasse, and ridge” of a nearby mountain range, “with a clarity and beauty that cannot be described.” Wellerstein notes that several eyewitnesses described Trinity’s light as “cosmic.” This was apropos, he says, for nowhere else, “except in the interiors of stars do temperatures reach into the tens of millions of degrees,” as they do during a nuclear explosion.
A team of astronomers recently tried to determine whether Trinity’s light might be cosmic in a different sense. The Trinity test involved only one explosion. But if there were many more explosions, involving many more nuclear weapons, it might generate enough heat and light to be seen from nearby stars, or from the deeper reaches of our galaxy—so long as someone out there was looking.
And so, the thinking goes, maybe we should be looking. If every intelligent species eventually stumbles on nuclear technology, and not all of them manage it well, then it might be possible to spot an apocalypse in the heavens. Or several.
There are tens of billions of galaxies in the observable universe, each one a sea of stars. When astronomers watch these stars closely, they see them wobbling, the way our sun wobbles when its planets spin around it, tugging on its center of gravity. Astronomers also see these stars dimming ever so slightly, as though an objects were passing in front of them, and this dimming occurs at predictable intervals, as though these objects were moving around the stars in regular orbits. For these reasons and others, astronomers now believe that nearly all stars play host to planets, and they are making plans to image these planets directly, by catching the faint light they give off with huge, ultra-sensitive telescopes.
What will this light tell us? A remarkable amount, it turns out. Light encounters all kinds of molecules as it makes its way through the universe, and it keeps a close record of these encounters, in its spectra. If sunlight were to beam through Earth’s atmosphere, and then out into the stars, it would travel with this detailed chemical record in tow. If, after some millennia, this earth-kissed light fell into a distant astronomer’s telescope, that astronomer would be able to determine what sorts of chemicals were present in our planet’s atmosphere. They would know that water vapor was present, and life too, because Earth’s atmosphere contains methane gas, breathed out by the trillions of organisms that live on its surface. Indeed, it’s precisely these sorts of “biosignatures” that Earth’s astronomers hope to find in the atmospheres of extrasolar planets.
Light from extrasolar planets might also tell us whether our universe is home to other tool-making beings. After all, some of our pollutants leave behind chemical traces that would never occur naturally. If we glimpsed these pollutants in a distant planet’s atmosphere, we could be reasonably certain that technological life lived on its surface at one time or another. And according to Adam Stevens, Duncan Forgan, and Jack O’Malley James from Cornell’s Carl Sagan Institute, we might be able to know whether they used their technology to destroy themselves.
In July, Stevens, Forgan, and James published a paper that asked what a distant, “self-destructive civilization” might look like through the business end of a telescope. To do so, they gamed out several dystopian science fiction scenarios in great detail. They calculated the brightness of the gamma rays that would flash out from a massive exchange of nuclear weapons. They asked themselves what would happen if an engineered pathogen ripped through a large population of human-sized animals. What gases would fill a planet’s atmosphere, if its surface were strewn with rotting corpses? And would those gases be detectable across interstellar distances?
I asked Jill Tarter what she thought of the paper. Tarter is the former director of the Search for Extraterrestrial Intelligence Institute and the inspiration for Ellie Arroway, the heroine of Carl Sagan’s Contact, played by Jodie Foster in the film adaptation. Tarter told me the paper was “getting a bit of buzz” in the SETI community. But she also urged caution. “The problem is the signatures are detectable for cosmically insignificant amounts of time,” she said. Distant stars burn for billions of years, sending a constant stream of light toward Earth, but the flash from a nuclear war may last only a few days. To catch its light, you have to have impeccable timing.
Stevens, Forgan, and James acknowledge the ephemerality of their extinction signatures. According to their paper, some will last only 30 years, and others less than that. And even if a signal were to stick around for a hundred millennia, it would still be a tough needle to find in the vast spatiotemporal haystack that is our night sky. The universe has been manufacturing planets for billions of years. The odds that you’d train your telescope on a planet just as its resident civilization winks out are, in Tarter’s words, “a lot worse than Vegas.”
To beat odds like that, you’d need to take a detailed census of the galaxy. You’d need to eavesdrop on billions of planets, and for long stretches of time, and the tech for that kind of survey just doesn’t exist yet, and won’t for a while.
But it’s conceivable, in principle, and that itself is a miracle of human ingenuity. It’s wild to think that we may one day know something about the various fates that await beings like us. And it’s a useful prod toward deeper thoughts, about the sorts of flashes we are starting to send into the cosmos, especially this year, as we mark the 70th anniversary of the Trinity test.
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