Have They Seen Us? | Space Time | PBS Digital Studios

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Matt O'Dowd (1:01)

A century of Earth's radio transmissions has now washed over thousands of other star systems, carrying with it some of our greatest broadcast masterpieces, as well as our worst reality TV.

What are the chances that alien civilizations have detected and even decoded these signals?

We are a young technological civilization. An advanced alien race monitoring our planet, even from the nearest neighboring star, would have seen nothing. Radio quietness. Until only around a century ago. Then, beginning with the faint sporadic buzz of the first experiments with wireless transmission, the radio brightness of this small planet would have bloomed into a continuous planet-wide cacophony of TV and radio broadcasts, satellite relays and radar pulses. This bubble of chatter is, as I speak, spreading out into the galaxy at the speed of lights. The unmistakable signature of an emerging technological power. The outer edge of this bubble is now over a hundred light years away, and it carries with it the first transatlantic radio transmission of Marconi himself. That edge may be hopelessly diffused, but it's followed closely by the much brighter 80 light year shell on which rides the footage of the 1936 Berlin Olympics, episodes of the Lone Ranger, and Orson Welles ill-fated radio adaptation of War of the Worlds. That shell has washed over several thousand star systems. Based on Kepler Space Telescope observations, that also means thousands of potentially habitable planets. Lots of chances for hypothetical civilizations to have learned of our existence.

So the question is, who could have seen us? What manner of civilization would be powerful enough to sense this bubble? What tech level would be needed to actually decode the signal it carries? If aliens arrive on our doorstep tomorrow, would they have been alerted to our presence by Nikola Tesla's first experiments? Broadcast footage of the moon landing? The phantom menace?

To answer the question of who can see us, it's helpful to first ask, who could we see? After all, humanity remains the one example of a technological civilization for now.

Our best astronomers have been engaged in the search for extraterrestrial intelligence, SETI, in earnest since the 60s, when Frank Drake first peeked at the stars Tau Ceti and Ceylon Eridani. Drake peered into the so-called water hole, a narrow frequency range in the radio spectrum between a pair of H and OH emission spikes, which itself is within the broader window from 1 to 10 gigahertz. There, the natural universe is especially quiet. Surely this is the window where aliens would choose to broadcast their messages welcoming us into the galactic community. Of course, Drake saw nothing. Decades of SETI programs followed, utilizing several of the world's great radio telescopes, like Arecibo in Puerto Rico and the Parkes Radio Telescope in Australia. These searches often focused on the water hole or the surrounding quiet frequency window.

But with the exception of some one-off oddities, the search was not successful. The 1977 WOW signal is the most compelling. A narrow-frequency radio blast detected in the water hole that still has no broadly accepted natural explanation. But no one ever saw it again. Apparently no one is trying particularly hard to get our attention. And it's important to understand that these searches could only have been successful if there were deliberate, powerful beacons emitted by an alien civilization. The radio leakage produced by their own internal broadcasts would be much harder to detect. Our best-targeted search so far, the SETI Institute's Project Phoenix, scanned 800 stars within 200 light years. At that range, it could have picked up a gigawatt beacon beamed directly at us. But it would only have picked up leakage from a civilization's internal broadcasts from within a couple of light years. If those broadcasts were the same strength as our own. In that range, there is exactly one star, ours. An alien civilization at exactly our current tech level would never have seen our TV bubble.

We have sent out more powerful beacons, like the stream of 10,000 tweets and celebrity videos that Arecibo blasted in the direction of the WOW signal. But that's a long shot.