Quantum Smell

There's an old joke that goes “My Dog's got no nose” “No nose? How does he smell?” “Terrible!” It's not very funny but it asks an interesting question, just how do we smell something? Well a controversial new theory suggests that the way we smell involves quantum physics. Following experiments with animals and human it challenges the long held notion that our sense of smell depends only on the shapes of molecules we sniff in the air Instead it suggests that it is the molecules' vibrations that are responsible for their perceived smell.

The traditional idea is that it is the molecules' shapes that is the link to how they it smells One way to test this is to use two molecules of the same shape, but with different vibrations. A recent study suggests that humans can in fact distinguish between the two. Another clue comes from the fact that molecules which include sulphur and hydrogen atoms bonded together all have a wide range of shapes, but they all smell the same, like rotten eggs. Now comes the new theory, this suggest that what actually determines the smell of a molecule is it's vibrations. The sulphur-hydrogen mystery then becomes absolutely clear. This theory explains the mechanism behind how we smell in this way. Molecules can simply be viewed as a collection of atoms on springs, those atoms can move relative to one another. Energy of just the right frequency, a quantum of energy, can cause the springs to vibrate. Now, all this has been suggested before, that it was these vibrations that explained smell. That mechanism is called "inelastic electron tunnelling" and the explanation is that in the presence of a specific molecule, be it the sulphur-hydrogen example from above, an electron within a smell receptor in your nose can jump, or quantum-tunnel, across and deposit a quantum of energy into one of the molecule's bonds, thus setting the spring vibrating. This is what allows us to smell the molecule.

This theory has received scepticism but one way to test the idea is to use two molecules of identical shape but with different vibrations. This can easily be done by replacing a molecule's hydrogen atoms with heavier deuterium. A previous study suggested that human participants could not distinguish between the two, and therefore the vibrations played no role in what we smell. But now a new study published a paper showing that fruit flies can distinguish between the heavier and lighter versions of the same molecule. So a repeat of the human study was done, using even heavier molecules, and it found that humans can indeed distinguish between the two molecules. This study has re-opened the controversy with more traditional biologists still not convinced, but many others are. It seems yet again that we may well live in a universe controlled by the laws of quantum mechanics.