Ernst Mach, the Austrian physicist-philosopher of the late 19th century, famously denied the reality of atoms. “Have you ever seen one?” he mockingly asked of atom advocates. Today many scientists speak with similar derision about the idea that the visible universe is not alone, but rather is only one of many universes—a single bubble in a froth of cosmic carbonation known as the multiverse. You can’t see these other universes, so the idea is not testable, multiverse opponents allege…
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