Breaking: Time Travel Without Mind-Bending Paradoxes Is Possible

If you’re looking to travel back in time to change the course of historical events—an option that might seem particularly attractive in late-stage 2020—be aware that your temporal tampering is likely futile, according to a recent study.


On the other hand, if you are a hapless Marty-McFly-type time traveler, you don’t have to worry about triggering the “grandfather paradox,” in which you prevent your own existence due to your actions in the past.


In both cases, any stopgap measures to divert the arrow of time will simply cause events to find a new way to produce the original outcome, reports the study, published this month in the journal Classical and Quantum Gravity.


“Agents could have free choice to make any action they wanted and no paradox would arise because the events just adjust themselves to remain consistent,” said Germain Tobar, an honors undergraduate student at the University of Queensland (UQ) who co-authored the study with physicist Fabio Costa, in a call.


To be clear, it’s not known whether time travel is possible in the first place. However, Einstein’s theory of general relativity allows researchers to speculate about what an observer with a time machine might expect from a journey away from the present, assuming it could happen.


Closed timeline curves (CTCs)—timelines that loop back to their starting point—are among the trippiest concepts to emerge from general relativity, because they suggest that an object could travel back in time and interact with its past self.


With the help of Costa, Tobar mathematically modeled whether such interactions within a CTC would produce meaningful inconsistencies in a given sequence of events. The project is one of many collaborations between students and faculty at UQ.


“UQ has a lot of opportunities for undergraduate students to get involved in research,” said Tobar, who is pursuing a bachelor’s of advanced science. “I’ve always been really interested in the topic, and it stood out above all the others.”


The new research builds on a 2019 study, co-authored by Costa, which concluded that “three parties can interact in such a way to be all both in the future and in the past of each other, while being free to perform arbitrary local operations”—in other words, without paradoxes.


“All previous attempts to study the motion of objects through these time loops have run into these paradoxes because the dominant paradigm in physics is you give a system an initial state, and then from that initial state, you calculate the full history of the system,” explained Tobar.

“If I throw a ball, I can calculate where and when it will be at any time,” he added. “But in these time loops, that just leads to a paradox.”


The 2019 study identified one logical process that allowed for time travel in CTCs without invoking the kinds of causal laws that lead to famous paradoxes. In the new research, Tobar and Costa broaden the scope of those initial findings, ultimately outlining multiple new processes and bolstering the hypothesis that paradox-free time travel may be consistent with fundamental cosmic laws.


The idea behind the two new studies “was to shift the perspective and try to describe the way things move without initial conditions,” Tobar said. “If we are successful in generalizing it in this way, we might be able to describe how things move through these time loops without any paradox.”


The findings hint at a rigidly deterministic universe that seems to overlap with the tropes of ancient Greek tragedies. The famous lines of the ancient play Prometheus Bound —“the strength of necessity cannot be resisted”—apparently also applies to interactions in CTCs.


Of course, all of these ideas are situated in an abstract framework because it is not currently possible to empirically test them out. CTCs are only predicted to exist in extreme regions of spacetime, such as the areas right outside of spinning black holes, so these concepts are likely to live in the world of theory for the time being.


“I can’t see in the immediate future any kind of way of experimentally verifying or observationally verifying that [CTCs] do exist,” said Tobar. “Either you need to go to a rotating black hole, or you need to create negative mass and negative energy to create one. Both of those things, I don’t think, are going to happen for a very long time.”

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