There are two predominant theories for how humans' sense of smell works. One, the more widely accepted theory, states that smell is dependent on the shape of molecules giving off an odor. However, a newer theory - called "quantum smell" and based on physics - postulates that smell is dependent on vibrations, stating that the molecules give off various vibrations to which the nose's receptors respond. That idea has recently gotten a boost by a new study recently published in the journal PLoS One.
The theory was first put forth by Luca Turin, who is also one of the authors of this recent study. As he explained to BBC, one of the lynchpins of his theory was the bonding between sulfur and hydrogen. While the bonds take several shapes, all of them smell like rotting eggs - important since, as Scientific American notes, the purpose of the smell is to alert humans of inedible decaying organic material. However, since the bonds are all different shapes, Turin says that pokes a hole in the theory.
One way to test the theory was to use two molecules of the same shape but with different vibrations. One study published in the journal Nature found that humans could not sniff out the distinction; on the other hand, a 2011 study published in the journal Proceedings of the National Academy of Science found that fruit flies could. In addition, the PLoS One paper recreates the one conducted in Nature. When the subsequent study recreated the Nature study, they found that humans indeed could not sniff the difference between the two molecules. However, when the experiment was retooled with larger molecules, humans could.
However, though scientists are taking the debate more seriously than they did in 1996, many are resistant toward joining Turin's team, equating it to "scientific suicide".