If you’re a fan of sci-fi movies, you’re probably familiar with the idea of alternate universes – hypothetical planes of existence with different versions of ourselves. As far from reality as this may seem, it is a question that scientists have considered. So how does fiction compare to science?
The many-worlds interpretation is an idea in physics that supports the concept of multiple universes existing. This stems from the way we approach quantum mechanics, which defies the rules of our ordinary world. Although it is impossible to test and is considered an interpretation rather than a scientific theory, many physicists believe it might be possible.
“When you look at the ordinary world, things are measurable and predictable – if you drop a ball off a roof, it will fall to the ground. But when you look at the very small scale in quantum mechanics, the rules stop working. Instead of being predictable, it’s about probabilities,” says Sarah Martell, associate professor in the School of Physics, UNSW Science.
The fundamental quantum equation – called the wave function – shows a particle occupying many possible positions, with different probabilities assigned to each. If you were to try to observe the particle to determine its position – known in physics as the “collapse” of the wave function – you would find it in one place. But the particle actually occupies all the positions allowed by the wave function.
This interpretation of quantum mechanics is important because it helps explain some of the quantum paradoxes that logic cannot answer, such as why a particle can be in two places at once. Although this may seem impossible to us, since we experience time and space as fixed, mathematically it adds up.
“When you make a measurement in quantum physics, you are only measuring one of the possibilities. We can work with this mathematically, but it is philosophically uncomfortable that the world ceases to be predictable”, a/Prof. said Martelle.
“If you don’t dwell on the philosophy, you just go on with your physicality. What if the other possibility were true? This is where this idea of a multiverse comes in.
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The quantum multiverse
As depicted in many science fiction movies, the interpretation of multiple worlds suggests that our reality is just one of many. The universe is meant to split or divide into other universes every time we act – whether it’s a molecule in motion, what you decide to eat, or your choice of career.
In physics, this is best explained by Schrödinger’s cat thought experiment. In the many-worlds interpretation, when the box is opened, the observer and possibly living cat split into one observer looking at a box with a dead cat and another looking at a box with a living cat.
“One version of you measures one outcome, and one version of you measures the other outcome. That way you don’t have to explain why a particular probability resulted. It’s just whatever could happen, happens, somewhere,” a/Prof says Martelle.
“This is the logic often portrayed in science fiction, as Spider-Man: Into the Spider-Versewhere five different Spider-Man exist in different universes based on the idea that there was a different event that established each one’s progression and timeline.
This interpretation suggests that our decisions in this universe have implications for other versions of ourselves living in parallel worlds. But what about the possibility of interacting with these hypothetical alternate universes?
According to the multiple worlds interpretation, humans could not interact with parallel universes as they do in the movies – although science fiction has creative license to do so.
“It’s a device used all the time in the comics, but it’s not something physics would have anything to say about,” a/Prof. said Martelle. “But I love science fiction for the creativity and the way little science facts can become a character’s motivation or a story’s essential crisis with characters like strange doctor.”
“If for nothing else, science fiction can help make science more accessible, and the more we get people talking about science, the better,” Prof. said Martelle.
“I think we’re doing a lot of good by putting hooks out there that people can grab. So if we can get people interested in science through popular culture, they’ll be more interested in science than we do.