As the world passed more than 1 million COVID-19 deaths this week, I suspect I am not the only one who has fantasized about how a time traveler might spare the world from a plague by going back to China’s Hubei Province in the latter part of 2019.
It turns out that traveling through time might be the easy part. According to calculations by a physics student in Australia and his academic adviser, time travel is at least theoretically possible – but changing the course of events is not.
I do not pretend to have mastered the math behind the work of Germain Tobar, a physics student at the University of Queensland, and physicist Fabio Costa, but the logic is straightforward enough to discuss here. Tobar focuses on the question of “closed time-like curves,” or CTCs, which Einstein’s theory of general relativity predicted. CTCs mean that an individual’s personal timeline could theoretically double back on itself – time travel, in other words.
CTCs function within a universe made up of any number of “spacetime regions.” Each spacetime region is the end result of whatever occurred before it, just as the moment when I write these words – or the moment when you read them – is the product of everything that has occurred before. Within our immediate spacetime region, we have freedom of choice; we are agents, to use Tobar’s term. The things we do as agents are a process by which we determine the endpoint of the spacetime region we inhabit. The region that follows will have the endpoint of this region as its starting point.
There is communication between spacetime regions, but only in one direction. I am writing in my present, but you are reading in my future. I can communicate with where you are, but you cannot communicate back to me at this moment. Even if you climbed in a time machine and visited me now to persuade me not to write this blog post, I will write it. Maybe I will write it at a different time, or choose some different words. But the post will exist in a close enough form to achieve the same result, because the post was the motivation for you to come back in the first place.
The word does not appear anywhere in Tobar’s analytical treatise, but in my layman’s mind, I envision his work as a sort of mathematical proof of destiny.
More prosaically, Tobar and Costa present their paper as the contradiction of the “grandfather paradox.” This is not only a major objection to theoretical time travel, but an ingredient in many science fiction stories. You can’t interact with your past self or past events in ways that prevent you from coming into being to eventually interact with your past self or those past events. Think of poor Marty McFly, in danger of fading away in “Back to the Future.” In the writer’s room, you can’t prevent the protagonist’s grandfather from meeting the protagonist’s grandmother – at least not without taking steps to fix it somehow before the story ends. You certainly can’t have the protagonist go back and kill her grandfather before he has any children (the paradox’s classic form).
In the academics’ work, though, the grandfather paradox is no longer an issue. Go back and do whatever you like; events may change, but significant outcomes will stay the same. What’s significant? At a minimum, whatever motivated you to time travel in the first place would qualify. As Tobar put it, “Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency.”
I pictured a train where individuals are free to move about as one car pulls the next one forward. Whatever happens within each car may change, but the track is fixed and the train will ultimately arrive at the same destination. You can stay in your assigned car and watch the scenery go by. With the benefit of a time machine, you could theoretically move between cars – and watch the same scenery go by.
The study’s authors note that time travel via mathematical equations is one thing; time travel in the physical universe is another matter. At first blush, it is disconcerting to think that even if we could go back to China a year ago, when the novel coronavirus was likely first making its jump to homo sapiens, there is still nothing we could do that would save the lives of those 1 million victims. Or at least, that is how I read the paper’s logical conclusion – although I suppose it all depends on where the boundaries of the “spacetime region” lie. There is freedom of action within the box. A bigger box would mean a wider range of freedoms.
Philosophically, at least in the broadest view, this is not news. We know we will all die eventually. That does not stop us from doing the things that bring us satisfaction and comfort in the meantime.
That is how I parsed the paper as I read it. Fatalism and apathy do not add anything to life while we’re living it. Maybe we can’t change the train’s destination. But we can all do our part to make the journey as comfortable and rewarding as possible, for both ourselves and our fellow passengers.