FIGURE 5.19 Two very different probability densities with exactly the same energy.
E = 13E0. This degeneracy arises from interchanging nx and ny (which is the
same as interchanging the x and y axes), so the probability distributions in the
two cases are not very different. However, consider the state with E = 50E0, for
which there are three sets of quantum numbers: nx
= 7, ny
= 1; nx
= 1, ny
= 7;
and nx
= 5, ny
= 5. The first two sets of quantum numbers result from the interchange
of nx and ny and so have similar probability distributions, but the third
represents a very different state of motion, as shown in Figure 5.19. The level
at E = 13E0 is said to be two-fold degenerate, while the level at E = 50E0 is
three-fold degenerate; we could also say that one level has a degeneracy of 2,
while the other has a degeneracy of 3.
Degeneracy occurs in general whenever a system is labeled by two or more
quantum numbers; as we have seen in the above calculation, different combinations
of quantum numbers often can give the same value of the energy. The number
of different quantum numbers required by a given physical problem turns out
to be exactly equal to the number of dimensions in which the problem is
being solved—one-dimensional problems need only one quantum number, twodimensional
problems need two, and so forth. When we get to three dimensions,
as in Problem 19 at the end of this chapter and especially in the hydrogen atom in
Chapter 7, we find that the effects of degeneracy become more significant; in the
case of atomic physics, the degeneracy is a major contributor to the structure and
properties of atoms.
5.5