Mirror mirror on the wall

How flipping reality back-to-front can turn science upside-down

Mirrors are endlessly fascinating, and not just for the serially narcissistic. Next time you’re shaving, plucking your eyebrows, or just wondering how you came to look so much like Ronnie Corbett, stare deep into the silvery otherworld and ask yourself: how do I know that that isn’t the real world, and that I’m not trapped behind the glass in the alternate-reality of someone else’s back-to-front bathroom?

Science has a word for this reflection—it’s known as ‘chirality’. An object is said to possess chirality if it is distinguishable from its mirror image. If an object is chiral, there is no combination of rotations or motions which you can perform which will result in it looking identical to its reflection.

A classic example of an object with this property of chirality, or ‘handedness’, is your hand. It doesn’t matter how you rotate or move your right hand, you simply can’t make it look like your left hand; nor will your right glove fit comfortably on your left hand. However, in a mirror, the reflection of your right hand does look just like your real-world left and, similarly, if you turn your gloves inside-out—which is neither a rotation nor a motion, but an inversion—you should find that your right glove fits your left hand, er, like a glove.

So why does science reserve a special term for this rather prosaic observation? Well, it turns out that if you look at physics and chemistry in a mirror, you might be surprised by what you see.

Some of the laws of physics are the same in the mirrorverse. For example, as long as I set the balls up the wrong way round to fool sport nerds, a game of snooker would look identical if you watched it back-to-front. Newton’s laws of motion, which govern the motion of everyday-size objects at everyday speeds, remain what scientists would call ‘invariant under a parity transformation’.

Left- and right-handed corkscrews by Leila Battison

Another classic example of an object with chirality is a corkscrew. Turn it clockwise, and you drill into the cork in a wine bottle; turn it anticlockwise, and you ease it out. Lefty loosey, righty tighty—right? However, in a mirror-world, the rule would become lefty tighty, righty loosey; the corkscrew flips in its orientation, and you’d need to turn the ’screw anticlockwise to bore into the cork. Although a trip to mirror-world is easy to deal with on a macroscopic scale, this inversion has profound implications on the tiny scales of quantum mechanics.

In fact, it turns out that subatomic particles such as protons and electrons act like tiny corkscrews. Every particle has an instrinsic ‘spin’ and, if it’s moving, a direction of travel. The relationship between the direction of spin and travel is known as a particle’s ‘helicity’. A particle spinning clockwise along its direction of motion would, like a corkscrew, be said to be right-handed. Matter particles, like protons and electrons, prefer their helicity to be left-handed, while antimatter particles, like antiprotons and positrons, would prefer to be right-handed.

Turning matter into antimatter is as simple as looking at it in the mirror and inverting the charge. The parity transformation reverses the particles’ helicity, while the charge swap explains why anti-electrons are called positrons: they’re positively charged. So, for example, an antiproton is negatively charged (in contrast to the positively-charged proton) and its helicity corkscrew spirals in the opposite direction.

Now as any good sci-fi geek knows, matter and antimatter are not happy bedfellows: bring one into contact with the other and both are annihilated in a burst of gamma radiation. However, one might naïvely expect from our present understanding of physics that an equal quantity of matter and antimatter should have been made at the beginning of time…so how is it that matter, which makes us up, also makes up almost everything we can see? Where has all the antimatter gone? Our best guess is that there was a tiny excess of matter left over after the cataclysmic cancellation, and the reason for this tiny excess of matter is that our Universe does actually behave ever-so-slightly differently from the mirrorverse. This means that matter and antimatter are not quite equal and opposite in every respect, and thus may either not have been formed in precisely equal amounts during the Big Bang, or else subsequent reactions may have given preference to one over the other. So, as beings made of matter, we owe our very existence to the concept of chirality.

Chirality in physics may be responsible for the existence of matter, but as carbon-based life forms, we owe more to mirror images than that. Most chemistry would proceed identically in the mirrorverse, but carbon is capable of creating handed molecules which can confound expectations.

Methane (top) looks identical to its mirror image. A chiral carbon (bottom) joined to four different things cannot be turned into its mirror image by any combination of movements or rotations.

Carbon is one of only a handful of elements able to make four bonds to atoms around it. Usually, it splays its bonds out into a tetrahedron—a triangle-based pyramid—with itself at the centre, and whatever it bonds with at the points. Imagine a carbon attached to four hydrogen atoms. This simple configuration is methane, the main component of the natural gas that probably powers your central heating. This looks identical to its mirror-image, and if you swap one of the hydrogens for something else, the molecule you’ve created still isn’t chiral—you could just rotate the mirror-image around until the non-hydrogen was in the same place as before. This will work once more: two hydrogens, something else and a third thing is still rotatable into its mirror-image; but a third replacement, meaning that the carbon atom is now bonded to four different things, transforms the atom at the centre into a ‘chiral carbon’, and assigns the molecule a handedness.

Now this is all well and good, and perhaps slightly nitpicking, until you realise that chemicals of one handedness can react in a very different way to those of the opposite handedness. For simple reactions, like setting a chiral hydrocarbon on fire, you won’t see a significant difference between handednesses. However, when you get into the complex molecules and processes which predominate in biochemistry, everything changes.

Perhaps the strangest observation relating to biological molecules is that almost all life on Earth comprises amino acids—the building blocks of proteins—which are left-handed. Why exactly this should be is, at present, uncertain. It could just be random; the first self-replicating molecules may simply have had a 50:50 chance of coming out that way, and that the left-handed ones might have been first by luck, spread across the Earth and, ultimately, evolved into every stitch in the elaborate tapestry of life. However, there are other theories: one, for example, suggests that our planet was seeded with amino acids by comets from deep space, and that the amino acids on these comets were broken down according to their handedness by circularly-polarised interstellar UV radiation—in other words, chiral starlight destroyed the right-handed molecules by giving them preferential sunburn.

Back on Earth, the strange fact is that we could, theoretically, construct identical-looking organisms which were totally incompatible with everything in our biosphere. Similarly, you could make a right-handed aubergine and cook it up into some chiral moussaka, but you’d be completely unable to digest it because the enzymes in your body can only process left-handed proteins.

There is a risk that these mirror meals may taste very different: chirality is key in our intimately-linked senses of taste and smell. For example, the compound limonene, extracted from citrus fruit, comes in two forms: right-handed smells of oranges, while its opposite number has an aroma reminiscent of lemons. However, perhaps the bigger risk is that mirror-foods may be poisonous. When scientists cooked up chocolate bars which contained only the mirror form of glucose, they were reported to taste the same as usual. Unfortunately, because this unnatural glucose could not be broken down in the stomach, it continued into the intestines where the remnant sugar turned what was a cheeky calorie-free choccy into a powerful laxative. So powerful, in fact, that this synthetic glucose is undergoing clinical trials for use exactly as that.

Through chiral chocolate and cosmology, the creepy mirror universe is seeping through the cracks in your bathroom tiles into our reality. Is that just reflected light, or your antimatter self? What does their toothpaste taste like? Are they more scared of you than you are of them? From the Big Bang to biochemistry, only chirality has the answer.

This was originally published in Issue 5 of Bang! science magazine, Trinity term 2010.


  1. Thanks for the wonderful article. I’m a writer who’s fascinated by chirality, in all its forms. Such a deep well to quench a thirsty imagination.

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