Most of you have probably heard of SETI -- the Search for Extraterrestrial Intelligence. SETI is an umbrella term that links dozens of different projects and approaches, but is most often connected to the SETI Institute, founded in 1984 specifically to address the question of whether extraterrestrial life exists, and if so, whether we could communicate with it.
Just detecting the evidence of an extraterrestrial intelligence (assuming there is any) is fraught with difficulties. To begin with the most glaring problem, there are an estimated 250 billion stars in the Milky Way alone, which makes for a hell of a survey area. And even if you start by looking only at the relatively nearby ones, there's still the question of what exactly you're looking for. Radio wave signals are an obvious choice, and we have radio telescopes that do exactly that, but to start with, that's an awfully broad band of frequencies (between 300 and 3,000 kHz). Even to pick it up would mean not only listening to the right region of space, but being tuned to the right frequency at the right time.
But there are other problems. Suppose there is an intelligent civilization on a star 25 light years from us, and they're sending out a radio signal of some sort. There are two possibilities, of which the first is that the signal comes from their own intraplanet communications, like the oft-discussed bubble of transmissions from our early radio and television, which is currently carrying snippets of I Love Lucy and Gilligan's Island that are sweeping past planets and stars seventy-odd light years away. The second is that the signal is a deliberately-deployed beacon, an interstellar lighthouse specifically for communication with other civilizations.
Each of these presents its own problems. In the first case, the bubble of radio transmissions gets fainter and fainter as it expands, according to the inverse-square law of intensity. So as funny as it is to think of some extraterrestrials on the third planet of Vega judging humanity by The Beverly Hillbillies, my suspicion is that by the time it arrived the signal would be so faint that it would be nearly impossible to detect. (And that's not even considering degradation of the signal from passing through interstellar dust and gas.)
The second sounds more promising, but it too has a difficulty. If you're beaming a signal toward another star, you get an improvement in intensity because the logical way to do it is to collimate the beam as tightly as possible. You're still subject to the inverse-square law, but a tightly-collimated beam has such a narrow cross-section and high intensity that it could retain its power for a great deal longer. (Consider that even back in 1962, the Lunar Laser Ranging experiment successfully collimated a laser tightly enough that they were able to reflect it from the Moon, and detect the reflected pulse back here on Earth 2.6 seconds later.) But the narrower your beam, the smaller the area of your potential target. You would have to have a good reason to choose a particular star, or at least a region of space, or your signal would miss detection entirely.
So broad signal/low intensity, narrow signal/smaller sample size. There doesn't seem to be any way around those equal-and-opposite problems.
There's also the difficulty of how exactly you could encode a message that would be understandable to a non-human intelligence. Once the signal is received, how do you make sure the aliens can figure out what it's saying? This question has most recently been tackled by a team led by Jonathan Jiang of NASA's Jet Propulsion Laboratory, and this is in fact why the topic comes up. Jiang has made a proposal to send out a message aimed toward the densest patch of stars in the night sky, in the direction of the galactic center in the constellation Sagittarius. To be successful such message would have to contain enough information to (1) tell its recipients that it's not just an anomalous radio blip from a natural source, (2) give an indication of how to translate it, and (3) tell the extraterrestrials a little about where and who we are.
Jiang's proposal addresses all three. It first contains an easily-deciphered binary code that links to our base-10 counting system, and lists off the first 24 prime numbers. (Shades of the wonderful movie Contact, in which SETI researcher Ellie Arroway recognized that there is no naturally-occurring process that would produce blips in prime-number groups.) Once that (hopefully) convinces the aliens that we're relatively intelligent, the message goes on to communicate our understanding of time (using a standard time interval -- the spin-flip transition of hydrogen -- that would hopefully be known to any technological species). There would be information using this universal clock telling when the signal was sent, which would tell the recipient how far away we are. Last, there would be a binary-encoded sketch of two humans, and some basic information about our biology and biochemistry, reminiscent of the gold plaques placed on Pioneer 10 and Pioneer 11 when they were launched in (respectively) 1972 and 1973.
There's also the question, of course, of whether alerting the aliens to our presence (and giving them directions for how to get here, no less) would be a good thing. Back in 2011, physicist Stephen Hawking warned us that first contact might not be quite as cheery as it was in Star Trek. "We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet," Hawking said. "I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach. If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans... They could be billions of years ahead of us technologically. If so, they will be vastly more powerful and may not see us as any more valuable than we see bacteria."
Which is both humbling and scary. The positive side of all this is that even if the aliens do turn out to be hostile, they're still very far away. Let's say that there is intelligent life on a planet orbiting Gliese 581 (the home of the first Earth-like planet ever discovered). Gliese 581 is 20.4 light years away, so it's in our general neighborhood. If we sent a signal to them saying hello, they'd get it 20.4 years from now, and we'd receive their response in (minimally) 40.8 years. If instead of just sending a radio response back they were offended by our sending them biochemistry and pictures of naked people, and launched an attack fleet, it'd be even longer before they arrived here. So even if they're hostile, we're safe for some time.
That's assuming that they haven't found some way to overcome the light-speed barrier and get here at superluminal velocities. In that case, we might well be fucked.
But, like Hawking, I still think we should do it. Finding out we're not alone in the cosmos would be the most stupendous discovery humans have ever made. Jiang and his team don't have a proposed date for beaming out our "Can you hear me?" message, but there's no reason why it couldn't be done soon. And if it does reach an extraterrestrial intelligence, we'll just have to cross our fingers and hope it's not the Daleks, Stenza, Sontarans, or the Crystalline Entity.