Classical physics is good at describing the motion of big things (such as planets), while quantum physics is needed to describe the behavior of small things, such as atoms. But if you get a big enough atom, its electrons should orbit around just like a planet orbiting the sun, and physicists have managed to make that happen.
To figure out whether quantum mechanics matches Newtonian mechanics at large enough scales, you need to find a system that's small enough to be quantum mechanically driven that you can scale up large enough that it shifts over to following Newton's laws of motion. To that end, researchers at Rice University have taken an atom of potassium and applied an enlargifying ray (an ultraviolet laser) to kick its electrons up to an absurdly high energy state.
How It Works
For electrons, higher energy states equate to orbits that are farther from the nucleus of the atom, and so when we're talking about "absurdly" high energy states, we mean electrons that are orbiting so far from their parent nucleus that the total size of the atom is about the size of a period. Yep, one of these. Right here. An atom that's hundreds of thousands of times larger than normal.
At these scales, the electrons orbiting the nucleus of the atom should start to obey the same mechanical laws as a little tiny solar system, but electrons are slippery little things that behave as both particles and waves and they don't like to be pinned down in one place for us to check and see where they are and what they're doing: instead, we can just get a probability of where we're more or less likely to find them. Ultimately, the best we can do is to collapse the wave function of an electron down into a more localized spot called a wave packet.
Once they'd created their giant atom, the researchers at Rice used a rotating radio frequency field to collapse the electrons down as small as possible, and then they used a final electric pulse to destroy the whole system, capturing an image of it in the process. By running the experiment tens of thousands of times, they were able to build up a movie of how the electron packet orbited the nucleus of the atom, and lo and behold, it looked exactly like the Trojan asteroids that orbit the sun out around Jupiter.
Why This Makes Sense
The Trojan asteroids orbit where they do, in the comma shape that they do, because of the interaction between the sun's gravity and Jupiter's gravity. Shrink that down to the scale of a single atom, and the electron wave packet orbits in the same place and the same shape due to the interaction between the electrical fields generated by nucleus of the atom and the rotating radio wave, which mimics the effect of a planet.
It's comforting, somehow, to see experimental evidence that physics behaves the way it's supposed to behave across such vastly different scales as a solar system and a single atom. Personally, I remember learning about atoms for the first time and picturing them exactly like little solar systems, with electrons orbiting nuclei like planets orbiting a star. As I learned more, though, that simplistic picture got shredded by crazy wave function orbitals, and atoms started to look like abstract art. So, I'm loving the fact that you can dig down even deeper, and with some tweaking, atoms really can operate exactly like solar systems after all.
Next up, the physicists at Rice are going to see if they can toss two separate wave packet "planets" into different orbits around a superatom, making an actual solar system. Meantime, you can get more details on this research in the video below.