Nowadays, the language and facts of the discipline are part of our common knowledge. If you commit a crime, your fingerprints could link you to the scene. If you shoot a gun, traces from the discharge will remain on your hands. If you handle explosives, residues will remain on your skin or surroundings. If your clothing touches somebody else's, minute fibres will be swapped: a shadow of you remains on them, and of them on you. If you leave a hair at the scene of your crime, it can be analysed using DNA testing which will identify you uniquely from millions of people.
Today, the knowledge of such details is taken virtually for granted by police, barristers, TV viewers and the readers of detective novels. Patricia Cornwell's character Kay Scarpetta is a forensic pathologist who is the key to the crime-solving. In the Seventies there was the American TV series Quincy MD, and, latterly in Britain, two TV series about forensic scientists: Dangerfield and Silent Witness, (the latter being a testament to the way that forensic science can, in effect, allow a victim to accuse their killer from beyond the grave).
Countless TV detectives rely on the accuracy of forensic evidence which is never questioned. Thousands more books, both fiction and non-fiction, detail the accumulation of scientific facts into an edifice of evidence that pins down the perpetrator of a crime. But the central feature in all these is the evidence's unchallenged status. It is science; and in the modern world, that means - by a syllogism we are only dimly aware of - that it must be right.
Ruth Rendell, author of the Inspector Wexford detective novels, says: "I don't use much forensic detail, because I'm not very interested in it myself," she says. "But yes, when I do, the assumption is that it would be right. Everybody, every reader and TV viewer, accepts that it would be right and ought to be right. It might make an interesting subject for a novel if it was wrong." One can hear the plot forming already: Wexford Led Astray, perhaps.
We accept the evidence easily because science has become knit so closely into our lives. Indeed, the processes used to carry out forensic tests are, technically speaking, routine. If they were groundbreaking science, they would be open to too much doubt to stand up in court. The centrifuge that was found to be contaminated is a standard piece of laboratory equipment. "People treat those things terribly," says a friend who works in a hospital laboratory. "You whack the samples in and spin 'em up and no one ever takes much care of them." Centrifuges are treated in laboratories like washing machines in homes with young children: long hours, little rest, even less respect. But they do the job. And the laboratories get their results, because the science is comparatively straightforward.
But does our culture's acceptance that so much detail can be accurately extracted from a few hairs and fibres mean that the modern jury enters the courtroom with an easy grasp of the technicalities? "Certainly not," says Anne Rafferty QC, chairman of the Criminal Bar Association. "All it might do is to prejudice them into thinking, 'Forensic evidence always solves it on the TV. When is it going to do that here?' It won't prepare them for close cross-examination by the defence of a pathologist about the alternative ways that an injury might have been caused."
She recalls a recent case that lasted five days, during which five different expert witnesses were called. The jury reached its verdict in two hours. "The professionals were left gasping: the jury simply couldn't have considered all the expert evidence in that time."
The problem is that nobody knows what weight juries do give forensic evidence. The Contempt of Court Act prohibits interviewing members of juries about they way they reached their decision. "We need to know, though," says Ms Rafferty. "We need a proper methodological study."
However, Neil O'May, a solicitor who is a partner at Bindmans, comments: "We know that juries really do listen to it. It has an air of impartiality and scientific method. The forensic evidence is a very, very important part of the case."
But the way that it is tested in court brings the worlds of law and science into a strange collision. Forensic science is a young discipline; the fact that fingerprints are unique was only pointed out in 1880. In contrast, our judicial system is centuries old, dating from a time when literacy was rare and people had to speak for themselves. Thus it still insists on the presentation of evidence in a courtroom by a person. The evidence cannot present itself; a witness must make it speak.
This can lead to the odd prospect of pathologists and scientists who are brilliant at their work, yet make prosecuting barristers groan inwardly because they cannot speak in the plain language that juries need. Some people think that this sort of confusion, caused by a surfeit of painstaking scientific detail from the prosecution, first inclined the jury in the OJ Simpson trial towards an acquittal. To some independent experts watching the trial on TV, the weight of forensic evidence seemed incontrovertible.
But perhaps the cross-examination of the DNA fingerprinting evidence - suggesting that a DNA match might not be as astronomically unlikely as the prosecution alleged - began to make the jury think that the test results were not as conclusive as they had expected. "It's a perfect example of how we don't know how much weight the jury gave to it," says Rafferty.
Yet even there, nobody was suggesting that the evidence was wrong per se; only that the prosecution was mistaken in its interpretation of what it meant. Yesterday's suggestion that 12 cases might have produced wrongful convictions hits very hard, because it undermines all our expectations about forensic testing. "This is the most prestigious laboratory in the country. It was unimpeachable," says Neil O'May. "Then we find from this report that, in effect, they went down to the Ministry of Defence car- boot sale six years ago [the contaminated centrifuge was bought in 1989, but was manufactured at least 10 years ago] and left it heavily contaminated, in their terms. Unless there's a really impressive investigation into the protocols, then there will be suspicion that this isn't the only lab with this problem."
There will be another effect. "Every forensic scientist who comes to court will be looked at in a different light after yesterday," O'May adds. "It has blown up the boffin."
The damage might be limited, but if this problem turns out to be widespread, the effect on forensic evidence's place in the law could be as drastic as when it was discovered that suspects' confessions were sometimes extracted with the toe of a police boot.
These days, we don't entirely believe the police, and now we don't entirely believe the scientific evidence. What is there left? "It's not as drastic as that," says O'May. "It just needs to be done right." But even so, there is the worrying feeling that a little more certainty has been chipped away from the world. We can only wait and see how big a gap will be left.