Sceptic James Randi is so convinced that homeopathy will not work, that he has offered $1m to anyone who can provide convincing evidence of its effects. For the first time in the programme's history, Horizon conducts its own scientific experiment, to try and win his money. If they succeed, they will not only be $1m richer - they will also force scientists to rethink some of their fundamental beliefs.
Homeopathy and conventional science
The basic principle of homeopathy is that like cures like: that an ailment can be cured by small quantities of substances which produce the same symptoms. For example, it is believed that onions, which produce streaming, itchy eyes, can be used to relieve the symptoms of hay fever.
However, many of the ingredients of homeopathic cures are poisonous if taken in large enough quantities. So homeopaths dilute the substances they are using in water or alcohol. This is where scientists become sceptical - because homeopathic solutions are diluted so many times they are unlikely to contain any of the original ingredients at all.
Yet many of the people who take homeopathic medicines are convinced that they work. Has science missed something, or could there be a more conventional explanation?
The Placebo Effect
The placebo effect is a well-documented medical phenomenon. Often, a patient taking pills will feel better, regardless of what the pills contain, simply because they believe the pills will work. Doctors studying the placebo effect have noticed that large pills work better than small pills, and that coloured pills work better than white ones.
Could the beneficial effects of homeopathy be entirely due to the placebo effect? If so, then homeopathy ought not to work on babies or animals, who have no knowledge that they are taking a medicine. Yet many people are convinced that it does.
Can science prove that homeopathy works?
In 1988, Jacques Benveniste was studying how allergies affected the body. He focussed on a type of blood cell known as a basophil, which activates when it comes into contact with a substance you're allergic to.
As part of his research, Benveniste experimented with very dilute solutions. To his surprise, his research showed that even when the allergic substance was diluted down to homeopathic quantities, it could still trigger a reaction in the basophils. Was this the scientific proof that homeopathic medicines could have a measurable effect on the body?
The memory of water
In an attempt to explain his results, Benveniste suggested a startling new theory. He proposed that water had the power to 'remember' substances that had been dissolved in it. This startling new idea would force scientists to rethink many fundamental ideas about how liquids behave.
Unsurprisingly, the scientific community greeted this idea with scepticism. The then editor of Nature, Sir John Maddox, agreed to publish Benveniste's paper - but on one condition. Benveniste must open his laboratory to a team of independent referees, who would evaluate his techniques.