Two mathematicians claim the logical inconsistency thought to lie at time travel's heart doesn't exist, and therefore it's theoretically possible one might be able to travel back in time and change reality. Unfortunately, time travelers might find the results frustrating.
If time is simply the fourth dimension, then it's natural to wonder why we can't move through it. “Einstein's theory of general relativity predicts the existence of time loops or time travel – where an event can be both in the past and future of itself – theoretically turning the study of dynamics on its head,” University of Queensland Honors student Germain Tobar said in a statement.
Many have been quick to point to the so-called “grandfather paradox” as a reason such travel can't occur. If a time-traveler prevents their grandparents from meeting, they will never exist, and therefore never be able to prevent the encounter. Numerous science fiction writers have made their time travelers dance around the problem, carefully changing things only in ways that don't prevent their journey.
Tobar was inspired by the equations Einstein created to describe these time loops. In Classical and Quantum Gravity, he and his supervisor Dr Fabio Costa show there is no logical requirement for time-travelers to be prevented from making non-trivial changes.
“Say you traveled in time in an attempt to stop Covid-19’s patient zero from being exposed to the virus,” Costa said. “However if you stopped that individual from becoming infected – that would eliminate the motivation for you to go back and stop the pandemic in the first place.”
The pair's solutions imply the universe would adjust. Although the model they describe in their paper involves a simplified world where cause and effect are represented by billiard balls bouncing off each other, they suggest more complex human environments operate the same way.
“In the coronavirus patient zero example, you might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would,” Tobar said. Although you might change history, it would only be in such a way you still exist and would want to make such a journey, avoiding any paradox.
Similarly, Tobar told IFLScience, a person who tried to launch themselves into a wormhole to travel back in time might miss slightly, damaging one eye in the process, which would then be the reason they couldn't target the hole accurately.
The pandemic example may make potential time-travelers question if the journey is worth the effort. Tobar acknowledges this but argues the changes might be unpredictable but could still be non-trivial.
If time travel is possible we might ask, in a version of Fermi's paradox, why travelers from the future haven't visited us, even to show up at events where they were specifically invited. “While time travel is logically possible, there could be further restrictions,” Tobar told IFLScience. “Alternatively, events readjust around anything that could cause a paradox so the paradox does not happen, so perhaps someone actually came back but word did not get out.” Instead of a government-run cover-up, as conspiracy theorists insist, the laws of physics might be hiding the truth.
Tobar added this work provides no guidance on how to time travel. Nevertheless, it could prove useful in other ways. The quest for Quantum Gravity, the theory that would unify General Relativity and Quantum Mechanics, is modern physics' holy grail. Costa told IFLScience that while the paper does not directly assist in finding Quantum Gravity, the different frameworks for dynamics developed in the paper could be applied to that search.