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Quantum mechanics offers one way that parallel universes could exist. A tiny subatomic particle can exist in a state called superposition, which means it is not in one specific place. Instead it exists in a range of spaces, each of which has some probability of being “correct”. Only when an experimenter observes the particle does its state become specified: it “collapses” from a range of possible positions into one.
Quantum physicists disagree over how to interpret the collapse. One interpretation, put forward by Hugh Everett in the late 1950s, is that the particle doesn’t really “choose” one single position. Instead, every possible option plays out in a different universe. Wherever we observe the particle is where it is in our Universe, but in other universes it is somewhere else. This became known as the “many worlds interpretation” of quantum mechanics.
There is no physical evidence that these other universes exist, and it is difficult to imagine what would constitute evidence. However, other interpretations of quantum mechanics have problems of their own, so the many worlds interpretation remains of interest.
If it is true, it offers a way to time travel into the past, and change things, without causing paradoxes. One of the first physicists to propose this was David Deutsch in a 1991 study. He argued that if you travel into the past and change something, you will create a second universe, parallel to the original one. In this second timeline, the altered version of history plays out – while in the original things go on as they always did.
On this view, you might be able to travel into the past, but “you can’t go back on your own worldline,” says Osborne.
Shoshany and his colleagues have developed this idea further. In a 2020 study, they showed that this version of time travel need not lead to infinite numbers of timelines. Infinities are generally a sign that a theory is wrong, says Shoshany, so this means it is more plausible. In February this year, they published a more realistic version of this model, which again avoided paradoxes and infinities. The following month, they explored the scenario in more detail, showing that a branch in the timeline would start out localised and gradually expand to affect more of the Universe.
It turns out that there are many possible scenarios that could take place if we were somehow to travel into the past. Right now, there is no way of conducting an experiment to determine which, if any, of these scenarios of time travel is correct. Finding the correct answer will require understanding how and why the arrow of time really exists. Until we have greater clarity on the laws underpinning the Universe, we can’t choose between our ideas about time travel.
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