A cover story on parallel universes?

Carl Sagan once admonished that extraordinary claims require extraordinary evidence. We’re certainly not claiming to present such evidence, but our cover story does address why some physicists consider that this darling of science fiction is nevertheless a respectable theory in science. They argue that Sagan’s admonishment has been met, in a way, because the hypothesis for other worlds emerged from the extraordinary evidence of the quantum world.

In our world, waves are waves and particles are particles. But in the sub-atomic quantum world, there’s no such distinction. That reality is dramatically illustrated by experiments that show electrons – particles of matter – interfere with each other just as they would if they were two ocean waves. Weirdest of all, a solitary electron can behave as if it existed in two places at the same time.

Physicists have never figured out what this means. Nevertheless, by 1926 they were able to mathematically describe the behaviour with a “wave function”. It has no precise position or speed until the moment you “detect” it and then it suddenly manifests as a particle.

However, from time to time, people have tried to understand what quantum mechanics says about reality. In 1953, Hugh Everett, a PhD student at Princeton University, had a flash of intuition. The weirdness of the quantum world could be explained by invoking the existence of other worlds. The moment you “detect” an electron, its probability wave splits into multiple worlds. In this world, you perceive it doing one thing but in another, a different you perceives it doing something different, in a way that has inspired any number of science fiction plots. But the “Many Worlds” concept, especially its recent articulation by Griffith University physicists Michael Hall and Howard Wiseman, does explain how a single electron might “interfere” with itself – it’s experiencing the effects of electrons from another world.

So is explaining something weird by proposing something weird a respectable way to approach physics? That remains hugely debated. On one hand, physics throws up weird stuff all the time – typically when maths is deployed to discern an underlying pattern in data. It was from the scrawl of Einstein’s general relativity equations that empty space was revealed as a fabric that could be dimpled by matter. But Einstein’s bizarre theory had a winning feature: it was testable. Physicists can’t really test for the existence of many worlds. But they can test to see if such models work to describe the quantum world, at least as well as the idea of a probability wave.

Weird and befuddling isn’t it? Take comfort in Richard Feynman’s adage: “I think I can safely say that nobody understands quantum mechanics.” And he was one of the greatest quantum physicists of the late 20th century. Perhaps it’s a matter of replacing one wacky idea with another. In this weird quantum world, may the best hypothesis win!