In the previous two articles of this series, I argued that a time traveler changing the past would create too many logical contradictions, and possibly some unsavory philosophical problems. But that doesn’t actually completely rule out time travel to the past – well, if it were scientifically possible.
There’s one more option for the effects of a time traveler’s actions in the past – that they don’t change anything, because those actions have already happened. I call this the deterministic interpretation of time travel. Everything in the past has occurred, even if it was caused by a time traveler from the future.
Using this form of time travel in works of fiction may sound boring, at first. Where’s the fun if there’s no way to actually change anything? But, when well done, this conceit can really add to the entertainment and enjoyment. It’s the backbone of Christopher Nolan’s film Tenet and its slogan, “What happens happened.” I love how this movie reveals something new about events we’ve already seen, as the backward moving characters again experience and sometimes even cause previously unexplained moments in the past.
Determinism is also the form of time travel writer Zack Kaplan chose for his comic book miniseries Forever Forward. He often uses it to foreshadow major plot developments, and the main character explains it fairly well on multiple occasions.
In a way similar to Tenet, deterministic time travel plays a clever role in Harry Potter and the Prisoner of Azkaban. Throughout most of the story, many mysterious occurrences fortuitously help out Harry and his friends, sometimes even saving their lives. Later (spoiler alert!), we find out that all these strange things were actually caused by Harry and his friends themselves, who magically traveled back in time.
This interpretation of time travel might look logically consistent and actually possible, but there are some major problems with it. The biggest paradox probably comes from Robert A. Heinlein’s short story “ ’—All You Zombies—’ ” , which is the basis of the film Predestination. Now brace yourself – the twist is somewhat disturbing, and again – spoiler alert! In the story, a man (revealed to be intersex) finds out that, through multiple uses of time travel as well as sexual reassignment surgery, he is his own father and mother. Yes, you read that correctly.
Surprisingly, Predestination‘s timeline contains no other logical inconsistencies. Everything the protagonist does while in the past has already been taken into account in the narrative. Everything that happens, happened. And yet, where did this person come from? They apparently sired and gave birth to themself. With what genetic material? The person has no ancestors. This paradox of information and/or material coming from nowhere, sometimes derisively referenced with the phrase “there’s no such thing as a free lunch,” is very hard to argue away.
Finally, whether a time traveler could change the past or not, I think Fermi’s famous question about extraterrestrials also applies to time travel. Simply put, where are all the travelers? If time travel to the past were somehow ever discovered, then it would eventually be perfected, allowing travel to any time. Surely someone would’ve traveled back to our time by now. Obviously, there are no time tourists.
I feel like I should apologize for being a killjoy. In this series, I’ve argued that the current state of physics and the logical paradoxes involving causality – inherent in all forms of travel to the past – basically rule out the possibility of any kind of meaningful time travel, in the ways we commonly speak of it. Sorry, it just doesn’t seem to be in the cards.
But you know what? That doesn’t have to stop us from enjoying a good work of science fiction, where the story’s the thing, and the rules can relax a little — if you let them.
AIPT Science is co-presented by AIPT and the New York City Skeptics.
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