How the Universe Differs From Its Mirror Image

After her adventures in Wonderland, the fictional Alice stepped through the mirror above her fireplace in Lewis Carroll’s 1871 novel Through the Looking-Glass to discover how the reflected realm differed from her own. She found that the books were all written in reverse, and the people were “living backwards,” navigating a world where effects preceded their causes.

When objects appear different in the mirror, scientists call them chiral. Hands, for instance, are chiral. Imagine Alice trying to shake hands with her reflection. A right hand in mirror-world becomes a left hand, and there’s no way to align the two perfectly for a handshake because the fingers bend the wrong way. (In fact, the word “chirality” originates from the Greek word for “hand.”)

Alice’s experience reflects something deep about our own universe: Everything is not the same through the looking glass. The behavior of many familiar objects, from molecules to elementary particles, depends on which mirror-image version we interact with.

Mirror Milk

At the beginning of Through the Looking-Glass, Alice holds her cat Kitty up to the mirror and threatens to push her through to the other side. “I wonder if they’d give you milk in there? Perhaps Looking-glass milk isn’t good to drink,” she says.

Alice was onto something. Just over two decades before the book’s publication, Louis Pasteur discovered while experimenting with some expired wine that certain molecules can be chiral. They can come in distinct left-handed and right-handed structural forms that are impossible to superimpose. Pasteur found that, while they contain all the same components, the mirror versions of chiral molecules can serve distinct chemical functions.

Lactose, the sugar found in milk, is chiral. While either version can be synthesized, the sugars produced and consumed by living organisms are always the right-handed ones. In fact, life as we know it uses only right-handed sugars — hence why the genetic staircase of DNA always twists to the right. The root of this “homochirality” remains one of the biggest mysteries clouding the origins of life.

Kitty couldn’t have digested looking-glass milk. Worse, if it had contained any bacteria with the opposite handedness, her immune system and antibiotics would have been ill suited to put up a fight. A group of prominent scientists recently cautioned against the synthesis of mirror-image biomolecules for this reason — if any were to escape the lab, they could evade every one of life’s defense mechanisms.

Shrinking Down

Continuing down the rabbit hole, we see traces of chirality all the way to elementary particles.

Pasteur’s work on molecules rested on a previous discovery by Augustin-Jean Fresnel, who in 1822 realized that different quartz prisms could send light’s electric field twirling in one of two directions — clockwise or counterclockwise. If each particle of light could leave a smoke trail in its wake, a right-handed screw of smoke would emerge from one prism and a left-handed screw from another.

Nowadays, physicists consider chirality a fundamental property of all elementary particles, just like charge or mass. The particles that don’t have mass are always traveling at the speed of light, and they also all carry an intrinsic angular momentum as though they’re spinning like a top. If the particles are flying in the direction of your thumb, their spin follows the direction your fingers curl — on either your right hand or your left.

The situation is a bit more complicated for massive particles, such as electrons and quarks. Because a massive particle travels more slowly, a speedy observer could overtake it and effectively reverse its direction of motion, thus flipping its apparent handedness. For this reason, when describing the chirality of massive particles, physicists often refer to the mathematical description of the particle’s quantum properties. When you rotate a particle, its quantum wave function shifts left or right depending on its chirality.

Almost every elementary particle has a twin through the looking glass. A negatively charged left-handed electron is mirrored by the anti-positron, a negatively charged right-handed particle.

In looking-glass world, Alice finds all logic turned on its head: People run in order to stay in place, and they celebrate “un-birthdays” on all the days they weren’t born. Similarly, our universe differs from its mirror image. The weak force — the force that’s responsible for radioactive decay — is felt only by left-handed particles. This means that some particles will decay in the normal world while their counterparts in the mirror would not.

Plus, there’s one particle that seems not to show up in the mirror at all. The neutrino has only ever been observed in its left-handed form. Particle physicists are investigating whether the right-handed neutrino exists or if neutrinos’ mirror images are simply identical, which could help explain why the universe contains something rather than nothing.

There’s a lot we can learn about our own world by peering through the looking glass. Just be careful not to drink the milk.