You don’t have to tattoo your field of study across your forehead. If you build your identity out of tribe memberships, attachments to people, and borrowed identities (even the identity of being a 'physicist' counts as a borrowed identity, from one generation to another), it’ll fall apart the first time the wind changes in the wrong direction. 

The real stuff that makes you, you, comes from how you move through the world, the way you keep chasing after things, the way you are around other folks, the way you think and feel, even when the ground shifts under your feet. 

Belonging isn’t some permanent tattoo either. Sometimes you find yourself in a pack, all chasing the same thing for a while, and then the road forks and everyone scatters. That’s just how it goes.