Determinism and Time Travel

Could You Go Back in Time and Kill Baby Hitler?

Introduction

Is causal determinism true? Is what’ll happen in the future predetermined by what’s happened in the past? The jury’s still out, but it’s a definite possibility. So let’s assume it is true for a second. In that case, it’s obvious that nothing would matter, right? The future’s already set in stone; whatever will happen will happen and there’s nothing anyone can do about it. We might as well just sit around and wait for the inevitable. It’s a depressing thought, right?

Probably! Thankfully, though, that’s not what causal determinism says at all. The thesis that everything is determined may well be true. In fact, I strongly suspect it is.¹ But a lot of people have a very inaccurate picture of what that entails. Granted, there is a sense in which, if determinism is true, the future really is set in stone, but it isn’t quite the sense you might think. And as we’ll see, the difference between the two sense is, practically speaking, pretty important!

In what follows, I’ll try to make it clear what causal determinism says and what it doesn’t say. And I’ll do it in the best way I know how: by talking about science fiction—in this case, sci fi scenarios revolving around time travel. Don’t worry, I won’t bore you with the complexities of general relativity or about how to resolve the grandfather paradox. I barely even understand the theory general relativity anyway, and I try to avoiding drawing attention to my own ignorance when I can!

No, my topic is a different one. To start, I’d like you to think about a question. Pretend that time travel is possible. Now assuming that that determinism is true, would you be able to travel back in time and change the course of the future? Think about it. No really, I’m serious. Stop reading right now and try to reason your way through it. Use a pencil and paper if you need to. Okay, now read out your answer. What is it? Did you answer no? I bet you did. If so, then congratulations! You’re wrong. Now let me explain why.

What Causal Determinism Says

As the American philosopher Dan Dennett has pointed out, people frequently confuse causal determinism with a very different view. He calls it “fatalism,” so I’ll do the same.² That confusion is exactly what I tried to illustrate above with the claim that what will happen will happen and there’s nothing we can do. As we saw, determinism doesn’t say that at all. So what exactly does it say?

Causal determinism is the view that the present state of the universe is determined by the conjunction of two things: first, the past state of the universe, and second, the laws of nature. Imagine the total state of the universe just a moment ago. Admittedly, this might be a little difficult. You’ll have to imagine account the state of every molecule, every atom, every particle in the entire universe, including its position and velocity. It’s a lot, I know, but try your best. I believe in you.

Now take it for granted that the laws of nature remain constant. Roughly, take it for granted that the regularities we’ve always observed in nature stay the same. Assume, for example, that the relations between force, mass, and acceleration won’t suddenly start yo-yoing around wildly, that large objects won’t just start popping in and out of existence willy-nilly, and that gravitational force won’t just vanish and send the planets careening out of their orbits.

If causal determinism is true, then the present state of the universe was determined by those two things. How things are right now, including the current position of every single particle in the entire universe, is the only way things could have been. It would have been physically impossible for things, including human thought and human action, to unfold any other way given how things were a moment ago and given the regularities in nature.

Here’s a fun way I like to illustrate this thesis. Imagine that we could simulate the entire universe at the moment right after the Big Bang in painstaking detail right down to the minutest particle. And then suppose that we got the simulation going. Maybe add that we can fast-forward it so we don’t actually have to wait for everything to happen at a natural pace. If causal determinism is true, what should we expect to see about 13.8 billion years down the line?

The answer is that we should expect to see exactly what we see now! The entirety of natural history would unfold in the exactly same way, such that, around the 13.8-billion-year mark, we’d find a guy just like me writing a short piece on determinism to post on an online platform just like this one. We’d find a user just like you reading it (and liking and subscribing, of course!) and having thoughts just like the ones you’re having right now (like thought that you should like and subscribe).

I said earlier that if causal determinism is true, there’s a sense in which the future is set in stone. And here we can see what that sense is. If determinism is true, then how things are right now is the only way things could have gone given the antecedent state of the universe. By the same token, whatever happens tomorrow or in ten or fifty or even ten thousand years will be the only thing that could have happened given how things are now.

What Causal Determinism Doesn’t Say

At this point you might feel like you’ve been had. I told you that the picture painted by causal determinism wasn’t a depressing one, but this sounds pretty depressing! Doesn’t it make you out to be the plaything of causal forces outside your control or understanding? To answer this question, we should distinguish between determinism and what we called fatalism above. This’ll help us get clear about what determinism doesn’t say.

Let’s be clear from the outset. Causal determinism is a theory about how the physical world operates. It says that events in the physical world are the product of prior events in conjunction with the laws of nature. This is eminently plausible. What I’m calling fatalism isn’t an alternate theory. No philosopher defends it, no physicist thinks it’s true. And the reason is that unless we make enormous unsubstantiated assumptions about what the world is like, it’s just not plausible.

The fatalistic view I’m talking about says that some events will happen no matter what anyone does. You see scenarios like this in fiction all the time. Call it fate, call it destiny, whatever, the point is, some events are just impossible to avoid. There’s no way to stop them from happening; you might as well try to stop the Sun from setting.

Here’s the kind of scenario I have in mind. A character learns some event has been fated to happen, say that they’ll die in a car crash. It’s literally unavoidable. If they make sure to drive very slowly, someone else will drive erratically and kill them. If they walk instead of taking the car, a drunk driver will take them out. If they resolve to hole up at home huddled up in a ball on the floor and wait things out, a car will somehow crash through their living room window and crush them. There’s no way out!

