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Attempts to provide a scientific foundation for homeopathy

Homeopathy was developed at a time when many important concepts of modern chemistry and biology, such as molecules and germs, were understood poorly if at all. In Hahnemann's day, many chemists believed that matter was infinitely divisible, so that it was meaningful to talk about dilution to any degree. Although the hypothesis of atoms can be traced back to the ancient Greeks, their size was not calculated until 1865 (by Josef Loschmidt).

We now know that, for example, a teaspoon of seawater (roughly 5 ml) contains about 160 mg of NaCl. The molecular weight of NaCl is 58.4, so by Avogadro's number, (or, in German-speaking countries, Loschmidt's number), 58.4 g of NaCl (one mole) contains 6.02×10²³ molecules. We can thus calculate that our teaspoon contains about 2×10²¹ molecules of NaCl. A 12C dilution of seawater will have about one molecule of NaCl per litre.

Thus homeopathic remedies diluted to more than about 12C are virtually certain to contain not even a single molecule of the initial substance. This is recognized by advocates of homeopathy, who assert that the essential healing power is not to be found in the chemical action of molecules, but perhaps in the arrangement of the water molecules, giving rise to the expression "the memory of water". In the homeopathic literature widely differing explanations are proposed for the alleged existence of this water memory. It appears that the community of homeopaths has not yet converged on a single explanatory paradigm.

For instance, homeopaths like to point out that water is not simply a collection of molecules of H₂O, but contain isotopologues, molecules with different isotopic compositions such as HDO, D₂O and H₂¹⁸O. Mass spectroscopy can indeed detect different isotopologues, but their relative concentration is the same before and after homeopathic treatment—the concentration ratios can only be changed by nuclear reactions—so that the presence of isotopologues in water cannot explain its memory.

Also the fact that the molecules H₂O appear in two proton-spin forms (ortho and para) has been suggested as a source of memory of water. These two spin forms, which appear in a ratio 3:1, are chemically non-distinguishable and are very difficult to separate or to convert into each other. It is therefore highly unlikely that a homeopathic treatment could change this ratio. Moreover, even if it is assumed that homeopathic tinctures would give somehow rise to ortho:para ratios other than 3:1, it requires much further explanation that these (chemically undetectable) ratios have different healing qualities.

Another suggestion is that double-distilled and deionized water contains trace amounts of contaminating ions. In particular, water, as a result of repeated vigorous shaking, might include dissolved atmospheric gases in the form of nanobubbles, molecular ions produced from water reacting with airborne contaminants, and silicates—tiny glass "chips". These possibilities are discussed in Ref. and Ref. But see for further discussion Ref. in which it is concluded that: "it [observed variation in NMR relaxation rates upon ultra dilution] might merely reflect a trivial air-dependent phenomenon, or an unsuspected bias, and should not be extrapolated to the so-called memory of water, often alleged to explain the effectiveness of homeopathy."

Some homeopaths believe that there might be an effect of successive shaking on water structure leading to "clustering" of water molecules. This contradicts the dominant scientific view that motions in liquid water are on the picosecond (10⁻¹² second) time scale and that such clusters could not live longer than a few picoseconds.

People sometimes wonder if the water used to make homeopathic medicines already has other memory imprints from its history prior to use in medicine. However, the water used by homeopathic manufacturers undergoes double-distillation, a process that homeopaths contend eliminates or substantially reduces previous memory. This raises the question: how do homeopaths know that this reduction is sufficient? If the presence of homeopathic qualities is below detection threshold, then surely the absence of such qualities is also undetectable.

In brief, for homeopathy to receive serious scientific consideration, there needs to be plausible explanations for the following:

• how the process of manufacturing a homeopathic remedy could yield a biologically active substance or solution
• why the principle of similars might apply in the case of homeopathic remedies
• how a biological mechanism could have evolved to recognize the specific nature of homeopathic remedies

There also needs to be

• clear and irrefutable evidence for the efficacy of homeopathic remedies, evidence that cannot be explained by placebo effects

These stringent demands are often summarised by the maxim "Extraordinary claims require extraordinary proof".

Meanwhile, homeopaths, working towards an elucidation of the effectiveness of their medicines, complain that they are up against a double standard in medicine and science. They point out that there is a long history of conventional medical treatments that are applied with positive effects, although their mechanism of action is not known. Only relatively recently, for instance, has it been understood how aspirin works, but before that doctors used it regularly despite an inadequate understanding of its actual mechanism. In due time, the homeopathic community asserts, the scientific basis of their methods will be unveiled.