One Major Problem With Time Travel May Not Be An Issue After All

Anyone who's seen "Back to the Future" should surely be familiar with the grandfather paradox. In the movie, Michael J. Fox's Marty McFly travels back in time only for a younger version of his own mother to fall for him. The film revolves around Marty trying to get the younger versions of his parents to fall in love again in order to secure his own existence in the future. This concept might have made for a brilliant premise for a beloved '80s classic, but it is also a very real paradox that theorists have tried to get around for decades.

French journalist and sci-fi writer René Barjavel first gave a name to the paradox back in 1943, when he asked what would happen if an individual traveled back in time and killed his own grandfather before that individual's parents were born. The paradox Barjavel was getting at was that if the grandfather was killed in the past, then the individual's parents would never be born, and therefore the individual themself would never be born and would be unable to travel back in time in the first place. 

The grandfather paradox has been a major issue haunting the discourse around time travel ever since it was first put forward. Some theorists have suggested it can be solved by imagining parallel universes being created every time the time traveler makes some sort of change — a fascinating idea in and of itself, but not enough to do away with the grandfather paradox entirely. However, it now looks as though this issue may have been solved by an Australian physics student, who has seemingly figured out a way for time travel to work without the grandfather paradox becoming an issue.

Back in 2020, an undergraduate student in Australia used mathematical modeling to show that time travel is possible without causing paradoxes, and without the need for a theory that posits multiple timelines. Germain Tobar, a physics student from the University of Queensland, carried out calculations that show paradox-free time travel is possible because events essentially rearrange and adapt themselves around whatever changes are made.

With the help of University of Queensland physicist Dr. Fabio Costa, Tobar published his modelling in Classical and Quantum Gravity, in a piece entitled "Reversible dynamics with closed time-like curves and freedom of choice." Speaking to the University of Queensland's news site, Dr. Costa summed up Tobar's findings, saying, "Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency."

The research itself is fairly complex, but Tobar laid out his approach to the University's press in simpler terms. He explained how classical dynamics says that "if you know the state of a system at a particular time, this can tell us the entire history of the system," before explaining how one of Einstein's major breakthroughs, the theory of general relativity, predicts the existence of time loops or time travel. This prediction upends classical dynamics as it posits that an event can be both in the past and future of itself. As Tobar put it, "The current science says both theories cannot both be true." So, with Dr. Costa supervising, Tobar set out to find a way to reconcile the two, and claims to have been able to "square the numbers."

While the math involved in squaring the numbers is quite in-depth, Dr. Fabio Costa provided a helpful example in his University of Queensland interview. In the theoretician's hypothetical scenario, he imagines an individual traveling back in time to prevent the spread of a disease, in this case COVID-19. The paradox comes into play when you consider that if this individual was successful in preventing patient zero from being exposed to the virus, that would prevent the disease from spreading, thereby removing the individual's reason for traveling back in time in the first place. 

It's here that Germain Tobar's work comes into play. According to the research, in this specific scenario something else would occur in order for the virus to be released, thereby reinstating the individual's future motivation for traveling back in time. As Tobar put it, "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." Essentially, Tobar's work suggests that space time would adapt to whatever action is taken by the time traveler, thereby removing any paradox. 

Thus far, time travel has been the stuff of science fiction. Various theories and potential methods for traveling through time have been put forward at various points, with one 2017 paper published in the journal Classical and Quantum Gravity describing how to build a time machine in a relatively simple way. Writing for ScienceAlert, University of Massachusetts, Dartmouth, physics professor Gaurav Khanna explained how his doctoral student, Caroline Mallary, offered "a very simple design" that involved "two super long cars" parked in parallel. In the theoretical experiment, one of these cars rapidly moves forward while the other remains stationary, with Mallary showing that a time loop can be found in the space between the two.

Still, even this model relies on objects known as singularities that have infinite density residing at the center of each car. Unfortunately, these theoretically "bare and observable" singularities are not thought to exist in nature, therefore making Mallary's model impossible to recreate, at least for now.

However, Stephen Hawking suggested in his final book that time travel may indeed be one day possible. What's more, while there are several myths surrounding black holes, it remains true that near the event horizon of these mysterious cosmological objects, a few hours could pass while 1,000 years might pass on Earth. This is due to Albert Einstein's general theory of relativity, which states that gravity affects the passage of time, and since gravity is highly concentrated around the lip of a black hole, time would pass much slower. All of which is to say that time travel is not beyond the realm of possibility, and if we ever figure out how to travel back in time, Germain Tobar's work suggests that we might be able to do so entirely paradox-free.