In 1989, a group of leading astronomical associations and academies issued a "Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence". The nine principles are a po-faced set of statements of the bleedin' obvious such as "the discovery should be properly recorded and monitored" and, if it appears to be credible, "national authorities and the rest of the scientific communities should be informed". The fact that the principles were drafted, debated and ratified with due solemnity demonstrates the power of the idea of alien life to evoke an amusing variety of harmless silliness.
Polls show that about half the public believe we have already been visited by extraterrestrials. Millions of people, mostly in the US, claim to have been abducted by them - this despite the US Government apparently making it illegal to have contact with off-worlders or their vehicles. Hundreds of scientists have made confident estimates of the probability they exist, listened for their signals, even planned their replies. And storytellers down the ages have beguiled us with tales of alien beings, almost all of whom bear startling resemblance to one or other earthly species.
This should not obscure the fact that none of the key questions that fascinates us about life elsewhere in the universe is currently answerable. We know that, by some definitions, there is intelligent life on Earth, though we don't know how long it may last. But ask: Is there life anywhere else in the universe? Is there intelligence? Will we ever meet? What would happen if we did? There is little to go on, save speculation.
Surendra Verma's book rounds up most of the speculation, past and present. His interest is in more or less scientific approaches to the issue, as opposed to the cultural fixation with aliens - there are no analyses of Men in Black , Close Encounters or Independence Day here. Verma reviews attempts at reasonable argument about life on other worlds, from the Ancient Greeks to the present day.
Are there reasonable arguments? Up to a point. Some fit the description only in the sense that it was once reasonable to argue about how many angels could dance on the head of a pin. Most seem motivated by wishful thinking. As Bernard de Fontanelle had his Countess remark in Conversations on the Plurality of Worlds in 1686: "You have made the universe so large that I know not where I am, or what will become of me." Nowadays, the universe is incomparably older and larger, and an even more daunting place to be alone in. And the notion that the Earth might be the sole location for intelligence can seem pretty depressing when you inspect the results of applying that intelligence so far.
So most contributors to the debate about extraterrestrial evolution want to believe that there is some other kind of life, somewhere. The wishful thinking shows in the ambivalent attitude to the sheer scale of things out there.
Most arguments that life on other worlds must exist boil down to variations on the idea that in a universe so old and so large even really unlikely things can happen lots of times. But the time and distance scales then tend to get elbowed aside when it comes to speculations about the possibility of actual contact.
But no one has a good answer to the question posed by the great physicist Enrico Fermi, from which Verma takes his title. If there are aliens in our cosmic neighbourhood, why aren't they here yet? Sticking to the evidence suggests that a kind of agnosticism is the most defensible position. It aligns with the view that while there may be a God, there is absolutely no reason to credit him with any special interest in humanity. Likewise, there might be life out there, but there is no reason to think it is either interested in us or capable of communicating with us.
Verma is a breezy guide to the prospects for encountering ET, an accurate summariser of others' views, with a little scepticism added when necessary. His book has no grand scheme, reading more like a series of feature articles, and occasionally strays into irrelevance. The downgrading of Pluto from its status as a planet is mildly interesting but off topic.
This volume is an amiable ramble with no particular destination. And that is in keeping with the uncertainty that still surrounds his subject.
Jon Turney is course leader for the MSc in creative nonfiction at Imperial College London.
Why Aren't They Here?: The Question of Life on Other Worlds
Author - Surendra Verma
Publisher - Icon Books
Pages - 240
Price - £12.99
ISBN - 9781840468069