What happens should you find yourself on the other side of a mirror, so that the girl staring at you from its surface becomes, well, you? Do you replace her? Does she replace you? Or do you come together into another, third girl altogether?
According to Alice, the heroine of Lewis Carroll's Through the Looking-Glass and What Alice Found There, what you see in the mirror is pretty much the same as what you get on this side-"only the things go the other way." Backwards, reversed, and inside out. A Knight who puts a right foot into a left shoe. A Queen you can only approach by walking backwards. A cake that is first passed around, and only then sliced. A place, in short, where your only hope of staying still is to run quickly. And the Alice that stares back from the mirror? Why, a backwards Alice, of course, who has to do everything in reverse to come out the same.
Through the Looking-Glass was published in 1871. In 1928, physicist Paul Dirac combined quantum theory and special relativity to describe the motion of an electron-and from that equation, posited the existence of an antiparticle for every particle. An antiparticle would match its fellow particle precisely, with one exception: it would be the opposite charge. When the discovery won the 1933 Nobel Prize, Durac went a step further: why couldn't there be an entire universe, he asked, made not of matter as we see it, but of its opposite, antimatter? In other words, why couldn't Alice's mirrored world actually exist, a Looking-glass House where everything is the same, except opposite?
Lewis Carroll, it seems, had the right idea after all. Why couldn't Alice have an entire world that reflected hers down to the smallest detail, waiting for her just on the other side of the glass?
Well, in theory she could. But in practice, we now know that the adventure would have been rather ill-fated, to put it mildly. Unbeknownst to Carroll, matter and anti-matter have never much liked each other. The moment that Alice, a girl of matter, pokes her hand through the mirror and poof! is magically whisked into the reflection, she will explode in a brilliant flash, emitting energy in proportion to her mass-E=mc2. In an instant, neither girl will exist; they will have cancelled each other out. And that would be the end of Alice's looking-glass adventure.
But, for the sake of argument, let's assume that Alice doesn't explode when her twin halves meet. Let's assume that she makes it through, and that the process instantly transforms her into her antimatter equivalent. What then? She'd look identical, but she would indeed act in a way that's opposite to her regular self-antimatter particles, after all, carry the opposite charge and are deflected in the opposite direction from their regular counterparts. Would Alice go backwards to go forward? In a way, yes, she would. Relative to her non-mirror, self that is.
Actually, though, Alice and anti-Alice may not be identical after all. If one girl were to tumble, she may not fall the same way as the other. Scientists are currently trying to see if matter and antimatter may not actually behave differently in gravity-a finding that, if true, would explain why there's such a radical imbalance between matter and antimatter in the world, or why we have many Alices and no anti-Alices walking around.
And what if Alice-the new, antimatter Alice, that is-were to offer her cat, Kitty, some looking-glass milk, as she proposes to do at the beginning of her adventure? Not a good idea. In its reflection, the milk's particle structure would be reversed-a mirror-image of the molecular structure of regular milk. And Kitty, you'll remember, is still fully mattered. Kitty plus milk…not a pretty sight. Both would go up in a loud bang before Kitty could even taste her beverage of choice. Oops.
Luckily, catastrophe is averted: Alice thinks better of the idea, speculating that, after all, "Perhaps Looking-glass milk isn't good to drink." No, it isn't. Not if you're not looking-glass yourself.
But if you are, drink up all you want. After all, you'll be able to survive in your new world indefinitely. Where you'll run into trouble is surviving outside of it. An anti-matter Alice is far more unstable than a regular Alice. In fact, until this year, she wouldn't have made it more than a few seconds, even in optimal anti-matter conditions, outside of her mirrored existence. Now, however, scientists may be able to study her for almost 20 minutes at a time, to try to see just how different, if at all, she is from her pre-mirror self.
For now, though, things in the looking-glass world are pretty much as you see; except they go the other way, of course. The only real question: if we tried to transform the Cheshire Cat into a version that could survive in the world of matter, what would happen to his smile?
Lewis Carroll was born on 27 January 1832. Today, he would have celebrated his 180th birthday.