WHAT happens after you die? I can name you 47 men who have tried to harness the rational horsepower of science to answer this most floaty question. Some were physicians, some physicists, some psychologists. Two were Nobel prizewinners. One is a sheep rancher. They have tackled it in labs, in hospital operating rooms, in barns behind their houses. Of them, only one, to date, has landed an irrefutable proof - not a suggestive nugget or an inexplicable anomaly, but the sort of answer you could plant your flag into and say, "Victory! Now I know for certain." The man's name was Thomas Lynn Bradford.

Though his background was in electrical engineering, Bradford's afterlife experiment involved gas, not electricity. On 6 February 1921, Bradford sealed the doors and windows of his rented room in Detroit, Michigan, blew out the pilot on his heater, and turned on the gas.

Finding out is easy. Reporting back is the challenge. For this Bradford needed an accomplice. Some weeks back, he had placed a newspaper advertisement seeking a fellow spiritualist to help him with his quest. One Ruth Doran responded. The two met and agreed, as The New York Times put it, "that there was but one way to solve the mystery - two minds properly attuned, one of which must shed its earthly mantle". The protocol was sloppy at best, for regardless of whether or not our mantle-shucking engineer came through on the telepathic wireless, Mrs Doran, for the sake of spiritualism or publicity, could simply have told the reporters that he did. But she did not lie. The Times ran a follow-up under the headline "Dead Spiritualist Silent".

A better-pedigreed variation of the Bradford experiment was undertaken by the physicist Oliver Lodge, once the principal of the University of Birmingham. Prior to his death in 1940, he devised the Oliver Lodge Posthumous Test. The goal, again, was to prove the existence of life after death. Lodge composed a secret message and sealed it in a packet (the Oliver Lodge Posthumous Packet) so that when, after his death, he told mediums (four of them, recruited by the Oliver Lodge Posthumous Test Committee) what the message was, their stories could be checked.

The packet itself was sealed inside seven envelopes, each envelope containing a clue the mediums could employ to jog the deceased physicist's memory should he forget his own secret. Instead, the clues merely irritated the mediums. The contents of Envelope 3, for example, read: "If I give a number of 5 digits it may be correct, but I may say something about 2 8 0 1, and that will mean I am on the scent. It is not the real number... but it has some connection with it. In fact it is a factor of it." Eventually the mediums walked off the set and the Posthumous Packet was torn open, leaving the committee with nothing for their efforts but a slip of paper bearing an obscure musical fragment and a gnawing suspicion that Sir Oliver had been a few envelopes short of a stationery set.

Of course, even had the mediums succeeded, one could never be certain whether they might simply have - via some discreet Oliver Lodge Posthumous Envelope Steaming - peeked. That is why, six years later, psychologist Robert Thouless turned to the science of encryption. Thouless, the president of the venerable, endearingly daffy Society for Psychical Research and an amateur encryption expert, encoded two phrases in what he was certain was unbreakable code. He announced the project and printed the snippets in the society's journal, inviting members and mediums to try and contact him after he died and obtain from his ethereal self the key to break the code, thereby proving that one's personality survives the change of scenery known as death. Thouless died in 1984, but the phrase remains a mystery, for although some one hundred people submitted what they believed to be the key, the results were invariably, to quote one Thouless Project report, "a meaningless jumble of letters". One party insisted that he had made contact with Thouless via no fewer than eight different mediums. Unfortunately, this man reported, Thouless had forgotten the key.

The dead-researcher approach is clearly not the way to go. A more promising tack might be to focus on those who have not quite died, but merely managed a sneak preview - in the form of a near-death experience. If someone could prove that the phenomenon is, in verifiable fact, a round-trip visit to some other dimension and not a mirage of the dying mind, that would surely be something to hang one's hopes on. But how does the person who claims to have glimpsed the beyond go about proving it? There seems to be no afterlife gift shop, no snow globes full of angel dandruff. Best to focus on one of those near-death trips that take the traveller only as far as the ceiling, enabling a reconnaissance-type view of one's corporeal hull down below. If one could at least prove that one had seen the details of the room from up there - and not remembered or hallucinated or some combination of the two - then that would at least establish the possibility of the seeming impossibility of a consciousness existing independent of its biological moorings.

