It was mid-October, 1863, when a budding mathematics lecturer from the University of Oxford ventured into London to meet with a publisher. He had just completed what would one day be seen as his magnum opus: a 12-chapter treatise on some of the most controversial topics in modern math. He had named his book .
Yes, most of us today (and indeed at the time) think of Alice as being much more a “harmless children’s fantasy tale” than an “exploration and lambasting of abstract math” – but underneath all the whimsy and there lies a genuine critique of what, at the time, was some of the most cutting-edge math around.
And it’s not complimentary: topology and projective geometry are ; abstract algebra leads to a ; even negative numbers are cast as unnatural and paradoxical. Carroll, , was not a forward-thinking scientist.
Which is why it’s so funny that today, his book is used to explain a that may or may not exist.
What is the Cheshire Cat effect?
You’d think that, given their field’s technical definition as “, I mean, the cat is , don’t ask me”, there wouldn’t be much that could surprise quantum physicists.
Yet, when a couple of papers seemed to show subatomic particles being separated from their own intrinsic properties – analogous to “colors going around without the objects that carry them,” Federal University of Minas Gerais physicist Raul Corrêa at the time – it seemed weird even by their standards.
“The possibility of separating a particle from one of its intrinsic properties […] is rather intriguing and questions a very basic everyday notion, by which the properties of things are always with the things themselves,” Corrêa said. “[S]till more intriguing, [is that] this completely weird phenomenon is said to happen in the physical world.”
The first example of this seemingly nonsensical phenomenon turned up , when researchers from Tel Aviv and the UK came up with an experiment that would divorce a photon from its polarity. Using a two-armed interferometer – a that gleans information from the interference of two or more superimposed light sources – the team claimed to show that a photon could travel with 100 percent certainty through the left arm, while its polarization would be detected in the right.
Like Carroll’s infamous Cheshire cat, the photon’s intrinsic property seemed to have been left behind by the photon – and the researchers embraced the literary comparison, beginning their paper with a direct quote from the book:
’All right’, said the Cat; and this time it vanished quite slowly, beginning with the end of the tail, and ending with the grin, which remained some time after the rest of it had gone.
‘Well! I’ve often seen a cat without a grin’, thought Alice, ‘but a grin without a cat! It’s the most curious thing I ever saw in my life!’
What’s going on?
“No wonder Alice is surprised. In real life, assuming that cats do indeed grin, the grin is a property of the cat – it makes no sense to think of a grin without a cat. And this goes for almost all physical properties,” the team wrote.
Similarly, “polarization is a property of photons; it makes no sense to have polarization without a photon,” they explained. Yet “in the curious way of quantum mechanics, photon polarization may exist where there is no photon at all.”
But ? While further experiments by other researchers seemed to support the idea – only one year later, a team from Vienna separating a neutron from its magnetic moment, providing what seemed to be the first experimental proof of the effect – other experts weren’t convinced.
“Most people know that quantum mechanics is weird, but identifying what causes this weirdness is still an active area of research,” Jonte Hance, a research fellow at Hiroshima University and the University of Bristol, earlier this year. “It has been slowly formalized into a notion called contextuality – that quantum systems change depending on what measurements you do on them.”
You may already know about wavefunction collapse – the quantum phenomenon where a wavefunction that exists as a superposition of eigenstates “collapses” into one single state upon observation. It’s the idea behind the Schrödinger’s Cat thought experiment: it’s not that Tiddles is either alive or dead and we just don’t know which – it’s both at once, up until we look at it.
Contextuality is similarly confounding: basically, it’s somehow the case that if we measure, say, a particle’s location and then its speed, it will give a different result from measuring the speed first and the location second. It’s a classic example of logic going haywire in the quantum universe – and, Hance and his colleagues argued, it’s the explanation for the Cheshire cat’s lingering grin.
“Different results are obtained if a quantum system is measured in different ways,” explained Holger Hofmann, a professor at Hiroshima University and, along with Hance, one of the authors of claiming to debunk the quantum Cheshire Cat effect.
“The original interpretation of the quantum Cheshire cat only comes about if you combine the results of these different measurements in a very specific way, and ignore this measurement-related change,” he said.
So much for quantum wonderland, then. Schrödinger’s cat may be alive, but the Cheshire cat is most likely dead.
At least – this version is.
The other Cheshire cat effects
Long before Alice’s feline friend started shedding fur all over the laws of physics, it had already bequeathed its name to some very different disappearing acts.
Take the coccolithophores, for example – a teeny-tiny organism just one cell big which spends its life building a mysterious chalky shell around itself, chowing down on carbon dioxide, and reproducing asexually. Goals, amirite?
But , with reproduction comes vulnerability. During their splitting process, they are uniquely susceptible to viral infections – and they’ve come up with a pretty ingenious solution.
“The haploid [with a single set of chromosomes] phase of [the coccolithophore] E. huxleyi is unrecognizable and therefore resistant to EhVs [E. huxleyi viruses] that kill the diploid [with two sets of chromosomes] phase,” reports describing the organism’s so-called “Cheshire cat escape strategy”.
“We further show that exposure of diploid E. huxleyi to EhVs induces transition to the haploid phase,” it explains. “These ‘Cheshire Cat’ ecological dynamics release host evolution from pathogen pressure and thus can be seen as an opposite force to a classic ‘’ coevolutionary arms race.”
But wait – what’s the smile in this analogy? And what’s all this about a red queen? Well, this phenomenon actually gets its name from a different part of Alice’s Adventures in Wonderland – when the Red Queen, midway through a croquet game, demands the Cat’s execution by beheading.
But of course, nothing could be so simple in Wonderland. This time, the cat is not vanishing, but appearing, face-first – and, in fact, face only. This made everybody “very uncomfortable,” Carroll wrote, with the executioner protesting that “you couldn’t cut off a head unless there was a body to cut it off from: that he had never had to do such a thing before, and he wasn’t going to begin at his time of life.”
Similarly, the coccolithophore logic goes, a diploid host can’t be infected if it’s only haploid – and so by “vanishing” the part of itself that makes it vulnerable, it lives to guzzle CO2 another day.
A grin without a cat
The Cheshire cat’s grin may not be separated from the cat in quantum physics – but strangely, it’s sort of possible in the real world.
“Normally, your two eyes see very slightly different pictures of the world around you,” explains the , for which the effect was designed . “Your brain analyzes these two pictures and then combines them to create a single, three-dimensional image.”
But sometimes, things get a bit complicated. When one eye sees one image, and the other sees a very different one, we get what’s called a – basically, your brain doesn’t know which one to concentrate on, so it alternates.
This version of the Cheshire cat effect exploits this neurological indecision by setting you with one eye looking at a partner, and the other – via a mirror – looking at a plain wall. You then “move your hand in front of the white surface as if passing a blackboard eraser over the surface,” the museum advises, and “watch as parts of your friend’s face disappear.”
See, since our jumpy monkey brains don’t know how to process the conflicting inputs, they try to figure out what’s most important and build a picture based on that. And what our brains think is important, usually, is anything that’s moving or changing – like your hand waving around in front of a plain background.
Your partner, meanwhile, can pretty safely be ignored, your brain figures. “Because the other person is sitting very still, your brain emphasizes the information coming from your moving hand, rather than the unmoving face,” the Exploratorium explains. “As a result, parts of the person’s face disappear.”
“No one knows how or why some parts of the face may remain, but the eyes and mouth seem to be the last features to disappear,” they note.
As Carroll so presciently wrote: the grin… remained some time after the rest of it had gone.
All “explainer” articles are confirmed by to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.