Modern physics is filled with ideas that defy common sense, but, nonetheless, have a strong claim to be true. A prime example is Einstein’s twin paradox, about identical twins who age at markedly different rates if one takes a trip at near-light speed. Another is Erwin Schrödinger’s thought experiment involving a cat that is arguably both dead and alive due to a fundamental feature of quantum mechanics.

But no theory strains credulity like “Many-Worlds,” the proposal that we experience only one branch of possibly infinite realities, which sprout like the limbs on a tree, splitting into numberless parallel universes—all of which are equally real. Officially called the Many-Worlds Interpretation (MWI) of quantum mechanics, this seemingly absurd idea may be what’s really going on in our multiverse.