QUANTUM SPIN?
In classical mechanics, you have two different types of angular momentum. Orbital Angular moment (the motion of the centre of mass) and spin (motion about the centre of mass). For the Sun and Earth, The movement of the earth around the sun would the orbital angular momentum, and the rotation of the earth about its axis, the spin angular momentum.
BACKGROUND:
Stern–Gerlach experiment.
In this experiment, a neutral beam* of silver atoms (silver atoms possess a
single electron in their outermost shell) is obtained from a suitable source
after collimation and is passed through a magnetic field which is
inhomogeneous, meaning that if any magnetic dipole passes through this field,
the net force due to this magnetic field on the two ends of the dipole would be
unequal; there would be a net force in some direction.
Now if the particles i.e. silver atoms were behaved like classical magnetic dipoles,
then the distribution of their angular momentum (vectors) would be random, and
hence continuous. Thus, a classical expectation would be that, upon defection
by the magnet, the atoms form a continuous smear on the detector/ screen (see
diagram below).
But, the observation was that the detector bore only two remarkably distinct smears, not a continuous one. This was puzzling, as angular momenta were thought to be continuous and not discrete or quantised. This was the first experimental measurement of what is known as Spin.
But, the observation was that the detector bore only two remarkably distinct smears, not a continuous one. This was puzzling, as angular momenta were thought to be continuous and not discrete or quantised. This was the first experimental measurement of what is known as Spin.
Problem
The problem lies in that, for the charge and size of
electrons in particular, their magnetic field is way too high. They’d need to be spinning
faster than the speed of light in order to produce the fields we see. As
fans of the physics are no doubt already aware: faster-than-light = no.
And yet, they definitely have the angular momentum necessary to create their
fields.
It seems strange to abandon the idea of rotation when
talking about angular momentum, but there it is. Somehow particles have
angular momentum, in almost every important sense, even acting like a
gyroscope, but without doing all of the usual rotating. Instead, a
particle’s angular momentum is just another property that it has, like charge
or mass. Physicists use the word “spin” or “intrinsic spin”
.
The spin of a particle has a very real effect on what
happens when it’s physically rotated around another, identical particle.
When you rotate two particles so that they change places you find that their
quantum wave function is affected. Without going into too much detail,
for particles called fermions this leads to the “Pauli Exclusion principle”
which is responsible for matter not being allowed to be in the same state
(which includes place) at the same time. For all other particles, which
are known as “bosons”, it has no effect at all.
Summary:-
Unlike
regular angular momentum, spin has nothing to do with actual spinning.
Instead,
a particle’s angular momentum is just another property that it has, like charge
or mass.
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