Q:

What does spin 1/2 physically signify..?
Is it something related to Angular Momentum of the fermion..????

- Mrinmoy (age 18)

assam, India

- Mrinmoy (age 18)

assam, India

A:

There are two kinds of angular momentum, both classically and quantum mechanically. The earth-sun system is a good classical example. The earth orbits the sun once per year and has orbital angular momentum proportional to the earth-sun distance times the relative velocity of the earth perpendicular to the earth-sun distance. It is a vector quantity. In addition the earth can have spin angular momentum proportional to the inverse of the length of a day times a quantity called the moment of inertia. Likewise in quantum mechanics you can have these two kinds of angular momenta, orbital and spin .

The major difference is that classically these quantities can be any value, whereas in quantum mechanics the orbital kind must be an integer times **ℏ** which is Plank's constant divided by 2π. See: Curiously, the values of spin angular momentum, s, can have half-integer values, e.g. s =* ***ℏ** x (0,1/2,1, 3/2 ...). The electron, a Fermion, happens to have s = 1/2* ***ℏ**

So when someone says that "electrons have spin 1/2" they imply the value in units of *h-bar.*

LeeH

So the classical distinction between the two types of angular momentum is a bit arbitrary, depending on how you choose to break up your description of objects into spinning wholes vs. orbiting parts. As Lee points out, quantum mechanically there's a real qualitative difference between the orbital and spin components. /mw

*(published on 08/27/2013)*