The physics professor who says online extremists act like curdled milk

Hate may be less like a cancer and more like bubbles, says Neil Johnson, who applies physics theory to human behavior

Lone wolves. Terrorist cells. Bad apples. Viral infections.

The language we use to discuss violent extremism is rife with metaphors from the natural world. As we seek to understand why some humans behave so utterly inhumanely, we rely on comparisons to biology, ecology and medicine.