Accueil - Science - Symmetry in nature
Date : 08 May 2025Categories : ScienceUnusual
Is there really symmetry in nature? It’s a question you obviously ask yourself every day while drinking your coffee! “Your right hand is the one with the thumb on the left”. My father often made this joke to me, which our dear teenagers, bred on social networks, would certainly classify as a “Dad’s joke”. But how about taking this joke seriously? Let’s talk about “chirality”. I can feel you all getting a little hot under the collar, as if to say, “Oh no, please, not science!”
When you put the palm of your right hand on your left, you realize that they’re different. The thumb of the right hand is then on the left and that of the left hand on the right. The hands are in fact the image of each other in a mirror, but it is impossible to superimpose them. Hands are said to be chiral. Can you follow me?
If the palm and back of the hand were identical, hands would not be chiral. All you’d have to do is flip one of them over to superimpose them. In the same way, let’s imagine twins who are perfectly identical, except that one is left-handed and the other right-handed. To differentiate them, we can symbolically draw the left hand of the left-handed twin and the right hand of the right-handed twin larger. Again, one is the image of the other in a mirror, but it’s impossible to superimpose them. I’d be tempted to take one of the twins and stick his buttocks in front, his face behind and so on, but that doesn’t seem advisable. It would be cheating anyway! It’s a bit like unstacking the pellets on a Rubik’s Cube©, or trying to fit a square into a round by forcing it a bit.
Studying the laterality of left-handed people is obviously fascinating, but finding out that they are chiral has, I have to admit, little practical or scientific interest. And anyway, it’s time to stop experimenting with twins. It’s a very different story when it comes to other chiral objects around us, and molecules in particular. Some of them are in fact chiral and exist in two versions, called enantiomers. Louis Pasteur is credited with identifying such molecules, thanks to the different effect each enantiomer has on light. He first observed this phenomenon on paratartaric acid crystals, which were themselves chiral.
Imagine a universe where everyone had either a right or a left hand. If you had to take the dish out of the oven with a symmetrical oven mitt (by which I mean that it wouldn’t be affected by the palm, back or buttocks thing), then everyone could do it. On the other hand, some people would be very annoyed if they had to go skiing with only a straight glove!
There’s such a thing as the crazy world of living organisms. Indeed, biological molecules have the particularity of existing in the form of a single enantiomer. This is true of the amino acids that make up proteins and the sugars in DNA. Just as a right hand, unlike a left, slips easily into a right glove, an enantiomer of a chiral molecule can interact with one biological molecule and the other. All this can have a variety of consequences. For example, one enantiomer of deltamethrin smells of dill, while the other has the scent of mint. More seriously, one of the enantiomers may have useful analgesic properties, while the other, on the contrary, may cause foetal malformations, as in the case of thalidomide.
Chirality is everywhere, not just in molecules. In fact, there are far more chiral objects than non-chiral ones! These include galaxy clusters, the Earth, snails, a lock or a corkscrew. And that’s where we understand the impact of chirality on the lives of left-handers, forced to live in a universe made up of all kinds of chiral objects adapted only to right-handers.
On the plus side, unlike biological molecules, nature has allowed left-handed and right-handed people to coexist.
This article was written with the help of my favorite right-handed scientist.