“Wheeler wasn’t the first to point out that quantum mechanics slips into paradox the minute you introduce a second observer. The Nobel Prize–winning physicist Eugene Wigner, for one, had emphasized it with a Schrödinger’s-cat-style thought experiment that became known as “Wigner’s friend.” It went something like this: Inside a lab, Wigner’s friend sets up an experiment in which an atom will randomly emit a photon, producing a flash of light that leaves a spot on a photographic plate. Before Wigner’s friend checks the plate for signs of a flash, quantum mechanics shows that the atom is in a superposition of having emitted a photon and not having emitted a photon. Once the friend looks at the plate, however, he sees a single outcome—the atom flashed or it didn’t. Somehow his looking collapses the atom’s wavefunction, transforming two possibilities into a single reality. Wigner, meanwhile, is standing outside the lab. From his point of view, quantum mechanics shows that until his friend tells him the outcome of the experiment, the atom remains in a superposition of having emitted a photon and not having emitted a photon. What’s more, his friend is now in a superposition of having seen a spot of light on the plate and not having seen a spot of light on the plate. Only Wigner, quantum theory says, can collapse the wavefunction by asking his friend what happened in there. The two stories are contradictory. According to Wigner’s friend, the atom’s wavefunction collapsed when he looked at the plate. According to Wigner, it didn’t. Instead, his friend entered a superposition correlated with the superposition of the atom, and it wasn’t until Wigner spoke to his friend that both superpositions collapsed. Which story is right? Who is the true creator of reality, Wigner or his friend?”