Our sense of smell is really quite incredible. Every time we take in a breath or taste food, countless molecules swarm into our nasal passages. As they move up the nasal tract, these visitors arrive at a patch of cells on which there are over 10,000 different kinds of docking stations. These cells are odor receptors, and each of them can register a different odor. Together they make up a chemical detector that is much more sensitive and versatile that anything we can come close to building.
In a paper published in the journal PNAS in February, the authors demonstrate through a series of ingenious experiments that smell can be sensitive enough to pick up on tiny differences in atomic vibrations.
The conventional theory of smell works somewhat like a lock and a key. The molecules are the key, and they ‘lock in’ to receptors that fit their exact shape and size. This is the shape theory of smell, and the basic idea had been suggested in the 1st century BCE by the Epicurean philosopher Lucretius. The idea has since garnered substantial evidence with the discovery of odor receptors, leading to the 2004 Nobel Prize in Medicine for working out the overall picture of how smell works.
An alternative hypothesis is the vibration theory. This proposes that smell works not by detecting the shape of molecules, but by measuring how the atoms in a molecule are vibrating.
Molecules are groups of atoms that are held together by chemical bonds. These bonds are somewhat elastic, causing the atoms in the molecules to constantly jiggle about. This is analogous to what would happen if you were to connect balls together with springs (something that physicists love to do). But the analogy breaks down at this microscopic scale, and one needs to resort to the laws of quantum mechanics to understand what is happening. It turns out that, similar to the balls and springs, molecules have certain ways in which they prefer to jiggle. They can stretch, rock, wag and twist around.
So, which is it? Does smell work via shape or vibration? The authors set out to address this question with flies.