**What is the origin of complex probability amplitudes?** - William Wootters

I begin this presentation with an attempt to explain the origin of probability amplitudes in quantum theory, but the explanation makes sense only if those amplitudes are real. This result provides motivation for studying the real-vector-space variant of quantum theory. I show how a particular model within real-vector-space quantum theory can produce the appearance of complex probability amplitudes. In this model, a special binary subsystem of the universe, called the universal rebit or “ubit,” plays the role of the complex phase factor. In a certain limit the effective theory emerging from the model mimics standard quantum theory, but if we stop short of this limit the model predicts the spontaneous decoherence of isolated systems.