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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: 5, Informative) by Immerman on Monday September 23 2019, @05:15PM (7 children)
A more fundamental question - can a single atom actually spin in any meaningful sense? Molecules can - but they have distinct nuclei tumbling around their shared center of mass.
Presumably they're not talking about the nucleus, which is very nearly a point-mass, seemingly incapable of sustaining a rotational inertia even remotely proportional to it's linear momentum, even if there were some mechanism for transferring a torque to it (and I don't think there is)
Which leaves the electron cloud - and how exactly would you even recognize rotation of a non-localized quantum standing wave? Sure, electrons have "spin", but the consensus seems to be that doesn't actually involve any sort of physical rotation.
(Score: 0) by Anonymous Coward on Monday September 23 2019, @06:50PM
Not yet. Torsion technology is still forbidden to this planet. Focus on social issues first.
(Score: 0) by Anonymous Coward on Monday September 23 2019, @11:16PM (1 child)
So the use of "spin" is just spin?
(Score: 2) by Immerman on Tuesday September 24 2019, @12:32AM
And quantum chromodynamics might "color" your opinion.
(Score: 2) by martyb on Tuesday September 24 2019, @07:38AM (3 children)
I can't answer your question, directly, but TFA links to this article [doi.org] and maybe you can find the answer, there?
Separately, I am aware that the nucleus is composed of protons & neutrons... I always thought they were in a stationary arrangement wrt each other, never mind the quarks which combine to form them, but now wonder if they, too, are in constant motion about each other? Much like how electrons are in constant motion around the nucleus?
This is way beyond anything I ever studied in school, so please forgive me if I have asked an impossible question out of ignorance!
Wit is intellect, dancing.
(Score: 2) by Immerman on Tuesday September 24 2019, @02:45PM (2 children)
I'm not sure anyone really knows what's actually going on inside the nucleus - it's *tiny*, around 10^5x smaller than the atom. I'm not sure we have the technology to do much more than confirm its existence though I'll freely admit that I could simply be ignorant on the subject. It does sound like they're bigger than they "should" be though - even a light-hydrogen nucleus is apparently twice the diameter of the solitary proton it's composed of, so something interesting would seem to be going on.
Given the relatively tight packing though, any motion wouldn't be anything like the motion of electrons around the nucleus, probably more like a bunch of balls in a sack, while an electron is... well, a quantum wave function bound in an interesting orbital shape around the nucleus. A free electron, like other fundamental particles, is generally considered to be a point-mass, and the nucleus is 10^5x smaller than the space occupied by a bound electron's wavefunction, making it effectively a point mass at that scale as well.
(Score: 2) by martyb on Thursday October 03 2019, @11:54PM (1 child)
Wait a minute... a hydrogen atom is one electron orbiting one proton. I can accept that an electron is not really a single "thing" but rather a cloud of possibilities of where it exists as it orbits the nucleus. And now you are saying that in an atom of hydrogen, while there is an electron present, the nucleus is twice as large as a proton?
IOW a proton is twice as large as a proton when it is in a hydrogen atom?
My mind is totally boggled!
Wit is intellect, dancing.
(Score: 2) by Immerman on Friday October 04 2019, @12:26AM
Yeah, go figure - it boggles me too.
Of course, I have no idea how you'd even measure the size of a nucleus within its electron cloud, other than bombarding it with neutrons and seeing what percentage hit something. So perhaps something about the electron cloud just makes the nucleus more "attractive" to neutrons? Or whatever else they're using to determine its size.