We are sitting here, a small group of us, in a tiny flat on Edinburgh's Royal Mile, staring fixedly at three blue cups balanced on top of a television. There is Ewan Irvine, a medium, Paul Rowland, a psychic investigator, and Ciaran O'Keeffe, whom insomniac couch potatoes will know as the sceptical parapsychologist in the popular TV series Most Haunted. The clock has just struck midnight and we are waiting for a poltergeist to reveal itself.
Apparently, the mischievous spirit has been moving things around when the (mostly American) tourists who stay here are out buying shortbread. So freaked out were some of the guests that they checked out early and caught the first plane back to the US. The flat, at the top of an old stone tenement and reached by a winding, coronary-inducing staircase, is said to have been the home of the 18th-century philosopher David Hume, who, it will be remembered, harboured the odd doubt about the existence of God and the afterlife. An irony, then, if the cup-throwing ghost turns out to be Hume's – a bit like that Rowan Atkinson sketch in which the Devil welcomes new arrivals to hell: "Atheists, over here, please – you must be feeling like a right bunch of nitwits."
Yvette can scream for Britain, the most innocuous sound being enough to set her off. You can find her most nights, staring eyes illuminated by the spectral light of a night-vision camera as she stumbles through the blackness of an allegedly haunted house.
"Oh ----," says Yvette in a most un-Blue Peter-like way, "what's that?"
"That" is a dripping pipe, a piece of falling plaster or the cameraman tripping over a wire, but to Yvette it's the cue to let rip like a banshee. Originally, Yvette was accompanied on the show by the self-proclaimed psychic Derek Acorah, who achieved the almost impossible in managing to be more irritating than her.
"My name is Jack!" Derek would shout suddenly in a Scouse accent, limbs stiffening as a passing spirit took possession of him. Yvette would ask Jack about his past life, and the spirit would come up with a story that had precisely nothing to do with the history of the haunted property being investigated.
"The things is, Yvette is genuinely scared," says Dr O'Keeffe, who spends his time away from TV studying the paranormal in a laboratory at the University of Toulouse. "I know because I took her to an allegedly haunted house when she was not being filmed and she jumped out of her skin even then."
"There were three theories about Derek: he really was possessed, he was faking it, or he was a bit…" Dr O'Keeffe taps the side of his head.
His opinion? "I'd rather not get into that."
The conversation helps pass the time. Ghost hunting, as newcomers soon realise, is basically a waiting game. Hoping to film the poltergeist is Paul Rowland, a paranormal investigator who believes that technology can unlock the door to another world. In addition to video and stills cameras, he uses an electromagnetic field detector, an ultraviolet light detector and sensitive microphones to pick up other-worldly phenomena.
"We are talking about something real, something that can be detected in the way that other forms of energy can be detected," he says.
Dr O'Keeffe dismisses Mr Rowland's equipment as pseudo science. "I have been actively involved in the field for the last 20 years and I have never encountered anything that leads me to believe that we are dealing with another world outside of this one," he says.
"Paul is a modern-day wizard who uses lights, beeps and technology. His motivation is genuine, but how do you make a detector when you don't know what it is you are supposed to be detecting?"
Earlier in the evening, Dr O'Keeffe acted as a guide in Mary King's Close, a warren of streets buried beneath the Royal Mile. It is a strange, unnerving place. The houses in the close were used as the foundations for buildings above, and so remain intact.
He draws his audience in with talk of ghosts, and then throws a marble into a corner where a ghostly child is said to play. It bounces back. It is, of course, a trick. Dr O'Keeffe explains how sleight of hand and suggestion can be used to manipulate the unwary. "Most people having experiences are genuine, but I believe they are the result of psychological and environmental factors."
Isn't it a bit, well, dispiriting to study something you believe to be the product of delusion or illusion? "Not at all. I am fascinated by the scientific answers to this phenomenon – why people believe, why they see what they see."
The night is wearing on and the cups on top of the television steadfastly refuse to move; David Hume or whoever is "out there" having none of it. Then, the microphone in the bedroom screeches into life, emitting feedback. Mr Rowland moves towards the microphone and, just as he is about to pick it up, the sound cuts out.
Nearing one in the morning, it is time to call it a night. As the ghost-hunters are beginning to yawn, suddenly off goes the microphone again. There is an Yvette Fielding-like screech. It's hardly conclusive proof of the supernatural but it's something to chew on.
Dr O'Keeffe is unmoved, and Ewan the medium – who when not communing with the Other Side works for a Lib Dem MP – isn't moved to come down on either side. But Mr Rowland remains optimistic that one day his box of tricks will find evidence proving the existence of ghosts. Until then, we must rely on Derek and Yvette and that God-awful scream. If anything can wake the dead, that can.