Subatomic Physics

Part V
Models
“A model is like an Austrian timetable. Austrian trains are always late.
A Prussian visitor asks the Austrian conductor why they bother to print
timetables. The conductor replies: If we didn’t, how would we know how
late the trains are?”
V. F. Weisskopf
Atomic physics is very well understood. A simple model, the Rutherford model,
describes the essential structure: A heavy nucleus gives rise to a central field, and
the electrons move primarily in this central field. The force is well known. The
equation describing the dynamics is the Schrödinger equation or, if relativity is
taken into account, the Dirac equation. Historically, this satisfactory picture is
not the end result of one single line of research, but it is the confluence of many
different streams of discoveries, streams that at one time appeared to have nothing
in common. The Mendeleev table of elements, the Balmer series, the Coulomb law,
electrolysis, black-body radiation, cathode rays, the scattering of alpha particles,
and Bohr’s model all were essential steps and milestones. What is the situation with
regard to particles and nuclei? We have described the elementary particle zoo and
the nature of the forces. Are the known facts sufficient to build a coherent picture
of the subatomic world? The theoretical description of nuclei is in good shape:
There exist successful models, and most aspects of the structure and the interaction
of nucleons and nuclei can be described reasonably well. Although many nuclear
properties can be obtained from first principles (e.g., through a time-dependent
Hartree–Fock treatment), the complexity of the many-body problem usually leads
to the replacement of such a description by specific models. They involve the known
properties of the nuclear forces but focus on simple modes of motion. Much remains
to be done until nuclear theory is as complete and as free from assumptions as
atomic physics. The particle situation is in about the same shape. Many properties
of the particle zoo can be explained rather well in terms of quarks and gluons. The
so-called standard model, which includes QCD for the strong interactions and the
electroweak theory of Chapter 13, can be used to fit much data.