Quantum Physics and Cheshire Cats

Jonathan Strickland

Darren Hendley/iStock/Thinkstock

We've said it many times on Fw:Thinking - the quantum world is weird. From electron tunneling to superposition to entanglement, things happen on the quantum scale that defy common sense. And just when I think I'm starting to get the slightest grip on what's going on at the quantum level, I learn something new that convinces me that I'm the equivalent of Jon Snow. I know nothing.

Today's new lesson comes in the form of a quantum Cheshire Cat. If you're familiar with the tales of Alice's Adventures in Wonderland, you know about the Cheshire Cat. This is a character who not only matter-of-factly tells Alice that "we're all mad here," it also is capable of vanishing either quickly or one part at a time, leaving its grin to hang in the air before fading away.

So what's a quantum Cheshire Cat? It's when a quantum particle behaves as if separated from one of its properties. Using a macro-level comparison, it'd be as if you had a red ball and suddenly the ball and its redness behaved as if they were separate things. Or perhaps it would remain red, but its roundness would be separated. It's bonkers, right? But on the quantum level, this can happen!

Now, when we're talking about quantum particles, we're not talking about color or shape. But other properties can behave as if they are separated from the particles that possess them. In a paper appearing in Nature Communications, researchers describe an experiment in which the scientists observed a quantum system that behaved as if neutrons followed down one pathway while the neutrons' magnetic moment traveled another. Magnetic moment, by the way, is an object's tendency to align with a magnetic field.

Here are some important things to remember about this announcement: we're talking about the behavior of a quantum system. The system as a whole behaves as if the neutrons and their magnetic moment are traveling down two different pathways. But we can't assert that any of the neutrons are actually behaving that way on a particle level. It could be possible that from the relative perspective of a particle, the magnetic moment remains firmly attached. Does this create a paradox when we shift over to the system-level perspective? You bet! That's quantum physics for ya!

It could be that we just haven't seen the full picture here and so it only seems to us that particles appear to behave one way on an individual basis but another way at a system level. And that's what makes me throw my hands up in the air (and yes, I wave them around as if I just did not care). The paper is up and available to read -- and I found it to be pretty entertaining once I accepted that the behaviors described made little sense to me.