Paul Davies is dry, calm and rational. But talking to him about his new book, The Eerie Silence: Are We Alone in the Universe?, soon leads into some pretty surreal territory.

When it comes to searching for evidence of life in outer space, he explains at one point, "I think we should try as many scenarios as we can." But, he adds reassuringly, this doesn't mean that "there is a good chance that we'll find a nuclear waste dump of an alien expedition or data loaded into our genomes by an interstellar virus".

Or take these startling speculations on alien psychology: "There is no reason why aliens would want to harm us unless they regarded us as a threat. They might invade to get hold of the resources on our planet, but it's been here for 4.5 billion years and they are leaving it a bit late to come and get them!"

So how did a well-known scientific populariser get to this point? A theoretical physicist working on dark energy, cosmology and astrobiology, Davies is a professor at Arizona State University. He recently set up a small cancer research project, where he hopes "to bring insights from physics and mathematics to bear on cancer, trying to break totally new conceptual ground, not just using gadgets to zap the disease".

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On this theme, Davies takes the trenchant view that "cancer research, with a few notable exceptions, has got nowhere in 40 years - because too much money has been spent on it. I increasingly feel, after a lifetime in science, that you can spend too much money on things. Doing things on a shoestring sometimes forces you to be more creative."

The problem, he suggests, is that laboratories with billions of dollars' worth of equipment, dozens of technicians and graduate students, papers to produce, grants to obtain and reputations to maintain often become focused on acquiring more and more information about one particular detail, such as a gene or set of genes. They are far less successful at changing tack to pursue fresh approaches or seeing the wood for the trees. So there may still be a place for shoestring science.

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For the wider public, however, Davies is most celebrated for books such as The Mind of God: The Scientific Basis for a Rational World (1992), The Last Three Minutes: Conjectures about the Ultimate Fate of the Universe (1994), How to Build a Time Machine (2001) and The Goldilocks Enigma: Why Is the Universe Just Right for Life? (2006), which reveal a rare talent for making physics mind-bogglingly vivid and exciting.

The last of these books surveys the major theories that have been advanced to explain the emergence of conscious and intelligent life: "a unique universe which just happens to permit life by a fluke; a stupendous number of parallel universes that exist for no reason; a pre-existing God who is somehow self-explanatory; or a self-creating, self-explaining, self-understanding universe-with-observers, entailing backward causation and teleology". Yet all of these, Davies concludes disarmingly, are "either ridiculous or hopelessly inadequate".

Such big issues have long preoccupied him. The Mind of God finishes with the sentence: "We are truly meant to be here." Asked to unpack this, he explains that he believes "the emergence of life and mind, and the ability of beings like ourselves to understand the Universe, is not a trivial and accidental feature that happens to have cropped up in one little corner of the Universe. I think it's fundamental to the workings of nature."

In an address he gave in 1995 after winning the Templeton Prize, awarded to a person "who has made an exceptional contribution to affirming life's spiritual dimension", Davies said that he viewed the Universe as "a coherent, elegant and harmonious expression of a deep and purposeful meaning. I believe the time has now come for those theologians who share this vision to join me and my scientific colleagues to take this message to the people!"

Such an underlying "meaning", however, is not much like the God of Scripture - or a God that many people would want to worship. Does he see it as offering emotional and spiritual as well as intellectual satisfaction?

"To a limited extent," Davies replies cautiously, "although it won't provide much comfort to the dying or bereaved, and won't help you solve deep ethical problems."

Nonetheless, he remains clear that he would "feel much happier about a Universe in which we are at home. It seems to give my own and humanity's existence a greater significance, a cosmic dimension. I don't think we are a monumental irrelevance."

The key question can be posed pretty simply: is life a freak accident or some sort of "cosmic imperative"? Davies' previous books have investigated this at the level of cutting-edge physics and high philosophical speculation. But we can also try to carry out a research programme.

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If human beings are alone in the Universe, it would show that life is just an amazingly improbable accident. Evidence of intelligent life elsewhere would suggest the opposite.

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Such a programme already exists and is celebrating its 50th birthday in 2010: Seti, or the Search for Extra-Terrestrial Intelligence. Davies is currently chair of the Post-Detection Task Group, the man who will have to start the ice-breaking conversation with any aliens that turn up. The Eerie Silence offers a report on the search so far and suggestions on how it needs to be rethought.

Seti is a classic example of shoestring science. Funded with comparatively small amounts of private money, it draws on the efforts of amateurs all over the world scanning the heavens for signs of radio activity. Yet Davies argues that it is based on a single, distinctly implausible assumption - "that ET is obligingly beaming messages to us".

This means that Seti will remain "a hard sell", he writes, as long as it amounts to "eavesdropping on an extraterrestrial civilisation on the premise that the aliens will still be using 1980s technology". If we hope to find life out there, surely there must be more promising alternatives?

Some of the book's initial suggestions sound concrete and practical. One of Davies' research areas, astrobiology, looks at whether all kinds of life on Earth form branches of a single tree. So far the answer is yes, but could there be examples of "weird life", perhaps bacteria living in the deep oceans or in volcanoes, where we have been unable or have not thought to look?

Once life exists, how likely is intelligent life? Peacocks' feathers and elephants' trunks may be one-off oddities, but eyes and wings, for example, seem to have evolved on several separate occasions, as if there were a niche waiting for them. Davies attempts to determine in which of these two categories intelligence may lie.

Yet as the book develops and he calls into question all our anthropic assumptions, the possible forms of intelligent life he postulates get stranger and stranger: time tourists; comet-stealers; self-reproducing machines; and even quantum computers located in "the coldest possible environment" that don't have "much interest in the physical Universe". If such entities had no desire to make contact with us, how could we deduce that they had passed our way?

Now it is only fair to point out that, although Davies remains "open to new evidence", his provisional judgement is that "we are probably the only intelligent beings in the Universe, and I would not be very surprised if the solar system contains the only life in the Universe".

Yet alongside such scientific caution, he frankly admits to a "wide-eyed schoolboy fascination" with the possibility of aliens. "Whether it is godlike quantum minds floating in the black emptiness of intergalactic space," he writes, "super-cyborgs riding commandeered comets, Matrioshka (Russian doll) brains hugging spinning black holes or humble planet-dwelling biological organisms with big brains and fancy technology, I'd like to hear from them."

Although all these are possible, in Davies' view, according to our current understanding of physics, they are nonetheless wildly improbable. So why should science devote time and money to pursuing million-to-one shots?

"It depends on the costs and the pay-off," he responds. "If it's not costing much and the pay-off is enormous, you may as well do it!

"People are sequencing genomes anyway, but there's no systematic search for possible messages in them. It sounds ridiculous - an alien message in the genomes! - but it's no more ridiculous than an alien message in radio waves. It costs nothing to search. The same applies to looking for anomalies in the geological record.

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"At the speculative frontiers of science, you do what you can do, particularly if you can do it for no money, and not what you expect to succeed. I would be truly staggered if there were an intelligent message in the genomes. It's such a crackpot idea that it has to be approached in a spirit of fun."