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Submitted via IRC for Bytram
Atoms spin backwards while flying along a surface
Have you ever noticed that when a car is filmed, sometimes the wheels appear to be turning backwards? For cars, having the wheels rotate in the opposite sense to the car's motion is an artifact. But, for atoms, it may actually happen.
Let's set the scene. A flat sheet of metal, hanging in the vacuum: the camera pans to see a single atom moving flat-out a few nanometers above the surface. The electrons surrounding the nucleus of the atom push the electrons in the metal away from the metal's surface, creating a kind of bow wave of charge in front of the nucleus and a wake of charge behind it. What we're looking at is the very picture of a quantum salt flat racer.
The forces that generate the bow wave and wake are carried by virtual photons that are exchanged between the metal surface and the atom. In the exchange process, the atom will emit a steady stream of real photons in the direction of travel. The momentum kick from launching these photons slows the atom. This is, ultimately, friction for a single atom.
The calculation for that scenario is old and only takes into account translational motion. But, the researchers asked themselves, does the atom also rotate? More carefully put, are the forces between the surface and the atom such that they might produce a torque?
The straightforward answer to this is no. Previous calculations showed that the photons emitted by the atom are linearly polarized, which means that they carry no spin momentum. That seemingly rules them out as a source of angular momentum that would spin the atom. If the atom were to start rotating, then something else has to provide the angular momentum. In the quantum world, this can only happen if electrons or photons carry away or deliver some angular momentum.
In this case, the researchers show that photons with spin angular momentum are emitted, meaning the atom has to start rotating to keep everything balanced.
But the equations also show that these photons can only be emitted opposite to the direction that the atom is traveling, which will cause the atom to accelerate. In other words, the atom doesn't just start to rotate, it is also speeds up in the direction of its motion. Indeed, on the face of it, all friction appears to have vanished, which seemed unrealistic.
(Score: 2) by Hyperturtle on Monday September 23 2019, @05:08PM
You can see the same effect on car wheels simply by going outside and looking out one of the cars windows and observing the other cars. Or even just looking at cars, but the effect is most pronounced when moving at speeds similar to that of the object being observed. This has to do with the mind's sense of focus/awareness. Otherwise a fast car is just a blur if you're close enough to be able to actually focus on the wheels/hubcaps. There are also hubcaps that spin seperately from the wheel, sometimes with cathode tube or RGB lighting, for that extra pimped-up bling some people want on their ride.
No need to find someone's recording of going outside and being exposed to the public, but uh I guess it is scary for some reporters to go out and report.
Speaking of which, the last time I drove, I had to suppress my natural instincts from years of playing games like Saints Row and Grand Theft Auto--after become frustrated in yet more road construction, I considered my options: punching the semi trailer driver and stealing the truck to ram through the obstruction up ahead and then making off in a front-loader before stealing a motorcyle to jump the ramp over a crowd of people on my way to the next waypoint on my ride.
This may be a good strategy overall, but such ingenuity and expression of creatively defying social conventions during rush hour is frowned upon where I live.