This is a highly debated topic in project management. Some say that project managers need to be subject matter experts in the project's topic, others disagree.
My answer is somewhere in between:
- If the project manager is experienced, then he can manage any project in any industry, without knowing anything about the technicalities of the project. He will base his estimates by triangulating estimates from team leaders, other project managers, and his past experience on similar projects.
- If the project manager is not experienced, then he should have some experience in the respective field that will make up for his lack of experience in project management.
In my opinion, the amount of necessary technical experience that the project manager needs in order to manage a project is inversely proportional to the amount of experience he has in project management. It's all a balancing act.