Time travel and the theoretical existence of multiple histories are at the centre of new research from Brock University.

Assistant Professor of Physics Barak Shoshany, along with fourth-year Physics student Jared Wogan, recently published a paper titled “Wormhole Time Machines and Multiple Histories.”

In true sci-fi fashion, the paper discusses a theoretical model of a time machine created using a wormhole — a ‘shortcut’ through space and/or time. A wormhole can connect two very distant points in space, such as two stars, enabling quick travel between them without the limitation of the speed of light. It can also connect two points in time, potentially enabling travellers to go back in time. This is called a wormhole time machine.

While physicists do not yet know if wormholes can exist in our universe, it is possible to use them in theoretical models in order to investigate hypothetical scenarios. In their paper, Shoshany and Wogan used a wormhole time machine to analyze potential paradoxes that may arise if time travel was possible.

“The most well-known type of paradox is called the grandfather paradox,” says Shoshany. “In this paradox, a time traveller, let’s call her Alice, travels back in time and prevents her grandfather from meeting her grandmother. This means Alice will never be born. But if Alice was never born, then she could not have gone back in time in the first place — so Alice will be born. In other words, Alice is born if and only if she is not born, hence the paradox.”

In their paper, Shoshany and Wogan discuss a similar type of paradox that arises when sending an object back in time through a wormhole. The object emerges in the past and then collides with its past self,  preventing itself from going into the wormhole in the first place.

“This is essentially a mathematically precise way to formulate and study the grandfather paradox,” says Wogan.

One proposed resolution for time travel paradoxes is Novikov’s self-consistency conjecture, which suggests the past simply cannot be changed, and any attempt to change it is doomed to fail. In this scenario, Alice can never succeed in preventing her grandparents from meeting — or perhaps it turns out that she is the one who caused them to meet in the first place.

Another possible way to resolve time travel paradoxes is by assuming the existence of multiple histories (or timelines). In this scenario, travelling back in time causes history to split into two independent histories: the original one and a new one. In the original history, Alice was born. In the new history, Alice was not born. The new history cannot affect the old one and since Alice came from the original (unchanged) history, there is no paradox.

“In a previous paper, with my former student Jacob Hauser, we showed that a particular model of time travel paradoxes is simply impossible to resolve using Novikov’s conjecture, and therefore it must be solved by assuming the existence of multiple histories,” says Shoshany. “Hence, we claimed that if time travel is possible, then multiple histories must also be possible, as there is no other way to resolve certain paradoxes.

“However, the model we used in that paper was simplistic and unrealistic. In the new paper, with Jared Wogan, we created a new model that is completely realistic — provided wormholes can exist in our universe, of course. This provides much stronger evidence for the claim that time travel necessarily implies multiple histories,” explains Shoshany.

Potential time travellers may find the idea of multiple histories disheartening, as it means their own history can never be changed; they can only create new histories. If the time traveller prevents a nuclear war, then the war will not take place in the new history, but it will still take place in the original history. There is simply no way to change that.

“There are several possible forms in which multiple histories may manifest. One of them is related to the Everett, or ‘many-worlds,’ interpretation of quantum mechanics, which, in simple terms, means there is an infinite number of histories and anything that can happen, does happen in one or more of those histories,” says Wogan. “One of the things that can happen is that a time machine appears and a time traveller comes out of it. So, if the Everett interpretation is indeed true, it would provide a natural way to resolve time travel paradoxes without introducing any new physics.”

If this quantum manifestation of multiple histories is true, then the history where the nuclear war didn’t happen already exists, even without the time traveller’s intervention, and, to make things worse, there are other histories where an even more terrible apocalypse event happened. This might mean that time travel could, in a sense, be pointless, at least as a form of changing history, says Shoshany.

Shoshany and Wogan’s paper is available here.