I am studying an on-line course, called Quantum Mechanics Concepts from DrPhysicsA. I am on the 3rd lecture on electron spin. The lecturer is very good and quite precise in his language, but I am hung up on the meaning of some terms and a concept. He describes axis of spin as indicated by a line in some direction and rotation around that axis by an arrowhead, according to the corkscrew rule. Fine. He goes on, though, to speak of spin up and spin down states. What is meant by this-- axis or rotational direction? I SEEM TO BE CONFLATING THE TWO PARAMETERS. The results (and predictions) of experiments based on alignment of AXIS of measuring devices relative to AXIS of spin show that spin can be either up or down. For instance, if for the prepared electrons, the arrow indicates clockwise spin, and the axis line is indicating some angle [relative to a measuring device that will only register vertical (z) direction] then some electrons measured collapse to "up" and some to "down". The down arrow would then indicate anti-clockwise spin for some electrons. How does AXIS direction take on a meaning of ROTATIONAL direction or does it?

 

John

 

thompsjc@gmail.com

 

It's the corkscew rule. Classically, clockwise would be spin up, and anticlockwise would be spin down. It gets weird in QM because nothing is physically spinning, but we use the same terminology.

Thanks, but I still do not get it. There is certainly a particle property called spin the units of which are joule s. There is another property called spin axis, the units of which are degrees or radons. Does up mean right?

Thanks, but I still do not get it. There is certainly a particle property called spin the units of which are joule s. There is another property called spin axis, the units of which are degrees or radons. Does up mean right?

The Spin axis does not have units, and is not a property, as such. It's a coordinate system reference. Spin up means the rotational axis is pointed up.