Source: http://theatlantic.com/technology/are...

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.

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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.