Let's build a beacon to tell aliens who we were

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Somewhere in the cosmos, 36 light years away from us in the direction of the Hercules constellation, a series of electromagnetic waves stretching across 30 million miles of space carries a message from Earth. Each of the 1,679 signals it contains falls in one of two frequencies ? an FM signal that translates to a bunch of ones and zeroes.

This message originated in 1974, when it was broadcast from the Arecibo radio telescope in Puerto Rico to commemorate the facility's renovation. The authors of the message, Carl Sagan and SETI founder Frank Drake, hoped that any aliens who happened to receive it might notice that 1,679 is the product of two prime numbers, 23 and 73, and if you arrange all the zeroes and ones in a grid of 23 columns and 73 rows, you get a series of simple, ASCII-like pictures, including a double helix and a crude image of a person. Whether or not an alien civilization could crack the code, they would at least notice something funny about these FM signals. They're 10 million times stronger than the background noise from our sun.

But no extraterrestrials will even get the chance. The Arecibo transmission was aimed at a system 25,000 light years away that will have long since orbited out of the signal's path by the time it arrives in the vicinity. The odds of any two civilizations ever overlapping in time are extremely small. Even if we left the Arecibo telescope squealing out its signal until its power ran out and its hardware rusted, there's virtually no chance that the emanations would get anywhere in particular, and hang around long enough to be seen or heard. The only way we'll make contact is if we can make a beacon that keeps going for millions or billions of years after we're gone.

The practice of sending signals to prospective alien neighbors is called active SETI or METI (Messaging Extraterrestrial Intelligence). In 1999, and again in 2003, a Russian scientist named Alexander Zaitsev took it upon himself to send powerful signals to nearby star systems using a high-power radio transmitter in Ukraine. This upset a lot of his colleagues, who thought it was a bad idea to announce ourselves to the cosmos ? more than we already do with leaked radio communications ? when we have no idea what an extraterrestrial civilization's intentions might be. Stephen Hawking has warned that we could face a Columbus-coming-to-America situation, with Earth playing the part of the Native Americans. Michio Kaku pointed out that Cortes conquered the entire Aztec empire in less than two years, with technology that was only perhaps 1,000 years more advanced. If a bunch of aliens showed up here, the tech disparity might be many times greater in magnitude.

I'll accept full responsibility if the Earth is wiped out by aliens, but I don't think Hawking or Kaku have much to fear from the Russian. His broadcasts are sporadic, and the odds that anyone would be in exactly the right place to detect them are functionally zero. Not until someone develops a signal that can be repeated for eons will there be any feasible threat from a slime-thirsty race of space goblins.

Still, there's no point in putting ourselves at unnecessary risk. If we're only trying to preserve Earth's legacy, as opposed to making contact with another civilization in our species' lifetime, there's a simple way to avoid this problem: Make the beacon into something like an interplanetary alarm clock. The device could be programmed to remain silent so long as it's receiving regular communications (presses of the snooze bar) from Earth. When it notices that the planet has been silent for, say, 500 years, the machinery kicks into gear.

Such a machine sounds absurd, and physicists I talked to disagreed over whether we have the technology to build something sturdy and energy-efficient enough to withstand such a deep expanse of time. I was at a dinner with a couple of plasma physicists a few weeks ago who thought we could create one right now if the funding came through. Others think we don't yet have the necessary know-how ? though most concede we could figure it out down the line ? that such a device wouldn't violate any steadfast physical law. In any case, a group of serious scientists has already started to attack the problems associated with building an ultra-long-term beacon.

The most fruitful work takes place on the blog Centauri Dreams, which is affiliated with a foundation that promotes interstellar travel. The charge is led by a pair of twin brothers, physicists James and Gregory Benford (the latter is also a science fiction author), who approach the field of alien-directed beacon-design first from an economic approach. In a paper in the journal Astrobiology, they lay out how one might build such a device without spending the whole GDP. No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs, they argue. But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon's message arrived. This more sensible approach ? a sort of Energy Star specification for SETI ? would save enough power to keep the beacon running for millions of years.

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