The question was “What is the most difficult concept to grasp in physics?” and the answer by Alejandro Jenkins is a good one: the motion of rigid spinning bodies. A lot of the well-known weirdness in quantum mechanics and relativity, where physics seems most unlike everyday experience, actually turns out to be pretty simple, either in terms of calculation (we regularly teach sophomores to do calculations in quantum mechanics) or in concept (the math of relativity is very complicated, but on a conceptual level, you can distill the whole thing to one simple principle). If you put the question to me now, and didn’t let me steal that answer, I’d likely be forced to go a little meta in response. In a lot of respects, the hardest thing to grasp about physics isn’t any of the principles of the theory, but the mindset of the people who do physics.
The Universe is driven by four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. The behavior of matter within the Universes governed by their interactions via these forces. If these four forces are truly the only forces there are, then by understanding these forces we should have a full understanding of how objects interact. So when matter behaves in a way that is odd or unexplainable, one idea that often gets considered is that there might be another force at work. Perhaps there is a fifth fundamental force we haven’t yet discovered. Take, for example, the mystery of dark matter. Dark matter was first proposed to account for the fact that the motion of of
