The nose knows, but how exactly is a matter of scientific debate. The conventional theory on olfaction is that olfactory receptors bind to odor molecules and that binding sends a signal to the brain about a particular smell.
But there's a different and competing theory in the wings: rather than binding, olfaction works through quantum vibrations. In a sense, fancy and exotic physics occur with every odor that you smell:
Yet here's a twist: odorant molecules typically contain many hydrogen atoms. And hydrogen comes in multiple forms, each very chemically similar to the others. But those different isotopes of hydrogen do strongly affect how a molecule vibrates. So deuterium, containing a hydrogen nucleus that has both a proton and a neutron (as opposed to plain-old-hydrogen that has just a proton), might help scientists discriminate between the proposed vibration and standard chemical binding theories of olfaction.
According to new research published today in PLoS ONE, human noses can sniff out the presence of at least some kinds of deuterium. Specifically, experimenters found regular musk molecules smelled different from ones that contain deuterium. "Deuterated" musks, says researcher Luca Turin of the Alexander Fleming Biomedical Sciences Research Center in Greece, lose much of their musky odor and instead contain overtones of burnt candle wax.
The finding represents a victory for the vibration theory, Turin says. And, he adds, it makes some sense, when you consider the purpose of our olfactory ability—whatever its mechanism is. The natural world contains millions of types of molecules. Some are good for us, and some are bad. The nose helps to distinguish one from the other. "Olfaction is trying to be like an analytical chemist," Turin says. "It's trying to identify unknowns." Chemists identify unknowns using spectrometers. Olfactory receptors, according to the vibration theory, act like little wetware spectrometers.
Take a whiff at the new study over at Scientific American: Link