Since the advent of radio and television broadcasts, humans have been sending chatter deep into the cosmos, ranging from Orson Welles' War of the Worlds broadcast to the latest Jersey Shore masterpiece. But is anyone out there listening?
No one has a definitive answer to that question, but a team of researchers has unraveled what kinds of signals that aliens are most likely to see (we can only hope that Jersey Shore flies under their radar). Additionally, relatively close alien eavesdroppers could detect our signals with a radio telescope as big as one soon to be built on Earth, according to the team's paper on the ArXiv.
Ironically, aliens are more likely to see our accidental signals from TV, radio and radar than our intentional, direct messages. So why do these accidental signals survive, and what sort of message, if anything, should we be sending into the unknown?
Several factors impact the "detectability" of a transmitted radio signals on distant alien worlds. The authors of the latest ArXiv article, which has been accepted for publication in the journal Space Policy, built upon previous work by Woody Sullivan to determine which messages will endure the trip through outer space.
The length of transmission, for instance, has limited the detectability of previous direct messages. Carl Sagan and astronomer Frank Drake helped create one such message in 1974 that was sent from the Arecibo Observatory (known as the Arecibo Message), but the message was transmitted only once. Consequently, potential alien life had a relatively low chance of detecting this mostly ceremonial message.
The most powerful radio transmissions from military and astronomy radars rank among the most likely messages to reach extraterrestrials. Astronomers use radar to bounce signals off nearby astronomical objects, such as asteroids, and detect them. These transmitted signals may reach much farther than intended, however.
So what are the odds that aliens are seeing these incidental transmissions? First, we have to determine the likelihood that intelligent life even exists within the range of our transmissions -- an extremely difficult endeavor alone. After that, we'd have to calculate the odds that aliens have technology advanced enough to detect our signals.
Frank Drake, one of the architects of the Arecibo message, tried to answer the aforementioned questions by creating an equation bearing his own name. With values for seven input variables about alien worlds, the Drake equation will reveal the number of alien civilizations in our galaxy.
But these variables, such as the average number of life-supporting planets per star, have only very speculative values. As a result, answers for the Drake equation range from 1 to 100,000,000 or more civilizations in our galaxy. That doesn't exactly narrow down the range.
Michael Busch, an astronomer at UCLA, and his colleagues have narrowed down the technology needed to detect our signals, however. With a square kilometer array like the one to be shared among New Zealand, Australia, and South Africa, aliens from 50 light years away could detect some of our TV signals, according to Busch and his team.
Some signals transmitted from the Arecibo Observatory would only be detectable up to five light years away. Lower bandwith signals from Arecibo could be seen 200,000 light years away, but we'd have to patiently wait for a reply. Alpha Centauri, our nearest star system, is about 4.2 light years away.
Detectability does not mean the same thing as intelligibility, though. Even if aliens can detect our signals, that doesn't necessarily mean they can see the content of those messages. Nonetheless, extraterrestrials would likely know that the signal came from an artificial source rather than an astronomical object, according to Busch.
If we know that we can reach potential alien lifeforms, what should we be telling them, if anything? The original Arecibo message relayed information about our number system, the constituents of DNA and our solar system that could be translated into a graphics. Others are trying to build upon this foundation.
Busch and a few other astronomers have set up a website called "Dear ET" in an effort to craft potential messages to alien worlds.
As humans trying to contact unkown worlds, we have to "start with a basic common vocabulary," Busch told Physics Central. This shared vocabulary likely involves mathematics and astronomy -- the tools used to contact one another.
In addition to suggesting messages, visitors to the website can try to decipher a (fictitious) message from an alien world. The current record for unraveling the test message stands at 12 hours, according to Busch.
Regardless of what we decide to send, we have plenty of time to do it.
"If ET is 100 light years away, you can take a decade to figure out how to say 'hi,'" said Busch.
If you want to keep up with Hyperspace, AKA Brian, you can follow him on Twitter.