Could You Go Back In Time And Kill Your Own Grandfather?

Let’s talk about Avengers: Endgame for a second. In case you missed all of 2019 somehow, or you’re one of those folks who are too sophisticated for comic book movies, here’s the plot.

Bad guy Thanos obtained the six Infinity Stones that, once combined, provide the very specific ability to click your fingers and remove half of life from existence. It is unclear whether the combination of all of these powerful stones confers any other powers besides this one, weirdly specific thing, but I digress. Anyway, Thanos did the deed. The universe’s population has been reduced by a factor of half.

So, the heroic Avengers need to travel back in time to find historical versions of the Infinity Stones – because Thanos destroyed them after getting up to his genocidal mischief – and undo what he did. But then, the best Avenger, no-bullshit Colonel James Rhodes, AKA War Machine, AKA Don Cheadle, contemplates a simpler solution to their entire problem. Why not just go back in time and strangle baby Thanos, before he got all murder-y?

An excellent question! One asked before by such luminaries as Jeb! Bush, who was tasked with considering the ethics of killing baby Hitler. “Hell yeah, I would, you gotta step up, man,” said the man some people thought would be President before he was roundly beaten by an even more unlikely, almost certainly worse, candidate.

Funnily enough, the question of time and time travel is one that has plagued and entertained both philosophers and physicists. It’s also a great way to give yourself a headache, so let’s dive into the philosophy of time travel and see if we can support baby murder!*

*This is not a sentence I thought I’d write when I was receiving my diploma from philosophy school.

Can Time Travellers Change the Past at All?

Okay, so let’s start off with the biggest, baddest time travel conundrum of them all: whether it’s even possible to change the past. There’s no point garrotting baby Thanos (or Hitler, I’m going to use the baddies interchangeably here, so try to keep up) if it’s not going to stick and the future is still going to be genocidey — unless you’re into it for other reasons, in which case this is not the column for you and you should seek other help, quickly.

The best-known objection to the ‘going back in time to change it’ strategy is that it gives rise to a paradox: a logically unacceptable situation. And this particular time travel paradox has a name: the ‘Grandfather paradox’.

Here’s how it basically works. Let’s say you discover time travel, and you decide to travel back in time to when your biological grandfather was a kid. Why? I don’t know. Maybe he was a kindly figure to you and you thought it’d be fun to see him as a snotty little shit. Anyway, you travel back in time. And then, through a weird sequence of events, you kill him.

According to Back to the Future, you’d start to fade out of existence. If your grandfather didn’t grow up and have kids, your parents would never be born, and neither would you. (These are the thoughts I imagine Marty McFly was having while his Mum was trying to bone him.)

Let’s take the consequences of killing your kid grandfather a step further, shall we? If your grandfather had died as a kid, you never would have been born, which means you never would have gone back in time and killed him as a kid, which means he would have lived, had kids, and you would exist.

So, here’s the paradox: if you go back in time and change it, then the reason you had for going back and changing time disappears. You kill your grandfather, but then you’ve got no grandfather to go back and visit.

And the same goes for Hitler, or Thanos, or the people who decided to discontinue Paddle Pop Thickshakes. If you go back and time to stop them, then they never did the horrible thing you wanted to go back in time to stop them from doing, which means you never went back in time to stop them, which means they did the horrible thing, which means… fuck.

That’s the paradox. And lots of people have concluded that this paradox is enough to show us that time travel — in the sense of going back and changing the past — is impossible.

What Good Is Time Travel In Averting Disaster Anyway?

Maybe there’s a way to resolve all this time travel paradoxical nonsense. There are still a bunch of other philosophical questions we’d need to ask about time, like: does the past actually exist? What is time anyway? But let’s park those because I have a headache.

If we could resolve the paradox, there is still the problem of baby Thanos/Hitler/Paddle Pop Thickshake discontinuer being killed preventatively — that is, before they’ve committed the crime we are stopping them from committing.

Now, granted, these offences are outrageous, and maybe sacrificing one innocent life to prevent these evils is justifiable (maybe you could figure this out with the use of trolleys and train tracks). There’s a school of thought that believes preventative and deterrent punishment are ethically justified as a way to prevent harms from occurring. Still, generally speaking, we don’t favour killing the innocent.

German philosopher Immanuel Kant rejected the idea that ethics was just about “the greatest good for the greatest number” (a view known as consequentialism). Instead he argued that ethics was about doing what you are duty-bound to do — such as tell the truth and don’t kill.

He once considered the question of whether you could lie to save someone’s life. A murderer asks you for the location of a certain baby because he wants to murder him. Can you lie to save the baby’s life? Kant argued that you couldn’t — because you can’t guarantee that your lie will save the baby.

If you send the murderer to the bowling alley knowing the baby is upstairs, who’s to say the babysitter hasn’t taken the baby to the bowling alley without your knowledge? Suddenly you’ve told a lie and the baby is still dead, so you’ve made the situation worse overall.

In the case of Hitler, you would need to be certain his death would prevent the rise of Nazism and the Holocaust. If — as some historians argue — the rise of Nazism was a product of a range of social factors in Germany at the time, then killing a baby isn’t going to reverse those social factors. Butchering the babe might even create another power, equal to or worse than Hitler.

And you’ve still killed a baby.

Just Stop Killing Babies, You Weird Time Traveller

Okay, last thought. In applied ethics, we think a lot about the principle of necessity. If you’re doing something that seems bad-ish, you need to show that it’s necessary if you want to get the ethical tick of approval.

And here, the Baby Hitler plan starts to unravel. If time travel is possible it seems unlikely to be necessary to kill baby Hitler as opposed to, say, kidnapping him, adopting him out to a nice family whose explicit job is to make sure he doesn’t get into politics, or even offering him a scholarship to the Vienna School of Fine Arts (which he unsuccessfully applied to before becoming an evil, abominable monster). It’s a route YouTube sketch show How It Should Have Ended considers for Baby Thanos — adopting him into the Stark family.

We’re probably not going to be time travelling any time soon, if ever. Still, there’s something weird about how every thought experiment we come up with — trolley problems, ticking time bombs, baby Hitlers — is about killing people and becoming murderers. I don’t know what it says about our moral imagination, but it’s probably nothing good.

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Overthinking It is a new philosophy column on Junkee that aims to answer the big questions. For more Overthinking It, head here.

Matt Beard is a philosopher, ethicist, and fellow at The Ethics Centre.