I don't quite understand why it's fysikens lagar and not fysiks lagar. It just sounds awkward to use the genitive with the definite from: One of the physics laws.
In some cases we like to use the definite form for abstract nouns where English prefers to have them indefinite. I answered a similar question yesterday here: https://www.duolingo.com/comment/8679677
We think that fysiken is something specific, I know what physics we're talking about, you know what physics we're talking about, it's not 'just any physics', so it should be definite.
Why not "den"?
Edit: I think I have understood. In this kind of cases it is always det in the beginning.
Would “It’s a law of physics” be reasonable, or is there a different Swedish phrasing that would parallel that?