Scientists may have an extra challenge when it comes to detecting alien civilizations: a time limit.
A new study suggests that intelligent aliens, if their technological progression is similar to that of humanity's, are likely to have moved away from noisy radio transmissions to harder-to-hear digital signals within a 100-year time frame. That offers Earth just a narrow window in which to pick up any signals from extraterrestrial civilizations.
"Based on the results that we looked at, if we assume that the civilizations are humanlike with similar technological progress to us, we calculate the probability of making contact is roughly one in 10 million," the study's lead author, Duncan Forgan, told SPACE.com.
The time it takes a planet to go "radio quiet" dramatically restricts the types of signal it sends into space and our chances for eavesdropping on them, said Forgan, a postgraduate researcher at the University of Edinborough in Scotland. [Poll - Is Earth Ready to Meet an Alien Civilization?]
Forgan and his team applied their technology-development time scale to a simulation of the galaxy, based on the assumption that the pace of an alien civilization's technological progress would be similar to that on Earth. Based on this simulation, the researchers determined the 1-in-10 million odds of humans accidentally stumbling across a transmission from aliens.
The researchers, whose study will appear in an upcoming edition of the International Journal of Astrobiology, focused their work on the expected eavesdropping capabilities of the Square-Kilometer Telescope, a radio telescope slated to be completed by 2023.
A radio century
In the early 20th century, the only options for communicating quickly over long distances were by telegram or wireless radio. As radio technology improved, so did the quality of its broadcasts and receivers.
"In the past, the detector would only take up a chunk of that energy, and the rest would go out into the universe," Forgan said. "Now, instead of firing off a huge amount of energy and picking up a trace, we send out a very small amount and soak up almost all of it, so the amount of lost energy is much lower."
Traditional radio communication is being replaced by innovations in the use of light to send digital signals through fiber-optic cable and the emergence of the Internet. The transition from analog to digital broadcasts, which in the last few decades recoded radio signals into formats that preserved their clarity over long distances, have further cut down on Earth's signal leakage into the universe.
The Drake equation
To determine the Square-Kilometer Telescope's chances of picking up a radio broadcast from intelligent extraterrestrials, scientists used simulations similar in structure to that of the famous Drake Equation.
While that formula (created by astronomer Frank Drake) spat out a single number for the amount of contactable civilizations by stringing together seven factors deemed critical to the formation of intelligent life, the program used probabilities to create a spatial distribution of a galaxy's stars and planets.
"What we're doing is approaching it from a different angle," Forgan said. "Instead of squeezing huge amounts of astronomical data into several numbers, we're going to use as much of it as possible."
The scientists attempted to create a statistically accurate picture of our galaxy by using data on typical star masses, locations, number and masses of planets, and their habitability. According to their simulations, civilizations on par with the technological development of humans could be separated by distances of at least 1,000 light-years or so.
One type of simulation examined a scenario in which all intelligent alien life became radio quiet — a realistic scenario, Forgan said, because signal leakage lessens steadily as technology improves. Another type looked at civilizations that Earth could see during the planet's entire lifetime.
"When you do that, the possibilities improve immensely; it goes from being a remote impossibility to being much, much higher," Forgan said.
The concept of radio quiet doesn't affect the change in distance, but it affects the time scale during which you can hear the signal, he added.
"If your target is broadcasting for 1,000 years, then this gives you more time to find it before the signal switches off than if they were only broadcasting for 100 years," Forgan said. "The 'radio quiet' concept reduces the time scale to a very low number, which means your chances of hearing it are very small."
What this means is that searching only for radio leakages could cause scientists to miss "quite a lot" of civilizations, Forgan said.
"These results aren't as new as you may think, but they are exciting because it shows that even the most powerful radio telescopes in the world will still struggle to find E.T. unless we design the search carefully," Forgan said.
Previous studies on the observation limits of the Square-Kilometer Telescope showed it could detect extraterrestrial signals from up to about 300 light-years away within two months, assuming that the other civilization's broadcasts were at least as strong as the military radar used by Earth's governments.
Will we ever detect life?
Scientists continue to use radio waves to search for life because of the scarcity of natural sources of radio waves in the universe, and the fact that they are less easily lost by absorption than other forms of electromagnetic radiation, which includes light.
Even the smallest snippet from an alien broadcast could count as evidence of an extraterrestrial intelligence.
"An artificial signal will have patterns in it that usually do not appear in nature, even if distorted," Forgan said.
Alternate search targets include technologies that possess a distinct radiation signature, such as the exotic matter created inside a particle accelerator, and technologies that have not been invented yet on Earth, such as communication by lasers or neutrinos.
Neutrinos are very light particles that constantly stream through our bodies and the Earth. (If anyone found a way to capture neutrino emissions in a device, Forgan said, the technology surely would replace Megahertz light waves for cell phones, because the signals would never be obstructed by a building.)
But doesn't mean humanity should give up on radio: Rather than look at the steady drip of leakage that results from our televisions, radio or radar, we could look for alien civilizations that are making a general effort to communicate with us, Forgan said.
That approach, he admitted, is likely to mean a long wait for any cosmic callbacks.
"If we send a signal right now, it will take four years to reach the nearest star," Forgan said. "It's much more likely we will receive a return message in hundreds or thousands of years.
"On the other hand, other civilizations may have a different outlook. They may be desperate to make communication with other civilizations."