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Spin as a key for QM

Spin is one of those things that we can understand intuitively. We can see spinning tops. We can imagine spinning electrical currents and their effect. Playing with electrical currents in magnetic fields didn’t always end up with expected results. Markus Ehrenfried has a good rundown to describe the situation answering the question What is Spin?

“In 1921 two physicists named Otto Stern and Walther Gerlach made an interesting experiment. They took a beam of electrically-neutral silver atoms and let it pass through a non-uniform magnetic field. This magnetic field deflected the silver atoms like it would deflect little dipole magnets if you threw them through the field. After passing through the field the deflected atoms hit a photoplate and made little visible dots.”
“The result of this experiment was totaly unexpected and very surprising. Keep in mind: those atoms were just coming out of an oven where silver was evaporated, they had no special orientation in space, therefore the spins of the outer electrons in these atoms should point into all possible directions in space. Depending on their orientation the magnetic force our little dipole magnets ‘feel’ is different and therefore the deflection is different. Some of them would be oriented in a way that the deflection is very strong, others would have an orientation which results in almost no deflection at all, so Stern and Gerlach expected a pattern like the one depicted as ‘Classical Expectation’ in the picture above: a spot on their photographic plate produced by many many tiny dots caused by silver atoms hitting the plate all over the place. What they got instead was a pattern like the one shown below: only a contour was hit by the atoms and in the middle was nothing!!! “

This experiment is one of many that yield results that stimulated development of ideas in quantum mechanics. Exploring the implications of these ideas is in the news right now as groups at the LHC and other high energy particle accelerators have announced that they have data to support a particle first suggested in the 60’s such as by Higgs and Eisele. That particle is a Boson and that type of particle is distinguished from those that make up the matter in the universe that are called Fermions. One difference between these two groups is in their spin. Fermions are half spin types that make up matter and Bosons integral spin types that make up forces.

That is why the quantum mechanical property of spin can be mind boggling. Not only is it ‘quantum’ but it ties in with the Pauli exclusion principle that leads to statistical behaviors and classes of those behaviors and all sorts of stuff that, eventually, gets to string theory and the standard model and other ideas that are mathematical constructs that seem quite distant from our everyday experience.

“Markus Ehrenfried was born in the 20th century and he will die in the 21st. He lives somewhere in the northern hemisphere. More than 99.9% of his mass consists of protons and neutrons which is why he thinks it is important to understand their spin structure. He is thankful that quarks end electrons have a spin of 1/2 — especially considering that the world would be a pretty boring place if quarks and electrons had integral spin. Markus admires the second law of thermodynamics and tries to comply with it whenever possible. He consumes a considerable amount of caffeine every day and likes cake, cookies and waffles. “

Check out his website!

For a more theoretical (i.e. math oriented) take on the topic, see Quantum mechanical spin – lecture 6 notes in PDF. A bit in between with a focus on history and magnetism is at Trinity College: Relativity and quantum mechanics bring understanding at last. That one gets into tape recorders and the mid Atlantic ridge.

Do a search on ‘understanding spin in quantum mechanics’ and you too will find all sorts of interesting reading material!

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