That's true, but there wasn't any physics back in Indy's day.. and wearing a hat is kinda like not-wearing-the-helmet-that-goes-with-your-HEV-suit. 
:Nitpick: - Physics has been around since the ancient Greeks started messing around with rocks. Indy's 20th century.
Come come now, if Indy had decided to go into theoretical physics in the 20s they'd have laughed at him and asked what in god's name that was. XD