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Even if you could go back, you can’t change the present

The theory of physics allows for time travel, but no matter how much we change the past, things always lead to the same future.
Going back doesn’t change what’s going to happen

Time travel may be a dream for many, but the bad news is that even if you could travel back in time, you can’t change what happened. That’s the conclusion of mathematical modeling by two researchers at the University of Queensland in Australia. But there is a bright side to the news. While the findings shatter our desire to change history, they also remove one of the major obstacles to time travel.
The logic goes like this: If what we do when we travel back in time doesn’t change anything, it doesn’t create some of the logical paradoxes that have led many experts to dismiss the possibility of time travel altogether. For example, when we travel back in time, we can’t stop our parents from falling in love and giving birth to us, or our own existence would be unfounded, as Martin McFly did in Back to the Future. In fact, we may not be able to change anything that has happened. So there is no logical conflict between who we are today and who we are back in the past. The logical obstacles to time travel have been removed, although the technical difficulties remain.
This idea has been around for decades, and Russian astrophysicist Igor Novikov has long proposed the “principle of self-consistency”, which states that if an event changes the past in any way, it has zero chance of happening: cosmic history does not allow for modification. Now, theoretical physicist Fabio Costa and Germain Tobar, who is doing a masters degree in mathematics at Cambridge University, have backed up the idea with calculations published last year in the journal Classics and Quantum Gravity. Their results, Costa says, “are like something out of science fiction.”
The risk of returning to the past

While no one knows how time travel is technically possible, Einstein’s theory of general relativity proves that the concept works, at least in theory. Specifically, the equations of general relativity allow the existence of closed time-like curves, which represent cycles of starting and ending points at the same point in space-time. An object that follows these closed time-like curves eventually returns to the time and place where it started, and may interact with its past self.
There is no guarantee, however, that such curves actually exist. Many prominent physicists have rejected the possibility of closed time-like curves – either naturally generated or manufactured by a time machine. “The laws of physics do not permit closed time-like curves.” Hawking wrote matter-of-factly in 1992. He half-jokingly calls this idea the “preservation chronology conjecture,” a feature needed for a “reassuring universe for historians.”
Even as a pure thought experiment, time travel faces many challenges. At the root of all this is the grandfather paradox. The paradox takes its name from the famous scene in which a man travels back in time and kills his own grandfather before he can leave any offspring. If this man is successful, he should not appear in the future as a descendant of his grandfather; But then, who killed grandpa? More abstractly, the grandfather paradox is the idea that any change to what has already happened can create logical inconsistencies.
The simplest solution to the grandfather paradox is to deny the possibility of time travel — if the laws of nature prevent us from going back in time in the first place, our actions cannot have consequences that defy logic. But Costa and Tobar’s model suggests another solution: once we go back in time, no matter how much we change what happened and try to create a paradox, things will always play out the same way, and eventually lead to a predetermined outcome. The math involved is fiendishly complex, but the researchers offer a popular example of how it could be applied to real life.
Nearly two years after the COVID-19 outbreak, many people might envision going back to the end of 2019 to prevent Patient Zero from becoming infected. But that would be illogical — if we could successfully prevent COVID-19, there would be no reason to go back, which is a variant of the grandfather paradox, and equally untenable. So we’re doomed: our flight to the source of the disease will be delayed, or we’ll slip on a banana peel while trying to catch the bats that spread COVID-19 to humans, or we’ll be stopped by some more mundane obstacle. The epidemic will continue to grow, we can be sure of that, because it actually does.
A story with no beginning

If this conclusion sounds strange, it’s because we tend to turn these puzzling causes and effects upside down — because COVID-19 eventually happened, we always step on banana peels when we go back in time. But the truth is, regardless of the outcome, the peel was already there. As Costa explains, “everything that ever happened happened once,” and it happened in a precise way. Our intuitive belief that the past can be changed stems in part from the idea that we imagine time travel as something outside the arc of history. But the truth is that time travel, like any other event, is entangled in history.
If you were to go back in time, you would always watch, always do what you’ve done. Reality, unlike a game, doesn’t give you as many opportunities to start “round one.” In a 1997 paper, “Are There Enough Banana skins for Time Travel?” by Australian philosopher Nicholas J. Smith, “If a time traveler were to travel back to some time in the past, he would have been there already.” He can influence the past, but he can’t change the ending. His actions will never change the course of events – it’s the only way things are going to go.
Although we’re used to thinking of time as linear, it’s an inherent human bias. It fits with what we know about time order, but “when it comes to time travel, this kind of reasoning doesn’t work because your story doesn’t have a clear beginning and end.” Costa said. Life is like pictures on a film, no longer side by side, but overlapping. “We told the whole story at once,” he says.
For physicists like Costa, there’s a practical lesson worth keeping in mind: if you study events in a time loop, you can’t extrapolate from initial conditions as you can in our familiar world. There are no “initial” conditions in this time loop, and even the word “initial” has lost its meaning. So Costa and Tobar changed their perspective. Their model starts not from some hypothetical beginning, but from a fixed set of variables that you can interpret as adult choices. From there, “physics will write the rest of the story.” Costa said.
So what’s the rest of the story? In a sense, people are still “free” and can act as they please. Once again, banana peels are not coincidences of some mysterious force calling on you to regulate your behavior. Instead, the peel will be there before you pass by. That’s why the world is the way it is today, not as a result of it. In the sacred river of time, Costa said, “the universe is just doing the only thing it can do, which is to align with itself.”
If time travel is feasible, it could cause other problems. For one thing, building a time machine might require an infinite amount of mass or negative energy, a source of energy we’re nowhere near tapping. Second, due to the nature of closed time-like curves, a time machine can only go back to the moment of its birth. So if we do manage to build or discover a time machine, our range will still be limited. You might want to give up your dream of riding a brontosaurus.
The complexity of time travel is enough to deter many scholars. “Time travel is not the most studied subject,” Costa said, “because it probably doesn’t exist.” But it remains a constant topic of wonder and fascination, and not for nothing. “What’s fascinating about time travel,” he added, “is that we can’t prove it’s impossible.”

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