12.7 Clebsch-Gordan coefficients

In classical physics, combining angular momentum from different sources is easy; the net components in the , , and directions are simply the sum of the individual components. In quantum mechanics, things are trickier, because if the component in the -direction exists, those in the and directions do not. But the previous subsection showed how to the spin angular momenta of two spin particles could be combined. In similar ways, the angular momentum states of any two ladders, whatever their origin, can be combined into net angular momentum ladders. And then those ladders can in turn be combined with still other ladders, allowing net angular momentum states to be found for systems of arbitrary complexity.

The key is to be able to combine the angular momentum ladders from two different sources into net angular momentum ladders. To do so, the net angular momentum can in principle be described in terms of product states in which each source is on a single rung of its ladder. But as the example of the last section illustrated, such product states give incomplete information about the net angular momentum; they do not tell you what square net angular momentum is. You need to know what combinations of product states produce rungs on the ladders of the net angular momentum, like the ones illustrated in figure 12.3. In particular, you need to know the coefficients that multiply the product states in those combinations.

These coefficients are called Clebsch-Gordan

coefficients. The ones corresponding to figure 12.3 are
tabulated in Figure 12.4. Note that there are really three
tables of numbers; one for each rung level. The top, single number,
table

says that the net momentum state is
found in terms of product states as:

The second table gives the states with zero net angular momentum in the -direction. For example, the first column of the table says that the singlet state is found as:

Similarly the second column gives the middle rung on the triplet ladder. The bottom

tablegives the bottom rung of the triplet ladder.

You can also read the tables horizontally {N.29}. For
example, the first row of the middle table says that the
product state equals

That in turn implies that if the net square angular momentum of this product state is measured, there is a 50/50 chance of it turning out to be either zero, or the 1 (i.e. ) value. The -momentum will always be zero.

How about the Clebsch-Gordan coefficients to combine other ladders than the spins of two spin particles? Well, the same procedures used in the previous section work just as well to combine the angular momenta of any two angular momentum ladders, whatever their size. Just the thing for a long winter night. Or, if you live in Florida, you just might want to write a little computer program that does it for you {D.66} and outputs the tables in human-readable form {N.30}, like figures 12.5 and 12.6.

From the figures you may note that when two states with total angular momentum quantum numbers and are combined, the combinations have total angular quantum numbers ranging from to . This is similar to the fact that when in classical mechanics two angular momentum vectors are combined, the combined total angular momentum is at most and at least . (The so-called “triangle inequality” for combining vectors.) But of course, is not quite a proportional measure of unless is large; in fact, {D.67}.