It is now half a century since the celebrated conference at Green Bank, West Virginia, on the subject of ‘Intelligent Extraterrestrial Life’, at which Frank Drake proposed what has become known as the Drake Equation for the number of civilisations that might exist in space. Unfortunately, there were no real values for most of the factors. Much has been discovered since, but, as Halley’s book makes clear, there are still too many unknowns to be able to give a real answer.
The book is based upon undergraduate seminars that the author has taught at the University of Minnesota, and accordingly includes the basic equations relating to many of the topics, although unfortunately many of these do not lead to conclusive results. For example, he gives the equations for determining the ‘black body temperature’ of a planet, a theoretical quantity which is supposed to approximate to the actual temperature, although for simplicity the calculation ignores both the albedo and the greenhouse effect. This, he observes, works fairly well for both the earth and Mars, but is completely wrong for Venus: its black body temperature is forty degrees hotter than the earth, whereas its real temperature is more than four hundred degrees hotter. Since we cannot know the albedo or the scale of the greenhouse effect for any extra-solar planet, we cannot be certain of its temperature. This may explain why the temperature of Gliese 581d (a planet of similar mass to the earth) has been variously reported to be forty degrees centigrade or minus fifty degrees centigrade.
Halley begins by looking at star formation. It is now known that no element heavier than iron can be formed in a star – everything higher up the periodic table is created only in supernovae, and this is important, because the latter include elements such as copper and iodine which, on earth, are essential for life. Though more than 500 extrasolar planets have been discovered in the past fifteen years, he admits that nothing is known of their chemical composition, so that it is impossible to say what fraction of these planets might be habitable.
A few years ago, the fashionable theory was that simple one-celled life could arise easily wherever the conditions were suitable, but that complex, multi-celled life, and hence intelligent life, would have so many problems to overcome that it might be very rare. Halley takes exactly the opposite view. Although the famous Miller-Urey experiment showed that amino acids, ‘the building blocks of life’, can readily be formed under certain conditions, for those amino acids to arrange themselves into functional proteins appears to be incredibly unlikely, even given billions of years for it to happen. This, he observes, if put into the Drake equation, would give a number of technological civilisations far less than 1, even for the entire universe. On the other hand, he argues that once life has formed it may evolve quickly. One example is the development of resistance to antibiotics in bacteria. “This evolution, which proceeds much as Darwin originally conceived the process, is very rapid and is a serious public health problem.” This only applies to single-celled creatures, but among the multi-celled the appearance of new species is on average one a century (though much of it “occurs in short temporal bursts”).
Another possibility is ‘directed panspermia’, that is, “that an advanced civilization facing death due to some event such as the expiration of its star as a supernova or red giant might choose to launch a directed, not randomly moving, probe loaded with the essential elements of its biochemistry toward a hospitable star.” This idea was, I believe, put forward in the 1930s in a science fiction novel by Olaf Stapledon, and then proposed seriously in the 1970s by Francis Crick of DNA fame. However, “This possibility is not of much help in resolving the question of the ultimate prebiotic origin of life, because it does not affect the 13.8 billion year time limit imposed by the age of the universe.”
The other approach to extraterrestrial intelligence is to look for direct evidence of it. Halley perfunctorily rejects UFO reports, besides which there is no evidence of ETs at all. Enrico Fermi famously demanded to know why, if there are aliens, they are not here? This implied that interstellar travel is easy. A mathematical model is given here for ‘diffusion’, that is, the rate at which the galaxy could be colonised, but it is entirely dependent upon how long it would take to travel from one star to another. So far, we cannot do that at all. Radio telescopes have been listening for intelligent signals from space for fifty years, without result.
The only other places in our own solar system for which there is now any hope of finding life are Mars, Europa and Titan. No life more complicated than single-celled is expected to be found on Mars, and so far landers have failed to locate even that. There is still some hope that Europa, a moon of Jupiter, and Titan, the largest moon of Saturn, might have subsurface oceans, but any life in them would have to be ‘extremophile’, and again single-celled. He does not, though, mention a hypothesis that was proposed a few years ago: methane, and one or two other chemicals that on earth are associated with life, have been found in the Venusian atmosphere. Now, whilst at the surface of Venus temperature and pressure are unbearably high, both decrease with altitude (as on earth), so that at a sufficient height living spores similar to terrestrial bacteria could live and flourish.
One point upon which I would disagree with Halley is the weight he gives to the failure of SETI, saying that this shows that there must be less than one hundred civilisations in the galaxy, probably fewer than ten. As Scientific American pointed out some years ago, for an omnidyne transmitter to be detectable one hundred light years away, it would require a power supply 25,000 times that of the whole of the United States. Extrapolating from this, if receivers of the sensitivity used by SETI were placed on Alpha Centauri, the nearest star to us, they would be unable to detect any radio signals from earth, perhaps leading someone to conclude that our planet is uninhabited. Of course, it would be possible, with moderate power consumption, to aim a unidyne beam at a particular star, indeed we have briefly done this ourselves a few times. But it is hard to see why aliens should direct messages specifically to us, unless they had a particular reason to think that someone would be listening.
At one point Halley remarks that neutrinos are ‘nearly massless’. Now, when neutrinos were first discovered, it was thought that they had no mass at all, and so would move at the speed of light. More recently, it was suggested that they might have a very small mass, so that their speed would be slightly less than that of light. As everyone will be aware, experiment now indicates that they move just slightly faster than light, confounding all accepted theories. This raises the possibility that there might be some way of sending information considerably faster than light. If so, then no civilisation that had discovered it would attempt to communicate across interstellar distances by the comparatively slow method of radio, so SETI would be sure to fail, even if the whole cosmos were crammed with life. -- Gareth J. Medway