We must begin our discussion by noting that the magnetic moment of an electron
is (according to the ``Uhlenbeck-Goudsmit hypothesis'')

(16.103) |

(16.104) |

(16.105) |

I don't have the time to go into more detail on, for example, what the Zeeman effect (splitting of energy levels in an applied magnetic field) is. In any event, it is strictly a quantum effect, and you should study it soon in elementary quantum theory, if you haven't already.

Thomas (who taught for years over at NC State) showed in 1927 that the discrepancy is due to a relativistic kinematic correction like that we previously considered. In a nutshell, the rest frame of the electron rotates as well as translates (boosts) and we must therefore take into account both kinematical effects. This results in an additional (Thomas) ``precession'' of the frames. When Thomas precession is taken into account, not only are both the fine structure and anomalous Zeeman effect in atoms accomodated, but a deeper understanding of the spin-orbit interaction in nuclear physics (and rotating frames in general) also results.

Let us begin by (naívely) deriving the spin-interaction energy. Suppose
the electron moves with velocity in external fields **E** and **B**. Then the torque on the electron in its rest frame is just

(16.106) |

As we will show very soon, the magnetic field transforms like

(16.107) |

(16.108) |

(16.109) |

The electric force is very nearly the negative gradient of a
spherically averaged potential energy . For one electron atoms this is
exact; it is a good approximation for all the others. Thus we will try using

(16.110) |

(16.111) |

The error is, in a nutshell, that we have assumed the electron to be in a ``rest'' frame (that is, a frame travelling in a straight line) when that frame is, in fact, rotating. There is an additional correction to vector quantities that arises from the rotation of the frame. This correction, in macroscopic systems, gives rise to things like coriolis force.

Let us recall (from classical mechanics) that if a coordinate system rotates
at some angular velocity
, the *total* rate of change of any
vector quantity is given by

(16.112) |

and should be perpendicular to and

Well, as I noted above, the expression we have given above for the time rate
of change of the spin was correct for the field and moment *expressed in
the rest frame of the electron*. In the *lab* (non-rotating) frame,
which is where we measure its energy, we therefore should have:

(16.113) |

(16.114) |

To answer that we must consider carefully what defines the ``rest'' frame of
the *accelerating* electron. We will do so by chopping the motion of the
electron into infinitesimal segments. If the electron is moving at velocity
at any instant of time , then at the electron is moving at
. To get from the lab frame () to the instantaneous rest
frame of the electron () we must therefore boost:

(16.115) |

(16.116) |

The coordinate frame precession is going to be determined by the Lorentz
transformation between these two (infinitesimally separated) results:

(16.117) |

(16.118) |

Then

(16.119) |

(16.120) |

(16.121) |

(16.122) |

To first order in
, we see that the total transformation
is equivalent to a boost and a rotation:

(16.123) |

(16.124) |

(16.125) |

(16.126) |

(16.127) |

Finally we see explicitly that at least for infinitesimal transformations,
a pure Lorentz boost
is equivalent to a
boost to an infinitesimally differing frame
followed by a
simultaneous infinitesimal boost *and* rotation.

Now comes the tricky part. The equation of motion for the spin that we began
with (in the ``rest frame'') can be expected to hold provided that the
evolution of the rest frame is described by a series of infinitesimal boosts
alone (*without* rotations). In other words, we have to add the
relativistic equivalent of counterrotating the frames (like we did above with
the
term). These ``relativistically
nonrotating coordinates'' are related to the instantaneous rest frame
coordinates of the electron by the infinitesimal boost

(16.128) |

(16.129) |

Thus the ``rest'' system of coordinates of the electron are defined by .
They are rotated by
relative to the boosted laboratory
axes . If a physical vector **G** has a (proper) time rate of change
of
in the rest frame, the precession of the rest frame axes
with respect to the laboratory makes the total time rate of change

(16.130) |

(16.131) |

The acceleration perpendicular to the instantaneous velocity appears in this expression because it is this quantity that produces the ``rotation'' in the infinitesimal transformation between frames that occured in the infinitesimal time interval. Note that this is a purely kinematical effect, and has nothing to do with the laws of nature, just like the non-relativistic ``coriolis force'' and ``centrifugal force''. If one wishes to relate the laws of nature as measured in some accelerating frame to those measured in a non-accelerating frame, then it is necessary to insert a fictitious ``force'' (or more properly interaction ``energy'') that is kinematic in origin.

In this case, curiously enough, the laws of nature are known in the
accelerating frame, and the fictitious force appears in the lab frame, where
it is not properly speaking fictitious. However, it *is* still kinematic.
That is, there is no actual energy associated with the fictitious interaction
(whatever *that* means); however, this interaction is necessary
nonetheless if we wish to obtain the equation of motion from the energy
equation alone without explicit consideration of the transformations of
frames.

To conclude, for electrons the acceleration is caused by the (screened)
Coulomb force on the electron that keeps it bound. Thus

(16.132) |

(16.133) |

With , *both* the spin-orbit interaction *and* the anomalous
Zeeman effect are correctly predicted in accord with what is experimentally
observed. Relativistic effects, which are generally thought of as being
``subtle'', are not subtle at all when it comes to kinematics. The
relativistic kinematic correction is as large as the other quantities
naturally present independent of the particular orbit or speed of the
electron.

This effect is even more pronounced in atomic nuclei. There the
electromagnetic forces are much weaker than the binding nuclear forces, and
can be neglected to lowest order. However, even uncharged neutrons experience
a spin-orbit interaction

(16.134) |

This is just a drop in the proverbial bucket of accelerated systems. Clearly, accelerated, relativistic systems have a much more involved structure than that described by the Lorentz transformations alone. This becomes even more so when Einstein's revered equivalence principal is invoked, so that gravitational force and ``real'' acceleration are not (locally) distinguishable. But that is general relativity and far beyond the scope of this course.