In this blog, I will cover what is a quantum spin, do electrons spin similar to planets and the Stern-Gerlach experiment; direct observation of the spin.
WHAT IS SPIN?
Spin is an inherent property possessed by the electron. However, it does not rotate. In quantum mechanics, we speak of an en electron as having an intrinsic angular momentum called spin. The reason we use this term is that electrons possess an angular momentum & a magnetic moment just like a rotating charged body.
DO ELECTRONS SPIN SIMILAR TO PLANETS?
IT IS MISLEADING TO IMAGINE AN ELECTRON AS A SMALL SPINNING OBJECT DUE TO THE FOLLOWING:
- An electron’s spin is quantified. It has only two possible orientations, spin up and down, unlike a tossed ball.
- To regard an electron as spinning, it must rotate with a speed greater than light to have the correct angular momentum[Griffiths, 2005, problem 4.25].
- Similarly, the electron’s charge would have to rotate faster than the speed of light to generate the correct magnetic moment[Rohrlich, 2007, Pg 127].
- Unlike a tossed ball, the spin of an electron never changes. It has only two possible orientations: spin up and down.
- In 1922, Otto Stern and Walther Gerlach performed an experiment sending a beam of silver atoms through an inhomogeneous magnetic field and observing their deflection.
- The result illustrated that the electron possesses a quantized intrinsic angular momentum i.e. spin. If the particles were classical spinning objects, one would expect random and continuous spin angular momentum vectors. Each particle would be deflected by an amount proportional to its magnetic moment, producing density distribution on the screen.
- Instead, the particles passing through the Stern-Gerlach apparatus are either deflected up or down by a specific amount.
To conclude, the quantum spin of fundamental particles like electrons is very different from the concept of spin in classical physics.