SETI: The state of the art
Posted: 7 April
“At this very minute, with almost absolute certainty, radio waves sent forth by other intelligent civilisations are falling on Earth.” – Dr Frank Drake, Intelligent Life in Space, 1962
Fifty years ago this April, radio astronomer Dr Frank Drake embarked on Project Ozma, the first ever attempt to detect the technological signature of intelligent creatures on planets orbiting stars other than our Sun. This was the first step along a path that one day may culminate with our realising that humanity is part of a galactic culture of intelligent life, scattered across the light years between the stars. Ozma was a pretty simple set-up; Drake only had a single channel (bandwidth) receiver on the 26-metre Green Bank radio telescope with which he targeted two stars – tau Ceti and epsilon Eridani – scanning at 1420MHz. This frequency translates to the microwave wavelength of 21cm that is emitted by neutral hydrogen gas, the traditional wavelength around which SETI searches are conducted. For six hours a day, from April to July 1960, he listened to these two stars, but heard nothing but static. Fifty years later, we are still searching. So, has SETI progressed at all over the last half century, or has it stood still, lost in the trappings of mid-twentieth century scientific thinking?The 85-foot radio telescope at Green Bank, West Virginia, where SETI began with Project Ozma. Image: NRAO.
The concept of searching for radio signals from ET was first developed by Cornell University’s Philip Morrison and Giuseppe Cocconi in their seminal 1959 Nature paper Searching for Interstellar Communications. In this paper, they wrote “we may presume that the channel used will be one that places a minimum burden of frequency and angular discrimination on the detector. Moreover, the channel must be highly attenuated in space or in the Earth’s atmosphere.” They concluded that the neutral hydrogen line was the best bet, as it also comes under a great deal of scrutiny from astronomers anyway, making it more likely that we will come across a signal. Ending their paper with what has now become a mantra for SETI – “The probability of success is difficult to estimate, but if we never search, the chance of success is zero” – Cocconi and Morrison advocated searching the closest Sun-like stars first. Frank Drake duly obliged, and SETI was born.
East meets west
A spirit of cooperation between East and West flourished within the SETI community, and in 1971 a ground-breaking conference between scientists from both hemispheres, including Carl Sagan, Frank Drake, Philip Morrison, Nikolai Kardashev, Iosif Shklovskii and 43 others, was held at an observatory in Armenia. Intriguingly, the Soviet scientists were more interested in beaming signals into space, whereas the Western astronomers were more content to listen. This fundamental difference between the two philosophies would erupt into controversy almost four decades later, but for now the 1971 meeting was a positive step forward; with more scientists becoming involved, SETI was progressing.The Arecibo radio telescope. Image: NAIC–Arecibo Observatory, a facility of the NSF.
The Armenian conference was followed up a year later by a blueprint for SETI called Project Cyclops. Ground-breaking and ambitious, it was a plan of where, how and what to search for, based on a series of conferences convened in the late sixties by NASA’s Exobiology Division at the agency’s Ames Research Centre (in the days when NASA was still officially interested in SETI). Edited by Bernard Oliver and Dr John Billingham, the Cyclops report called for an array (or ‘farm’) of a thousand radio dishes, each one a 100 metres across, to listen to the microwave sky for those signals that Frank Drake is positive are falling on Earth every day. Such a forest of radio dishes have rivalled the Square Kilometre Array, planned to be built during the next decade, but was an idea borne out of over-enthusiasm for the burgeoning Space Age and the explosion of large radio telescopes around the world, including the giant 300-metre Arecibo ‘bowl’ in Puerto Rico, which will forever be linked with SETI. Whilst finances meant that the Cyclops Array was a non-starter, in 1974 Arecibo was the location of the first ever message sent to the stars on behalf of humanity, as Drake led a team of scientists that beamed a pictogram detailing human beings and where we come from towards the globular cluster M13, twenty-five thousand light years away.
