Find the odd one out!








That last one a little tricky ?

Well I bet you could smell the difference in a second!

Hopefully you figured out that each set of images above shows rotations of the same 3D object in space except for one mirror image of that object. Such objects which are different from their mirror image are called chiral (as opposed to, say, a sphere, which is identical to its reflection). The last set is a schematic representation of the molecule carvone. It has this property of chirality, and comes in 2 forms, a ‘left handed’ and a ‘right handed’ version. One version R-carvone smells like spearmint, while its mirror S-Carvone smells like caraway seeds.

To use an example from Lewis Carroll – If Alice has some spearmint chewing gum in the drawing room, it smells (and tastes) like spearmint:


However, when she passes through the looking glass, the reflection of that chewing gum smells of caraway seeds instead


Also note, before she passed through the mirror, Alice’s reflection could smell the reflected gum as spearmint. It was only when real world Alice smelt looking-glass gum that it became caraway flavour.

That is because Alice herself is chiral, and not just in the obvious sense of being left or right handed – at a molecular level, the enzymes and amino acids in her cells have a ‘handedness’.

Because of this chirality of human biology the left and right handed versions of chiral molecules can have dramatically different effects on us (think of trying to shake someone’s right hand with your left). For example the Thalidomide tragedy was caused by a poor understanding of the different effects of the mirror forms of the molecule. So to answer our initial question in the title of this post (a quote from Through the Looking Glass in which Alice is talking to her cat) – No, looking glass milk probably is not good to drink.

Strangely, although some people are left handed and some are right handed, at this molecular level, all people, and indeed all life on earth, have the same handedness. Amino acids produced artificially in a lab have a 50% 50% mix of left and right handedness, so why are there no looking-glass people ?

The answer may lie in fundamental physics. Forces such as gravity and electromagnetism are understood to act the same as their looking-glass counterparts (they conserve parity) and for a long time such parity conservation was thought to be a universal law. However, in 1957 it was astonishingly found that one of the four fundamental forces of nature – the so called ‘weak interaction’ which affects radioactive decay – violates this parity. At the very deepest level it appears that the laws of nature must have a particular handedness. Some think that perhaps this profound asymmetry of the universe is what caused life on earth to develop the chiral way it has.

Curiouser and curiouser!

for more on chirality try Chris McManus’ book “Right Hand, Left Hand “, It doesnt go that deeply into the physics, but it has lots of interesting stuff about handedness in culture and medicine. For a superb non-technical introduction to the maths and physics involved watch this lecture by the great Richard Feynman