Angular Momentum: P.A.M Dirac QM Principles Explained

[jamie.j1989]

Hi, In the principles of quantum mechanics by P.A.M Dirac it says on page 149,

For dealing with an angular momentum whose magnitude is 0.5(h_bar), it is convenient to put
m=0.5(h_bar)a
how is this if (m.m)^0.5= (3/4)^0.5*(h_bar)? Thanks

 

[Greg Bernhardt]

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

 

[Simon Bridge]

"the magnitude of $\vec m$ is 0.5" (in units of hbar) means $\vec m\cdot \vec m = 1/4$ - so I think the answer to the question is "it cannot be".

Probably the passage has been misread, those two expressions appear to be for different states - but there is not enough of the book quoted to make the context clear.

 

[Jilang]

Agree. The second expression looks like the value sqrt(m(m+1)) with m =1/2.

 

[Simon Bridge]

The book being used was published in 1930 - I think the best advise is to get hold of a more recent publication.
They did a lot of things back then, like the above, that can be confusing to the modern student. The recent texts take advantage of advances in education, as well as understanding of QM, made in the last 80-odd years.