Nate raised his hand and said "Professor a ghost is the remains of a person that died, and they can;t move on for whatever reason. Whether it's they want to get revenge on the person that killed them, or they may not even know that they are dead. A ghost would normally haunt the area that they died in. Ghosts normally don't attack people they just like to scare people. They can be preffered to be seen as an apparition or they can stay invisible. But I think that a physics can see and/or sense them even when they are invisible."