Quantum Mechanics Is Putting Human Identity on Trial

“Even in principle, one cannot demand an alibi of an electron!”
Hermann Weyl, The Theory of Groups and Quantum Mechanics (1950)

Have you ever heard the story of Martin Guerre?

He lived with his bride and newborn son in Artigat, a small village in the Pyrenees foothills of Southwestern France. In 1548, at the age of 24, after being accused by his own parents of theft, Martin Guerre disappeared, leaving his family behind. Eight years later, after his parents had passed away, Guerre returned home, reuniting with his wife, son, and fellow villagers.

Over the next three years, Guerre and his wife, Bertrande, had two more children. All was going swimmingly until a foreign soldier came through town and claimed that the man who had returned was not the real Martin Guerre, but an imposter named Arnault du Tilh. The accuser claimed to have fought alongside Guerre in the Spanish army, and said that Guerre had lost a leg in battle. Bertrande ignored the accusation, certain that the man with whom she was living was, and had always been, her husband. But soon Guerre’s uncle and Bertrande’s stepfather joined the foreign soldier in accusing the man of forging Guerre’s identity, and took him to trial.

If electrons were distinguishable, all would be chaos.

It is a story that has persisted in our collective imagination—having been made into a movie, a musical, a historical novel, a TV series, and a Simpsons episode—because it strikes an ever-raw nerve: There is a sense in which our notions of identity are insecure. How can we be sure who someone is, even someone close? How can we be sure who we are, or that we are? What can identity mean in a world that is constantly changing?

The early vitalist philosophers had a ready answer: Each of us is distinguished by a divine soul, our physical bodies mere puppets animated by our invisible selves. But science has eroded this answer, and sought identity in the physical body itself: At a microscopic level, promises the reductionist dream, there must be something to distinguish each one of us from another. A hard-nosed foundation for our identity, one made of molecules and atoms.

That path, however, is far shakier than it might seem. Cast your gaze on Guerre, standing there in the courtroom. Zoom