Now we’re starting to get to the really meaty stuff. We talked about the phase symmetry of the complex scalar field:
which basically wants to express the idea that the physics of this field only really depends on the length of the complex field values and not on their phases. But another big principle of physics is locality — what happens here doesn’t instantly affect what happens elsewhere — so why should the phase change be global?
To answer this, we “gauge” the symmetry and make it local. The origin of the term is fascinating, but takes us too far afield. The upshot is that we now have the symmetry transformation:
where is no longer a constant, but a function of the spacetime point .
And here’s the big problem: since varies from point to point, it now affects our derivative terms! Before we had
and similarly for . We say that the derivatives are “covariant” under the transformation; they transform in the same way as the underlying fields. And this is what lets us say that
and makes the whole Lagrangian symmetric.
On the other hand, what do we see now?
We pick up this extra term when we differentiate, and it ruins the symmetry.
The way out is to add another field that can “soak up” this extra term. Since the derivative is a vector, we introduce a vector field and say that it transforms as
Next, we introduce a new derivative operator: . That is:
And we calculate
So the derivative does vary the same way as the underlying field does! We call the “covariant derivative”. If we use it in our Lagrangian, we do recover our symmetry, though now we’ve got a new field to contend with. Just like the electromagnetic potential we use the derivative to write
which is now symmetric under the gauged symmetry transformations.
It may not be apparent, but this Lagrangian does contain interaction terms. We can expand out the second term to find:
Our rules of thumb tell us that if we vary the Lagrangian with respect to we get the field equation
which — if we expand out as if it’s the Faraday field into “electric” and “magnetic” fields — give us Gauss’ and Ampère’s law in the presence of a charge-current density .
The charge-current, in particular, we can write as
or, in a gauge-invariant manner, as
which is just the conserved current from last time with the regular derivatives replaced by covariant ones. Similarly, varying with respect to the field we find the “covariant” Klein-Gordon equation:
and, when this holds, we can show that .
So we’ve found that if we take the global symmetry of the complex scalar field and “gauge” it, something like electromagnetism naturally pops out, and the particle of the complex scalar field interacts with it like charged particles interact with the real electromagnetic field.