On the morning of June 28, 1914, a Bosnian Serb student named Gavrilo Princip stood outside Moritz Schiller’s delicatessen near the Latin Bridge in Sarajevo. Sometime after 10:45 A.M., a motorcade carrying archduke Franz Ferdinand, heir to the throne of Austria-Hungary, passed within meters of Princip, who drew his 0.38-caliber pistol and fired. One bullet struck the archduke in the neck. He was rushed to the military governor’s residence for medical treatment, but by 11:30 A.M. he was pronounced dead.
The assassination helped spark World War I. Historians view history as a series of interconnected but highly contingent events—built of myriad and mostly unseen chains of cause and effect. If Princip’s gun had jammed, the thinking goes, the archduke would have lived, and Europe’s subsequent history may well have been very different. Fiction writers have long been enthralled with these what-ifs (known to philosophers as “counterfactual histories”): What if Hitler hadn’t flunked out of art school? What if the Germans had developed the atomic bomb before the Americans? What if John Lennon had never met Paul McCartney? What if an asteroid hadn’t wiped out the dinosaurs some 65 million years ago and reptiles still ruled the Earth?
Such contingencies presume, of course, that things could have been different—either because a person exercising their free will could have chosen another course of action (Princip could have chosen not to pull the trigger) or because random events (such as the asteroid strike) could have unfolded differently. But is this attitude compatible with physics? Do the natural laws of the universe allow for free will?
Scientists and philosophers have been arguing over the question for centuries and are often torn between two competing poles. Some think, Yes, you obviously have free will. (Aren’t you already four paragraphs into a story that you freely chose to read?) Others think, No, you can’t possibly have free will because the laws of physics say that whatever happens was determined by what happened immediately before—and the happenings within human minds are no exception. Recently a new argument for why quantum mechanics is even more deterministic than physicists might have thought has sparked the debate anew.
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