Ferrous Wheels and Quantum Corrals
I was inspired by a very bad pun I heard this morning. Hidden inside of it is an important lesson on the quantum nature of electrons.
What you’re looking at is a famous experiment in which iron atoms were manipulated by the atomic needle tip of a scanning tunneling microscope until they formed a perfect circle on a copper surface (the bottom one is an artist’s interpretation, however). That’s already cool, manipulating individual atoms to form a sort of “ferrous wheel” of their own … but it gets cooler!
See the waves in the center of the corral? I’m reminded of the ripples on the surface of a pond when a pebble is thrown in. They form on the atomic scale due to the quantum nature of electrons. If you’ve religiously read everything I’ve ever posted, you know that atoms are mostly empty space and that electrons exist like waves rather than tiny points flying around. Copper’s electrons exist in a very particular pattern of orbitals that is represented like this:
When the iron atoms in that corral up there form their circle, the electron waves in the copper surface interact and intersect in such a way that a standing wave is formed. You are literally looking at the quantum nature of electrons! Each electron in an orbital can only exist in a certain wave, otherwise the other electrons in that atom would interfere with it.
Let Brian Cox tell you more about those standing waves with a little help from his friends.
Earlier this week, we were treated to rock star physicist Brian Cox explaining how everything in the universe is connected via the Pauli exclusion principle. If you haven’t seen it, go watch it so you’re up to speed.
The clip was from his “A Night With The Stars” televised lecture, which is something that would never make it on TV in the US, and that is sad.
In it, he rubbed a diamond and postulated that the PEP demanded that any changed energy states in the electrons in that chunk of carbon would be transmitted to all other electrons in the universe, so as to have no two in the same energy state. He’s a great speaker, but many people have accused him of screwing up in this demo.
Sean Carroll has the best write-up of why Cox’s take is a bit off on his blog, Cosmic Variance. It’s long, and made of words, but you’ll learn something fantastically interesting if you read it. The take-home:
In terms of explaining the mysteries of quantum mechanics to a wide audience, which is the point here, I think the bottom line is this: rubbing a diamond here in this room does not have any instantaneous effect whatsoever on experiments being done on electrons very far away. There are two very interesting and conceptually central points worth making: that the Pauli exclusion principle helps explain the stability of matter, and that quantum mechanics says there is a single state for the whole universe rather than separate states for each individual particle. But in this case these became mixed up a bit, and I suspect that this part of the lecture wasn’t the most edifying for the audience. (The rest of the lecture still remains pretty awesome.)
Physics, theoretical or otherwise, is a cruel and confusing mistress. I’m glad there’s people like Brian and Sean so that we get to have these conversations.
Previously: I translate Brian Cox’s voice and it’s funny.
Electron perfectly round to one part in a million billion, experiment finds. If you think that’s cool, you should see the calipers they used.