Why do I say that fatalism in this sense is implausible? Well, as I hope you can already see, it requires us to make really weird assumptions about how the world works. It really looks like there’d have to be supernatural forces for events to be fated or destined in this sense. Unless there are gods or demons or maybe just the Force pulling strings behind the scenes, it’s very hard to see how our attempts to avoid certain future events could be thwarted like this!

So now we get to the difference between determinism and fatalism. Causal determinism doesn’t say that what happens will happen no matter what anyone does. That doesn’t make any sense! How could that possibly be true? Instead, it says that what happens will happen precisely because people will do what they do.

Now of course, if causal determinism is true, then people will do what they do because they want what they want, and they’ll want what they want because of their physiology and environment, and that their physiology and environment will be the way they are because of a host of biological, social, and historical factors, and so on ad infinitum. But the point is that future events most certainly won’t occur the way they do unless people do what they in fact end up doing!

The Mission to Kill Baby Hitler

I find that just about the best way to explain the difference between causal determinism and fatalism is to consider what it would entail for time travel. Actually, looking at scenarios involving prescience of the kind featured in Minority Report (2002) or Dune (2021) works just as well. But let’s stick with time travel for now.

Time travel is an enormous headache. It just raises too many questions: Would you travel to an earlier point in the same timeline? Is it like a causal loop so that there were two of you the whole time à la 12 Monkeys? How does a causal loop like that even start existing? Would you instead create a brand-new timeline? But then des that mean the two timelines exist in parallel so you aren’t actually changing anything in the original one?

Instead of wading into this mess, let’s just sidestep it entirely. Never mind the paradoxes and contradictions, never mind if any of it is possible. Just pick whatever model of time travel you happen to favour and run with it. Imagine that you were to go back to the past with the intention of changing things are in the present. Might as well go with an audience favourite: suppose that you went back in time to kill Baby Hitler. Now the question is: Would it be possible for you to succeed and, in so doing, prevent World War II?

I really don’t mean to spoil things for you here, but yes. The answer is yes. Let’s try to reason through it. Remember, determinism says that, given some past state of the universe, the state of the future is determined. Now suppose you travel back to eighteen ninety-two or something. What now? Well, the past state of the universe has changed! There’s someone there—you!—who wasn’t there before! And from that, a different future may follow.

Of course, you may succeed in changing the future only a little bit. Suppose you step out of your time machine in Austria and immediately get run over by a horse and buggy. Well, now there’s a dead body around that wouldn’t otherwise have been there, so technically, you have changed the future (Congrats, by the way!). It’s just that you haven’t changed it by much. But nothing in principle prevents you from succeeding at your mission.

On an implausible fatalistic view, in contrast, something might prevent it. If Hitler has been fated to lead Germany headlong into an armed conflict with the Allied Forces, then there’s just nothing anyone can do about it. It’s not just that something might, by a stroke of misfortune, cause you to fail, but that something must cause you to fail, be it a horse and buggy, a serial killer, a fatal illness, or whatever. Something must happen to prevent you from completing your mission.

“Now wait a minute,” you might be thinking, “If determinism is true, then doesn’t that mean that I was determined to travel back and time? Doesn’t it mean it’s the only thing I could have done given the antecedent state of the universe?” Yes, exactly! Now you’re getting it! On the deterministic view, if we manage to develop time travel and send you back in time to kill baby Hitler, then that was the only thing that could have happened. There’s no other way history could have gone!

What does it mean exactly? Well, imagine rewinding things all the way back to the Big Bang again. What would happen? Exactly what did happen! Hitler would rise up, millions would be killed in World War II, then about eighty years later, we’d develop time travel and you’d go back to the nineteenth century, killing baby Hitler and so stopping World War II. Just like very, very confusing clockwork!

Conclusion

What’s the lesson here? Well, the main one is that causal determinism doesn’t say that what will happen will happen no matter what anyone does. Again, that doesn’t make any sense! In our scenario, World War II happens precisely because people do what they do in the decades leading up to it. If you manage, by travelling back in time, to change what people do, then you may well prevent it!

Is the resulting picture a depressing one? I guess mileage may vary. Causal determinism says that you’ll do what you’ll do because of antecedent factors like what your beliefs and desires are. But why’s that depressing? Take the example of ordering a pizza. Granted, it may be shameful and borderline sexually perverted to like Hawaiian pizza. But is it really depressing to think that the reason you ordered the Hawaiian pizza is that you desired one?

I can’t speak for you, but it’s very hard for me to see why it would be depressing! But in any case, whether it’s depressing or not, it certainly isn’t a good excuse to do nothing. Though of course, if you end up doing nothing, it’ll be because your desires and beliefs caused you to. But at the same time, just by pointing that out to you, I might be causing those beliefs and desires to change. Kinda makes your head spin, doesn’t it?

The bottom line is this. Even if what happens must happen, we can’t know what will happen until we get there. Although the course of the future may be determined by the state of the past, it is, once again, determined by what we do and not despite it. Here it looks to me like the Danish philosopher Søren Kierkegaard got it right. We can understand the complicated interactions of cause and effect when we look at life backward. But to get there, we have first to live it forward.

1. As more scientifically informed readers might already know, quantum mechanics might pose an obstacle to strict determinism. There are deterministic interpretations of quantum mechanics; however, the standard interpretation is indeterministic. Even if it turns out to be indeterministic, though, what might be true is near-determinism: determinism at the macro-level coupled with indeterminism at the micro-level.

2. Daniel Dennett, “I Could Not Have Done Otherwise—So What?” The Journal of Philosophy, Vol. 81, No. 10 (1984), p. 554.