Ack! I'm cringing! "The hydrogen atom consists of a heavy,
essentially motionless proton " w0efjwefj sorry sorry sorry ...
but 0jw0r9jw ok. I know ... I know it's _necessary to consider
it as such and that it *works* .... I mean you have to hang your
hat on something but. r023j02j3r03r00oijpojoiejpfw Sorry
sorry... sorry. Ok.. sorry. It works. It works. I know it works.
I just get all WAIT-WHAT?! when I see something like
"essentially motionless". I suppose we have to freeze all
spacetime in order to make these things work.. and we can make
up for it later on but that sentence made my not-enough-caffeine
mind rebel. Ok, I can read on. But I had to express smile
emoticon Still, 147 pages in, it's the first thing I read that
made me cringe. smile emoticon The fact that it took me _that
long_ to cringe, and even without doing the math and the
exercises, damn good educational material. Not that I'm in a
position to judge but I enjoy it anyway smile emoticon == Ah!
More good connections to where theory and reality depart
somewhere. These are the things I'm looking for: Little
reminders to the reader just where they're standing and also
where they're NOT standing. "The photon is a quantum of
electromagnetic radiation; it's a relativistic object if there
ever was one, and therefore outside the scope of nonrelativistic
quantum mechanics. It will be useful in a few places to speak of
photons and to invoke the Planck formula for their energy, but
please bear in mind that this is external to the theory we are
developing." It's easy to get caught up in the excitement and
not realize that the text is in the world of non-relativistic
quantum mechanics. It's a nice little reminder 'cause it's easy
to get your head in the clouds as things begin to click into
place more and more. == In classical mechanics, a rigid object
admits two kinds of angular momentum: orbital (L = r x p),
associated with the motion of the center of mass, and spin (S =
Iui). associated with motion about the center of mass. For
example, the earth has orbital angular momentum attributable to
its annual revolution around the sun, and spin angular momentum
coming from its daily rotation about the north-south axis. In
the classical context this distinction is largely a matter of
convenience, for when you come right down to it, S is nothing
but the sum total of the "orbital" angular momenta of all the
rocks and dirt clods that go to make up the earth, as they
circle around the axis. But an analogous thing happens in
quantum mechanics, and here the distinction is absolutely
fundamental. In addition to orbital angular momentum, associated
(in the case of hydrogen) with the motion of the electron around
the nucleus (and described by the spherical harmonics), the
electron also carries another form of angular momentum, which
has nothing to do with motion in space (and which is not,
therefore, described by any function of the position variables
r,0,<f>) but which is somewhat analogous to classical spin (and
for which, therefore, we use the same word). It doesn't pay to
press this analogy too far: The electron (as far as we know) is
a structureless point particle, and its spin angular momentum
cannot be decomposed into orbital angular momenta of constituent
parts (see Problem 4.26). 21 Suffice it to say that elementary
particles carry intrinsic angular momentum (S) in addition to
their "extrinsic" angular momentum (L). =
