Natalie Portman was on the London set of the Thor sequel, called Thor: The Dark World, today. Her leading man in the franchise, Chris Hemsworth, has been spotted in costume wielding his famous hammer this week too. Work on the action movie comes after Natalie spent time shooting opposite Michael Fassbender in Austin. The project with Michael required that Natalie Portman go blond, but she's now back to her natural brunette hair. Natalie's thrown herself into new endeavors this year, also working with Christian Bale on Knight of Cups.