Equivalence and Duality in Electromagnetism

In this paper I bring the recent philosophical literature on theoretical equivalence to bear on dualities in physics. Focusing on electromagnetic duality, which is a simple example of S-duality in string theory, I will show that the duality fits naturally into at least one framework for assessing equivalence---that of categorical equivalence---but that it fails to meet a necessary condition for equivalence on that account. The reason is that the duality does not preserve empirical content in the required sense; instead, it takes models to models with dual empirical content. I conclude by discussing how one might react to this.