Steve Donnelly, professor of experimental physics at Salford University, is about to read my mind. “Think of a two-digit number between 1 and 50, ” he says, “with both digits odd.” I think of 33. “But the digits must be different, so 33, for example, wouldn’t be allowed.” I think of 37. “Don’t choose it yet, let me concentrate. Now think of it.” I think of 39. “For a moment I had an impression of 37, but now I’m sure it’s 39.”

It’s a simple trick, exploiting the woeful predictability of the human mind, but nonetheless impressive to the victim. For mind-reading is just one skill Donnelly has picked up in a parallel career of scrutinising astrology, UFOs, dowsing, psychic powers, ghosts and other weird ideas which are not normally the province of respectable physicists. Donnelley’s curiosity about so-called “paranormal” phenomena was sparked in 1973 when, as an MSc student at Sussex University, he saw a TV programme in which psychic Uri Geller bent cutlery to the pop-eyed amazement of mathematical physicist John Taylor. Donnelly’s attempts to investigate Geller-type phenomena in his laboratory came to nothing – “I think it was probably more hopeful expectation than good sense” – and he became increasingly sceptical of psychic claims of all kinds. “The turning point was when I realised that many conjurers could replicate feats performed by psychics, ” he recalls. “Why should psychic powers manifest themselves as conjuring tricks? Surely there are ways in which one could demonstrate psychic powers that are not replicable by a conjurer.”

After gaining a PhD in ion beam interactions with solids at Salford University, Donnelly worked in Belgium, the US and Australia, before returning to Salford in 1986. Alarmed to find that public fascination with the paranormal had ballooned while he was away, he joined UK Skeptics, a loose association devoted to the scrutiny of paranormal claims. Since 1989 he has been co-editor of its journal, The Skeptic (the US spelling seems obligatory in this field) and is now one of the few British scientists ready to appear on TV and radio to talk about such things. As he says, “Someone needs to be available to put the rationalist point of view.” For example, a TV company recently asked him to examine a home-video sequence that appeared to show balls of light making crop circles in a corn field. Hollywood special effects experts had declared that it could not have been easily faked. “They said it couldn’t be done without a million dollars of resources, ” Donnelly says, “but I showed it could have been done with a sheet of glass and a small computer.” Yet most of the bizarre claims increasingly featured in popular TV programmes go uncontested, to the point where simple party tricks are held up as challenges to science. To those who regard such stuff as harmless nonsense, Donnelly has a chilling rejoinder. Just before Easter he was asked by a TV producer to comment on an apparently whimsical story about an alien spaceship hiding behind Comet Hale-Bopp. “It was based on a photograph by an amateur astronomer showing a star near the head of the comet. Lens aberrations had distorted the image into a classic UFO shape.” A few days later 39 members of a US cult killed themselves in the belief that the “spaceship” was coming to take them away from their material bodies. Against that appalling perspective, Donnelly’s fears for rationality do not seem excessive.

Yet he is concerned not to be seen merely as a debunker. “I am very much an experimental scientist, ” he explains. “For me it is the quality of the experimental evidence that counts. If there was significant evidence from well-controlled experiments that demonstrated, for instance, that the position of the planets could influence human personality, I would be persuaded by the evidence that there was something worth investigating even if there was no known theoretical means whereby that could happen. I know of no area of purportedly paranormal phenomena where that is the case.” Are some scientists hostile to paranormal claims because, paradoxically, they fear that among the dross there might be a germ of truth? After all, if any one of these claims were verified, it would have shattering implications for physics. “To be on the cusp of a paradigm shift is a way of getting your graffiti on to the walls of the universe, ” Donnelly counters sagely. “That is the place to be for a physicist. A lot of highly respectable scientists jumped into cold fusion even though that would have involved significant rewriting of what we know about nuclear physics. They carried out experiments, looked at the evidence and moved back out again. If there was a reasonable expectation that there was a new force or field giving rise to telepathy, for instance, physicists would want to get involved in it. But people who have looked at it seriously realise that the evidence is not there.”

Donnelly’s public skirmishes on the wild side of science seem to have done no harm to his real job – he gained a university chair last year – and by day he continues his research on ion beams and thin films, including the study of 10nm craters left by single ion impacts. His latest project is a virtual reality interface for a scanning probe microscope which will let the user fly over the bumpy surface of a specimen and, some day, pick up and stack atoms like grapefruit on a market stall.

After a quarter century of observing the paranormal scene, Donnelly shows no signs of flagging. He even toys with the idea of a new undergraduate course – Physics with Paranormal Studies, perhaps. So is he still receptive to the possibility of something startling? “Inevitably if you look at something for 25 years and everything points in a particular direction, you are not as open-minded as you were, ” he concedes. “But I like to think that the day the evidence is put before me I would treat it as respectfully as I would have done 25 years ago.”