And that is why there is, yes there is, a laptop computer duct-taped to the highest monitor in a cardiology operating room at the University of Virginia in Charlottesville. The computer has been programmed to show, during the duration of each operation, one of 12 images, chosen at random and unknown to anyone, including the researchers. The laptop is flat open with the screen facing the ceiling, such that the only way a surgery patient might view the image is as a disembodied consciousness. As patients come out of anaesthesia, psychologist Bruce Greyson interviews them about what they remember of their time in the operating room. So far there have been no surprises. Other, that is, than the surprising cooperation of a team of cardiac surgeons. Heart surgeons who believe that a consciousness can occasionally perceive things in an extrasensory manner, independent of a brain and eyeballs, are less rare than you might think.

But even then, how would we know that the near-death experience isn't a hallmark of dying, not death - a stopover, not a final destination? How do we know that several minutes later the bright light doesn't dim and the euphoria fade and you're just flat-out non-existent? "We don't know," concedes Greyson. "It's possible it's like going to the Paris airport and thinking you've seen France."

The other way to approach the afterlife proof is to consider not the destination but the vehicle: the soul (or consciousness, if you like). If the soul were something you could weigh, like a pancreas or a wart, then proving that it abandons the corpse at death would be a simple matter of placing a dying person on a scale and watching to see if the needle went down at the moment he died (while also accounting for the minute amount of weight lost via moisture in exhalations and sweat).

This is exactly what a Massachusetts physician named Duncan MacDougall did, beginning in 1901, using a tricked-out industrial silk scale. His post at a tuberculosis sanatorium provided MacDougall with a steady source of study subjects. He weighed six men as they died, and there was, he said in a series of articles in American Medicin e, always a down-tick of the needle. However, only one of his trials went off without a significant hitch. Twice the authorities barged into the room and tried to stop the proceedings. Oafish accomplices jostled the scale. Subjects died as the scale was being zeroed. And so MacDougall's claimed proof - that the soul exists, and that it weighs about 20 grams - is really no more than anecdote.

Ninety-some years later, a sheep rancher in Bend, Oregon, tried to replicate MacDougall's work. When a local hospital rebuffed his solicitation for terminal patients, Lewis Hollander Jr turned to his flock. Interestingly, he found that sheep momentarily gain a small amount of weight at the moment they die. Suggesting that the answer to the question "What happens when we die?" might in fact be: "Our souls go into sheep."

Of course, it's a stretch to think that the weight of a soul would register on a scale built for the likes of livestock or bolts of cloth. But what if you were to get your hands on a scale calibrated not in ounces or grams, but in picograms - trillionths of a gram? If you consider consciousness to be information energy, as some do, then it would have a (very, very, very, teeny tiny) mass. And if you were to build a closed system, such that no known sources of energy could leave or enter undetected, and you rigged it up to your picogram scale, and put a dying organism inside this system, then you could, in theory, do the MacDougall. In the course of researching a book about these various efforts to prove that there is (or isn't) an afterlife, I met a Duke University professor, Gerry Nahum, who would very much like to undertake a consciousness-weighing project of his own (offing not sheep nor men but leeches). Though he taught gynaecology and obstetrics, Nahum has a background in thermodynamics and information theory and has even worked out a 25-page proposal of exactly how to do it, if only someone will fund him the $100,000 he estimates it will cost.

If consciousness is energy, then I suppose you don't need proof that it survives death, because proof already exists: the First Law of Thermodynamics - energy is neither created or destroyed. Though it's hard to take much comfort from this. Who wants to spend eternity as a blip, a gnat's fart, of disordered energy, with no brain at your disposal to help you remember or imagine or solve the Sunday crossword? What would it be like? Would there even be a be ? Nahum uses the analogy of the computer: perhaps you'd be the operating system, stripped of its programs and interfaces. Heaven as the back of the closet where the broken-down Dells and Compaqs go.

If we are to eventually have our answer, our proof, it will no doubt come to us courtesy of quantum theory, or whatever takes its place. Few of us will understand it well enough to take much comfort, however, if indeed comfort is what it offers. I recommend that you enjoy life without worrying about the "after" bit, and keep in mind that one day altogether too soon, bad luck or genetics will hand you the answer. In the meantime, be nice to sheep.

- New Scientist, 18 Nov 2006 by Mary Roach. A writer based in San Francisco, Roach is the author of Spook: Science tackles the afterlife (W. W. Norton, 2006) and Stiff: The curious lives of human cadavers (Thorndike, 2004)