A fter a century of attempts by Black activists and lawmakers, President Joe Biden signed the Emmett Till Anti-Lynching Act on the White House lawn, surrounded by Black politicians, clergy, and nonprofit leaders. The new federal law makes lynching a hate crime. Representative Bobby Rush, the bill’s sponsor in the House of Representatives, called the moment “a day of enormous consequence for our nation.” But I had questions. Doubts really. Do white supremacists kill Black people because we did not have a federal anti-lynching law? If not, then does Congress think that such a law will be a deterrent? Will...