Meanwhile, we were still listening. Cyclops had recommended that the search continue at wavelengths ranging from the 1420MHz of neutral hydrogen to the 1662MHz of hydroxyl, which put together constitute a water molecule. This ‘water hole’ would surely be poetically attractive to any lifeform that understood the importance of water for life to survive. The 1970s saw almost 30 new radio searches, including numerous all-sky surveys, targeted searches of hundreds of stars, surveys of the galactic centre and globular clusters, and various astrophysical phenomena in the hope that they may turn out to be the work of advanced civilisations. Following our closest ‘hit’ to date, the 1977 detection of the sadly never-seen-again ‘Wow!’ signal, expectation was rife that a message would be detected any day.
We’re still waiting for that day, but it hasn’t been for lack of trying. In 1984 the SETI Institute in California was founded, which began acting as a contractor for NASA. Searches continued at Arecibo, Jodrell Bank, Green Bank and a host of other radio telescopes. In 1992 NASA began funding a SETI survey called the High Resolution Microwave Survey (HRSM) to the tune of 100 million dollars over ten years. HRSM, no doubt given a dull name so as not to conjure up images of little green men, was set to be the biggest and most concerted effort to find ET yet, utilising both NASA’s Deep Space Network antenna in Mojave, California, and Arecibo.
Then disaster struck at the very heart of SETI when US Senator Richard Bryan campaigned for HRSM to be scrubbed; an easy scalp for his political ambitions. In 1993 the US Congress cancelled NASA’s SETI efforts, with Senator Bryan declaring “This hopefully will be the end of Martian hunting season at the taxpayer’s expense.”An artist’s impression of the Square Kilometre Array, with radio dishes all the way to the horizon.
However, the conviction of SETI researchers has always been stronger than that of those who pour scorn over the subject, and the SETI Institute set into action. HP’s Bernard Oliver made a few calls to some of his buddies – William Hewlett, David Packard, Intel Corporation’s Gordon Moore (of Moore’s Law fame) and Microsoft’s Paul Allen. In one afternoon, Oliver had swiftly secured 20 million dollars to keep the SETI Institute afloat as a private, not-for-profit organisation and allow the search to continue.
The ATA is also a precursor to similar arrays that are in the planning stages elsewhere in the world – the Atacama Large Millimetre Array (ALMA)at the European Southern Observatory in Chile, and the Square Kilometre Array (SKA), which will operate between 100MHz and 25GHz and comprise a collecting area of a million square metres across several hundred ‘stations’ that will each be the equivalent of a 100-200 metre dish. When constructed, this forest of dishes will be an incredible sight, towering out of the plains of either South Africa or Australia (the final location has yet to be decided) and, although designed for straight-forward astronomy rather than for SETI, if there are any signals falling on Earth’s Southern Hemisphere it will find them. Or rather, you might find them; given the incredible amount of data that will be collected by the SKA, undoubtedly much of this will be disseminated across the eight million SETI@home users – a public science project that began in 1999 and is one of SETI’s greatest successes.
With modern technology we can search millions of radio channels (bandwidths) simultaneously, compared with only having one channel to search as Frank Drake had during Project Ozma, as well as having the ability to detect pulses from nanosecond lasers (it should not escape your attention that this year is also the fiftieth anniversary of the invention of the laser). To date we’ve found over four hundred planets orbiting other stars – SETI ran for thirty years without even knowing if there were other Solar Systems out there – with the promise of finding Earth-like worlds within the next few years. Meanwhile, our understanding of how planets and stars, both crucial to the later development of life, are formed has grown tremendously. We’re beginning to piece together the parts of the puzzle at an accelerated rate, and if we’re to believe Frank Drake, there are at least 10,000 civilisations out there waiting for us to pick up the phone. The SETI Institute’s Dr Seth Shostak reckons that with the likes of the ATA and the SKA, and as SETI technology advances exponentially following Moore’s Law of the doubling of computing power every 24 months or so, we’ll have found definitive proof of ET’s existence within the next two or three decades. So hold onto your hats – the future is going to be an exciting ride.
Stay tuned throughout April for more SETI articles.