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Quantum Mechanics

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Editor’s note: The term spin, like “atom” which means “undivided,” comes to us by way of historical misunderstanding. Spin is more like “orientation” and is considered a property of particles, like mass or charge.

from the documentary “Atomic Physics and Reality”:

“Aspect's experiment involved the polarization of photons. Polarization means that light oscillates only in specific planes. A polarization filter will allow light to pass only if it oscillates in a specific plane."

from https://quantumatlas.umd.edu/entry/spin/

Spin: It's more about a particle's identity than its merry-go-round motion.

Quantum particles have a quirky property called spin, which, as it turns out, has almost nothing to do with things like a bicycle wheel or the Earth spinning around an axis.

The name "spin" derives from an early and quite reasonable explanation for an unusual observation: Quantum particles, such as electrons, behave like miniature magnets. At the time, physicists knew that the motion of electrically charged objects produced north and south magnetic poles. From this, they concluded that quantum particles must also be spinning.

Quantum spin got its name because of the relationship between moving electrical charges and magnetic fields, as in the earth's core.This turned out to be wrong.

For one thing, physicists soon determined that an electron would have to be spinning much faster than the speed of light (not allowed in this cosmos) to produce the effects they were seeing.

For another, it seems that an electron has no actual size. It’s a dimensionless "point particle," with no observed structure down to a billionth of a billionth of a meter. Without structure, what does it even mean to spin on an axis? Weirder still, we now know that electrically neutral particles, such as neutrons, also have spin.

The Stern-Gerlach Experiment
Originally performed in 1922, the Stern-Gerlach experiment provided scientists with the first hint that quantum particles had an unrecognized kind of magnetism. In the experiment, researchers fired silver atoms through a magnetic field and measured where they landed on a screen. While an ordinary magnet could end up anywhere (depending on its strength and orientation), the atoms only showed up at two discrete spots.

SEE THE VIDEO, a simple explanation of the the Stern-Gerlach experiment

In fact, spin is an intrinsic property of all quantum particles—not just neutrons and electrons. It provides a way to categorize quantum stuff, like how we lump living things together into families and phyla according to their common characteristics. The catch is that non-quantum objects do not have this property—which makes it difficult to find a familiar analog.

Like many things in the quantum world, spin is quantized, which means that the spin of a particle is limited to particular values—0, 1/2, 1 and 3/2 are fine, but 1.349 is not. The allowed values separate every particle in the universe into two categories, depending on whether their spin is an integer like 1 or a half-integer like 1/2. You’re either a boson or a fermion. There are no other options...

Editor’s note: One physicist explained “quantized spin” like this. A major league baseball pitcher might throw a fast-ball at 81.2 mph; then, another at 80.4; and another at 82.0. In other words, the speed of the ball is all over the map. But when the silver atoms pass through a magnetic field and land on a screen at the back, there are only two “fast pitches,” so to speak; for the atoms, depending on their up or down spin, causing them to land either near the top of the screen or the bottom, with nothing in the middle. This means that the spin, like so many concepts in quantum mechanics, are quantized, issuing is definite, discrete amounts.

This quantum restraining order, known as the Pauli exclusion principle, enforces a rule on the electrons bound to an atom: No two can orbit the nucleus of an atom in precisely the same way. This rule has far-reaching consequences in physics and astrophysics, but its importance extends into chemistry and beyond. The reluctance of electrons to share the same quantum properties is the reason for the structure of the periodic table of elements.

Spin has an orientation that reveals itself when a particle encounters a magnetic field. So if you send an electron through a magnetic field and measure its spin, you’ll find that you only ever get one of two results. Physicists call these "spin up" and "spin down," where "up" and "down" are directions defined by the magnetic field (like the one used to deflect spins in the Stern-Gerlach interactive above). Electron spins will always end up pointing either up or down after a measurement—never at some angle in between.

Posted on October 29, 2011 by The Physicist 

Q: What is “spin” in particle physics? Why is it different from just ordinary rotation?

Physicist: “Spin” or sometimes “nuclear spin” or “intrinsic spin” is the quantum version of angular momentum.  Unlike regular angular momentum, spin has nothing to do with actual spinning.

Normally angular momentum takes the form of an object’s tendency to continue rotating at a particular rate.  Conservation of regular, in-a-straight-line momentum is often described as “an object in motion stays in motion, and an object at rest stays at rest”, conservation of angular momentum is often described as “an object that’s rotating stays rotating, and an object that’s not rotating keeps not rotating”.

Any sane person thinking about angular momentum is thinking about rotation.  However, at the atomic scale you start to find some strange, contradictory results, and intuition becomes about as useful as a pogo stick in a chess game.  Here’s the idea behind one of the impossibilities:

Anytime you take a current and run it in a loop or, equivalently, take an electrically charged object and spin it, you get a magnetic field.  This magnetic field takes the usual, bar-magnet-looking form, with a north pole and a south pole.  There’s a glut of detail on that over here.

A spinning charged object carries charge in circles, which is just another way of describing a current loop. Current loops create “dipole” magnetic fields.

If you know how the charge is distributed in an object, and you know how fast that object is spinning, you can figure out how strong the magnetic field is.  But in general, more charge and more speed means more magnetism.  Happily, you can also back-solve: for a given size, magnetic field, and electric charge, you can figure out the minimum speed that something must be spinning.

It’s not too hard to find the magnetic field of electrons, as well as their size and electric charge. Btw, these experiments are among the prettiest experiments anywhere.  Suck on that biology!

Electrons do each have a magnetic field (called the “magnetic moment” for some damn-fool reason), as do protons and neutrons.  If enough of them “agree” and line up with each other you get a ferromagnetic material, or as most people call them: “regular magnets”.

Herein lies the problem.  For the charge and size of electrons in particular, their magnetic field is way too high.  They’d need to be spinning faster than the speed of light in order to produce the fields we see.  As fans of the physics are no doubt already aware: faster-than-light = no.  And yet, they definitely have the angular momentum necessary to create their fields.

It seems strange to abandon the idea of rotation when talking about angular momentum, but there it is.  Somehow particles have angular momentum, in almost every important sense, even acting like a gyroscope, but without doing all of the usual rotating.  Instead, a particle’s angular momentum is just another property that it has, like charge or mass.  Physicists use the word “spin” or “intrinsic spin” to distinguish the angular momentum that particles “just kinda have” from the regular angular momentum of physically rotating things.