Quantum Smell, a controversial theory saying that the way we smell involves a quantum physics effect, has received a boost, following experiments with human subjects.

The theory challenges the notion that our sense of smell depends only on the shapes of molecules we sniff in the air.

Instead, it suggests that the molecules’ vibrations are responsible.

A way to test it is with two molecules of the same shape, but with different vibrations. A report in PLOS ONE shows that humans can distinguish the two.

Tantalizingly, the idea hints at quantum effects occurring in biological systems – an idea that is itself driving a new field of science.

But the theory – first put forward by Luca Turin, now of the Fleming Biomedical Research Sciences Centre in Greece – remains contested and divisive.

The idea that molecules’ shapes are the only link to their smell is well entrenched, but Dr. Luca Turin said there were holes in the idea.

He gave the example of molecules that include sulphur and hydrogen atoms bonded together – they may take a wide range of shapes, but all of them smell of rotten eggs.

“If you look from the [traditional] standpoint… it’s really hard to explain,” Dr. Luca Turin explained.

“If you look from the standpoint of an alternative theory – that what determines the smell of a molecule is the vibrations – the sulphur-hydrogen mystery becomes absolutely clear.”

Molecules can be viewed as a collection of atoms on springs, so the atoms can move relative to one another. Energy of just the right frequency – a quantum – can cause the “springs” to vibrate, and in a 1996 paper in Chemical Senses Dr. Luca Turin said it was these vibrations that explained smell.

The mechanism, he added, was “inelastic electron tunnelling”: in the presence of a specific “smelly” molecule, an electron within a smell receptor in your nose can “jump” – or tunnel – across it and dump a quantum of energy into one of the molecule’s bonds – setting the “spring” vibrating.

But the established smell science community has from the start argued that there is little proof of this.

One way to test the idea was to prepare two molecules of identical shape but with different vibrations – done by replacing a molecule’s hydrogen atoms with their heavier cousins called deuterium.

Leslie Vosshall of The Rockefeller University set out in 2004 to disprove Dr. Luca Turin’s idea with a molecule called acetophenone and its “deuterated” twin.

The work in Nature Neuroscience suggested that human participants could not distinguish between the two, and thus that vibrations played no role in what we smell.

But in 2011, Dr. Luca Turin and colleagues published a paper in Proceedings of the National Academy of Sciences showing that fruit flies can distinguish between the heavier and lighter versions of the same molecule.

A repeat of the test with humans in the new paper finds that, as in Prof. Leslie Vosshall’s work, the subjects could not tell the two apart. But the team then developed a brand new, far larger pair of molecules – cyclopentadecanone – with more hydrogen or deuterium bonds to amplify the purported effect.

In double-blind tests, in which neither the experimenter nor the participant knew which sample was which, subjects were able to distinguish between the two versions.

Still, Prof. Leslie Vosshall believes the vibrational theory to be no more than fanciful.

“I like to think of the vibration theory of olfaction and its proponents as unicorns. The rest of us studying olfaction are horses,” she said.

“The problem is that proving that a unicorn exists or does not exist is impossible. This debate on the vibration theory or the existence of unicorns will never end, but the very important underlying question of why things smell the way they do will continue to be answered by the horses among us.”

And although many more scientists are taking the vibrational theory seriously than back in 1996, it remains an extraordinarily polarized debate.

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