There's an idea that suggests all the universe's electrons are actually one particle forever traveling backwards and forwards in time. It's a simple, elegant idea that solves some of physics's biggest mysteries. There's only one tiny problem. It's complete nonsense.
This is the story of that bizarre thought experiment and John Archibald Wheeler, the brilliant, largely unsung physicist who came up with it.
Like so much of the quantum world, electrons are strange. What's worse, they're all strange in exactly the same way. Every electron is identical to every other electron. They all have the same mass, the same electric charge, and the same spin. (For more on what these terms mean, check out our field guides here and here.) Electrons are just one of the indistinguishable particles – other examples include photons, neutrinos, protons, neutrons, and indeed most of the subatomic particles.
This isn't a trivial point, either. Not only is it impossible to tell electrons apart based on their physical properties, it's essentially impossible to tell them apart at all. This is because determining specific electrons by their position would require measuring their trajectories with exact precision, and the laws of quantum mechanics forbid this. Between measurements, electrons in the quantum world are probabilistic, defined by wave functions that give the odds of finding that particle in any given position. When the wave functions of multiple electrons overlap, it becomes officially impossible to determine which of the electrons was the one that was originally measured.