p**n
发帖数: 10 | 1 What is the meaning of spin when no magnetic field is
applied or no equivalent
magnetic field exists. We know the spin is always related to
the magnetic field
in order to have some observalbe physical effect. Some cases
in point are
Stern-Gerlach experiment and spin-orbit interaction.
But when we consider an ensemble of some kinf of
non-spinless particles, how
do we know the preferred direction of alignment of spins,
(which is called
polarization). This is a very intersting question. I and my
ac | f*******d
发帖数: 339 | 2 I think you have some misconception of spin. Spin HAS
NOTHING TO
DO WITH magnetic field. The only connection between spin and
magnetic
field is that for some particles (such as electron, proton,
neutron)
it is easier to measure spin using Stern-Gerlach experiment
because they
have magnetic moment. However, there are also particles
which have
spin but no (or very small) magnetic moment, for example
photon,
neutrino, etc. The spin of these particles could be
measured, but not
using SG type experim | p**n
发帖数: 10 | 3 Thanks for your attention.
It is well known that spin is intrisic property of a
particle. Neutrio is immune
to the strong and em force, thus one of the ways to
determine the spin is using
cross section. Actually, the concept of spin originastes
from the interaction
with magnetic field, but we generate this idea to all
particles,(even to some
particles that are not governed by the em force). I will not
comment on this here
Quantum field thoery explains well.
What I am driving at is that the polar | rg
发帖数: 12 | 4 I didnot claim spin is meaningless without M field, but I claim the
prefered direction of spin is meaningless without M-field.
say, the sample is in mixed spin state, but the way we choose the eigenbasis
to describe it is arbitary. The perfered eigenbasis is not only unaccessible
but also meaningless.
【在 f*******d 的大作中提到】
: I think you have some misconception of spin. Spin HAS
: NOTHING TO
: DO WITH magnetic field. The only connection between spin and
: magnetic
: field is that for some particles (such as electron, proton,
: neutron)
: it is easier to measure spin using Stern-Gerlach experiment
: because they
: have magnetic moment. However, there are also particles
: which have
| p*****e
发帖数: 58 | 5 Spin is a property of a particle just like mass, charge, ...
It exists no matter if you measure it or not.
In Quantum physics, you can measure the "expectation value",
of the component of a particle's spin under certain
conditions, such as a magnetic field. The expectation is
the result of the energy (Hamiltonian) distribution
(Bose-Einstein or Fermie) under the conditions.
【在 p**n 的大作中提到】
: What is the meaning of spin when no magnetic field is
: applied or no equivalent
: magnetic field exists. We know the spin is always related to
: the magnetic field
: in order to have some observalbe physical effect. Some cases
: in point are
: Stern-Gerlach experiment and spin-orbit interaction.
: But when we consider an ensemble of some kinf of
: non-spinless particles, how
: do we know the preferred direction of alignment of spins,
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