The largest radio telescope ever to be constructed will struggle to listen in on extra-terrestrial civilizations like our own, according to two astronomers in the UK. Their calculations suggest that when the Square Kilometre Array (SKA) starts work in 2022 it will find it difficult to tune into radio signals from alien civilizations with Earth-like technology. The finding, they say, is further evidence that scientists must take a multidisciplinary approach to the hunt for intelligent life that doesn't just rely on detecting radio signals.

The Search for Extra-Terrestrial Intelligence (SETI) has been patiently eavesdropping on the galaxy for potential alien signals for over 50 years, so far without success. As one of its many scientific objectives, the SKA will join the search in 2022, hoping to answer the age-old question of whether our civilization really is unique.

However, research by Duncan Forgan, at the University of Edinburgh, and Bob Nichol, at the University of Portsmouth, suggest that its chances of finding human-like civilizations are slim: just one in 10 million.

They built a computer model of a mock Milky Way galaxy to see how many intelligent civilizations it could likely support. "We wanted to give as strong as an assessment as we could for using the latest radio telescopes for SETI," Forgan told The pair threw into the mixing pot the latest data on, among other things, stellar evolution, planetary system formation and habitable zones – the area around a star warm enough for a planet to have liquid water on its the surface.

10,000 civilizations per galaxy

To calculate the best-case scenario for SKA success, they optimistically assumed that if an Earth like planet sits in the habitable zone it would always go on to host intelligent life. From this they were able to populate the galaxy with intelligent life by assigning stars random properties from a statistical distribution. Having run the model 30 times they found the average galaxy would be home to about 10,000 intelligent civilizations.

"We now have a data set of galactic civilizations over time and space," said Forgan. "But there are factors which can prevent a civilization from being eavesdropped on: the civilization could destroy itself or be extinguished by an asteroid impact. However, more likely is that an advance in technology could make them harder to detect, " he added.

On Earth, we have been leaking radio signals into space for almost a century and any nearby civilization could eavesdrop on our signals. Indeed SKA could detect us if it were placed anywhere up to 100 parsecs – 326 light-years – away. However, as our technology is improving, and the power required to generate such signals is decreasing, we are moving from a "radio loud" to a "radio quiet" planet.

Too old for loud radio

With these factors in mind Forgan and Nichol combined their galactic population findings with the constraints on mass extinction, based on the Earth's fossil record, and the idea that a civilization is only "radio loud" for its first 100 years. They found the chances of radio communication between us and an Earth-like or a short-lived civilization, within the 100 parsec sensitivity limit of the SKA, to be one in 10 million.

However, this finding only applies to civilizations with technology akin to our own; it does not rule out stumbling across signals from a more highly developed civilization. "This is only one small part of SETI. Other SETI searches work on the assumption that they are looking for longer lived civilizations that emit, for whatever purpose of their own, rather stronger radiation," stressed Alan Penny, a SETI researcher at the University of St Andrews.

Forgan would like to see more resources put into other methods to sit alongside and complement conventional radio SETI searches. "The way we are approaching SETI is quite one-dimensional. There will always be a place for radio communication but we are getting close to its limit and we should find other ways to try as well," he said.

The research has been accepted for publication in the International Journal of Astrobiology and a preprint is available on arXiv.