My dentist says you can't have a gold crown atop a tooth which has a silver filling since it creates a magnetic. other opinions?

My opinion\advice: Technically and theoretically, two dissimilar metals in the presence of saliva which is acidic may create a galvanic response causing some 'shock'. In reality, this has never been a problem for me or my patients. It can happen and is often mild and temporary. Other options are to replace the silver filling with a composite filling, or use a ceramic crown.

Electrolysis: A gold crown can be done over a silver filling. However, there is a possibility of electrolysis occurring when 2 different metals are put in a liquid environment like the mouth. A small electric charge will pass between metals--more likely to occur between a silver filling and a gold crown rather than in a single tooth with a silver core covered by a gold crown.