When I found a copy of Carl Sagan’s Contact in a secondhand bookstore, and somebody gave me another Sagan book on extraterrestrial life signed by Carl himself, I started wondering again whether we are alone in the universe. Living, as we do, on the only planet in the solar system known to be inhabited, and thinking of the relative insignificance of our yellow dwarf star, the Sun, among the myriad stars in the Milky Way galaxy, it is hard to believe that the Earth is unique. Surely those vital parameters, which are essential for the development of life as we know it, must exist somewhere else too?
Over the last 40 years, a massive search has been under way for signs of extraterrestrial intelligence. Leave aside the accounts of UFOs and reports of the abduction of humans by aliens. The real search scans the universe for radio signals from nearby stars like our own Sun. Signals which cannot be explained away as “noise” or astronomical phenomena could be generated by some extraterrestial intelligence. (It must be remembered that we have been sending radio messages into space for over 50 years now; extraterrestrial civilisations may already be bewildered by some of our early soap operas!) Both Pioneer 10 and 11 and the two Voyager Spacecraft are also now travelling beyond the solar system, and may just cause consternation, in the very distant future, in some alien sky.
Radio waves, like all forms of electromagnetic radiation, travel at the fastest speed known to man, the speed of light (300,000 km per second), and they are not significantly degraded as they pass through gas and dust. So for the past 40-odd years, radio astronomy has been used to investigate “nearby” stars, those within 75 light years (about 750 million million km) of Earth. Even at this speed, a message sent today from a civilisation near the closest star to Earth, Proxima Centaurus, would not arrive for more than four years, and any signals received today from the outer boundaries of the search would have been transmitted 75 years ago.
In 1960, Frank Drake used a radio telescope at the National Radio Astronomy Observatory in West Virginia, USA, to listen to two solar-like stars, Tau Ceti and Epsilon Eridani, though without success. He is noted for formulating the “Drake Equation”, which estimates the number of technologically advanced civilisations which might exist in our Milky Way galaxy. The number is based on the rate of solar-type star formation, the fraction of stars which have planets suitable for life, and factors related to the surmised development of intelligence and technology in extraterrestrial civilisations.
The rate of solar-type star formation is important, since stars more than one-and-a-half times larger than the Sun would have lifetimes shorter than the time taken to develop intelligent life on Earth. Stars less massive than the Sun have longer lives, but they are unsuited for the formation of life as we know it. That is why the Search for Extraterrestrial Intelligence (SETI) project confines itself to Sun-like stars.
When all the variables in the equation are considered (hotly and with considerable debate among the various schools of thought), it is estimated that, out of the hundreds of billions of stars in the Milky Way, there are only 10 technologically advanced civilisations from which we could receive messages. Some astronomers argue that there is only one advanced civilisation in the Milky Way — us.
If extraterrestrial civilisations are attempting to communicate with us by radio waves, it is important to consider what frequency, free from interference, they might use. SETI pioneer Bernard Oliver suggested that the noise-free frequencies emitted by hydrogen and the hydroxyl radical (OH), in a region of the microwave radio spectrum called the “water hole”, might be a possibility. If extraterrestrials are sending messages into space, it is considered likely that they would transmit on these frequencies.
NASA’s ambitious HRMS (High Resolution Microwave Survey) project scanned the entire sky searching for Morse Code-like pulses, or continuous waves like the carrier waves for TV and radio broadcasts. But, one year after it began operation in 1992, the US Congress made a political decision (to save tax dollars!) to discontinue all further support for searches for extraterrestrial intelligence.
This decision did not kill SETI, however. The programme is now independently funded by non-governmental organisations and private individuals. Among the current SETI projects is Project Phoenix, which returned to Arecibo, Puerto Rico, the site of the largest radio telescope in the world, in March 2002.
Project Phoenix is currently the world’s most accurate and comprehensive search programme for extraterrestrial intelligence, and examines nearby stars for signals in the frequency range 1200 to 3000 MHz. It scans 3.6 billion channels and can listen to 57 million channels simultaneously, every four-and-a-half minutes. With its tremendous sensitivity to faint signals, Phoenix can easily distinguish terrestrial signals from potential extraterrestrial signals. Duplicate determinations are made in the Jodrell Bank Observatory in England. These determinations by two widely separated telescopes allows precise analysis of stellar signals.
The University of California at Berkeley operates a SETI programme called SERENDIP IV (Search for Extraterrestrial Radio Emission from Nearby Developed Intelligent Populations). This is a “piggyback” on the 21cm receiver of the 350-metre dish at the Arecibo Observatory, installed in June 1997. It succeeded SERENDIP III, which began operations at Arecibo in 1992. This piggybacking approach allows full-time observation of 4.2 million channels every 1.7 seconds in a narrow 12 MHz band centred at 429 MHz. The improved SERENDIP IV examines 168 million channels every 1.7 seconds in a 100 MHz band centred at 1.42 GHz. The data then travels to Berkeley for assessment.
Some prominent astronomers, like retired professor Michael Hart, believe that extraterrestrial intelligence is simply wishful thinking. But SETI shows no signs of flagging. Since May 2001, half a million people have joined the SETI effort by downloading free SETI software that allows their PCs to sift through radio telescope data for alien signals.
So the search for extraterrestrial intelligence continues. Last March, I had the privilege of talking to Seth Shostack, an astronomer at the SETI Institute in Mountain View, California, and author of Sharing the Universe: Perspectives on Extraterrestrial Life. As the SETI Institute’s Public Programs Scientist, Seth gives more than 100 talks a year and has written hundreds of articles on science and technology. He confirmed that no signal which could be attributed to extraterrestrial intelligence has been received to date at any of the SETI research stations. But he was confident that this picture would change in the next 10-20 years with ever-increasing improvements in equipment and techniques.
What would happen if such a signal was received ? Apart from the worldwide excitement, Seth thought the first step would be the construction of a larger and better radio telescope — the Arecibo telescope might not be adequate for the further investigations required. We talked about the scientific, social, political and religious implications of such a discovery — I was pleased to find that such an eminent authority in such an exciting field of research could be so friendly and informative with an insignificant terrestrial like me.
The bottom line? All the various participants in SETI may be looking for an astronomical needle in an enormous haystack. But what if the needle is there? In fact, how can it not be there?
