Anybody out there?

Whether mankind is alone as an intelligent life form in the Universe is a question that has intrigued man through the ages. It is only since the last 50 years that there has been a scientific and systematic search for evidence.

The Search for Extra-Terrestrial Intelligence (SETI) is the collective name for the many programmes, institutions, groups, individual amateurs, all watching the Universe for evidence, through electromagnetic radiation, of inhabitants of other worlds. Noted British physicist Paul Davies, , in his recent book, ‘The eerie silence, are we alone in the Universe?’ talks about the history, the different approaches, the issues, and every other aspect of this quest.   

In 1959, Giusippe Cocconi and Philip Morrison published a landmark paper (in Nature) in which they analysed the kind of radio signals that we need to look for.  The authors reasoned that assuming that there was an intelligent community in the Universe interested in communicating with a similar, technologically advanced life form, the radio signals they would use must be such that they could reach far off worlds and were likely to be detected. Very low frequencies, like 1 megacycle/sec, as well as frequencies higher than those corresponding to molecular absorption, about 30,000 megacycles/sec, would get absorbed in atmospheres and would not be suitable.

Even so, searching for a weak signal of unknown frequency can be challenging. The 1959 authors then reasoned that the intelligent extraterrestrial source would choose a convenient standard frequency, and one that is well known to intelligent listeners in the Universe, within this permitted range. A candidate is the outstanding radio frequency emission line of neutral hydrogen, at 1,420 megacycles/second or a wavelength of 21 cms.

In the matter of the power that the source may have, the authors noted that in the ‘galactic plane’ (of the Milky Way), there was 40 times the background noise, in the range of 21 cms wavelength, than off the galactic plane. Hence, a weak signal would be difficult to identify in the galactic plane and the right place to look would be towards the nearest stars off the galactic plane.  

What kind of signals?

In this matter, the recommendations were purely speculative. The signals, it was argued, would be in pulses not far off from once a second, to suit bandwidth and rotating devices. As the distances were in tens of light years, for the signal as well as the response, each signal would perhaps last a few years, and then to repeat. The content may be a series of pulses numbering prime numbers or simple arithmetic sums (or differences). The nearest stars, and of this kind, there were a few hundreds within say 50 light years, were the first towards which attention should be devoted.

In April 1960, American astronomer Frank Drake started systematic search for extraterrestrial radio signals with a 26-meter dish at Green Bank in West Virginia. Many others joined in and Drake later set up the radio telescope at Arecibo in Peurto Rico.

This was the start of SETI, now grown to an international effort, mostly privately funded, of scientific institutions and even individuals in several countries. While amateurs often work with just telescopes, to look for visible signs, the main apparatus for SETI is the radio telescope, an array of dish antennas spread out over several kilometers, with the signals received being brought together and processed by computers, to mimic a receiving mirror so many kilometers in diameter.  The important centre of activity is now the Allen Telescope Array a system of 350 networked dishes (42 are operational so far), being set up in Northern California.

Eerie silence

The trouble is that through all these 50 years, no acceptable ET radio activity has been detected. This is not by itself surprising as the best supporters of SETI do not hold out great hope of detecting signals in real time. Noted scientist and science fiction author Carl Sagan suggested that there may be an altruistic civilisation trying to make contact with the Earth.

Even if there were one such, 500 light years away, for instance, says Davies in his book, the civilisation, if it were to beam a signal towards the earth, would be addressing an earth in the 16th century. This is before the industrial revolution and all the alien civilisation would see on the earth is agriculture and maybe the Great Wall of China. If they were to wait till we also learnt about radio signals, this would be 400 years later, and then, the signals would take 500 years in transit! Sagan’s dream may not be for a millennium.

When Frank Drake started out in 1960, he devised a formula, which, even if it did not calculate anything, did show how far we are from understanding what we are about. The formula is dominated by the probability of life emerging and then of intelligent life, which are completely unknown. The probability of life emerging has long been considered to be exceedingly low, what has happened on the earth being regarded as astronomical fluke. And yet, now there is a counter view that emergence of life is fairly likely to happen if the conditions are ‘earth-like’. 

For this hypothesis, while we have no evidence in other worlds, it may make sense to look for repeated instances of life arising on our earth itself. This is the search, as part of SETI, that has now caught on, not life as we know it but at the microbial level, a population of organisms of a radically different biochemistry. If we should find anything like it on the earth, it would be a powerful reason to expect the same in other worlds too.

Technology footprint

Davies suggests that we look for indirect evidence of intelligent life. That is to say, not for signals beamed at us by the alien race, but other signs that they are there. For instance, humans have significantly modified the earth over the last so many centuries. Is it not conceivable that another civilisation may have done the same thing on their world? In 1950, physicist Enrico Fermi had suggested that an intelligent alien race would be able to spread across the galaxy in far less time than the age of the galaxy. Hence if there were any such in the Milky Way, they would have reached the Earth by now! This is called the Fermi Paradox and there are many resolutions to say why this has not happened!