The whole course of history changed in one single swoop.
When you start thinking about time travel, you pretty quickly get to the central and most difficult question: can you change the past?
Say by whatever means I have travelled to the past and found my grandfather when he was a teenager. I point a loaded gun at his head and pull the trigger. (This always seems to come down to killing grandfathers. If time travel is ever invented, will we start killing our grandchildren in pre-emptive self-defence?) What happens next?
What happens physically is that the hammer of the gun strikes the firing pin, this in turn strikes the rear of the cartridge, setting off a rapid exothermic chemical reaction in the propellant. The pressure of the resulting hot gas accelerates the bullet out of the end of the gun barrel in the direction of my grandfather’s head. On impact, the bullet penetrates my grandfather’s skull and enters his brain, where it decelerates and spins, transferring energy and momentum to the brain tissue and disrupting the physical structure of the brain, before exiting through the other side of the skull. This damage to the brain causes my grandfather’s central nervous system to shut down, quickly leading to the irrevocable cessation of his respiratory and circulatory systems.
In short, he dies.
The important point about this sequence of events is that every step along the way depends only on the physical conditions right then and there. How the gun came to be pointing at my grandfather’s head, the date of manufacture of the bullet, the history of the chemicals in the propellant – none of these things matter.
And this is the essence of the Grandfather Paradox. The laws of physics are local, each step following on from the step before, and yet, looked at globally, the result seems contradictory. How can I kill my grandfather, if killing him means I can never be born?
Physicists investigating the theory of time travel have a simpler – and less bloodthirsty – model that captures the same essential point. Imagine a region of spacetime that somehow functions as a time machine. Exactly how it works is not the issue, so long as it has the property that an object can enter it at a time t2 and emerge at an earlier time t1.
Now imagine shooting a billiard ball into the time machine at t2. It emerges at t1, as expected. But what if you arrange billiard ball’s trajectory at t2 such that, when it comes out at t1, it strikes its earlier self, deflecting it away from the time machine such that it never enters it at t2 after all?
At this point, there are basically three responses.
1. Time Travel Is Impossible
This neatly gets around all the problems by simply saying that the setup is impossible in the first place. You just can’t build a time machine, and that’s that.
While elegant, this response does have the problem that, as far as anybody can tell, the laws of physics that are currently known do not rule out time travel. So this answer is in effect saying “some as yet undiscovered law of physics will one day prove me right”, which is entirely possible, but not terribly satisfactory. I mean, you could say that about anything.
2. Only Self-Consistent Time Travel Is Possible
In this case, the ball is not deflected away from the time machine at all. Instead, when the ball emerging from t1 strikes its earlier self, it deflects the earlier ball just enough so that, when it enters the time machine at time t2, it is travelling at just the right angle and speed to deflect its earlier self into the time machine such that it emerges travelling at just the right angle and speed to deflect its earlier self into the time machine such that it emerges travelling at just the right angle and speed… and so on.
In the context of attempted grand-patricide, the scenario would be something like this. The bullet strikes my grandfather in the head, he collapses and I get back into my time machine cackling diabolically. But the shot does not kill him – he recovers, but is never quite the person he was, and suddenly the mild mental disabilities that had always afflicted him ever since I could remember but which the family never spoke about are all explained.
This gets rid of the contradictions, but it can’t help but seem a little contrived. Some influence must act across all of history to prevent actions that would otherwise be physically possible, and it’s not at all clear what that could be. We have the same problem as in the previous response, only much worse. It’s not so difficult to imagine that we will discover a law of physics that rules out time travel – it’s harder to imagine the kind of as yet unknown force that could act in this remarkable way.
3. Divergent Timelines
We looked at the many-worlds hypothesis in the last post. It’s the idea that, whenever an event has multiple possible outcomes, all of these outcomes occur simultaneously in different universes, with the universe splitting at the point of measurement. This is an attempt to explain the odd behaviour of measurements in quantum mechanics, the physical theory of microscopic systems, but it turns out that it can also resolve the apparent paradoxes of time travel.
In this many worlds or branching universes picture, there is no problem with you shooting your grandfather. Well, apart from the usual legal and moral issues involved in shooting anyone in cold blood, but we’re assuming here that you’re fine with those. The universe simply branches into two – one branch containing your grandfather’s slowly cooling corpse, the other containing you standing there with a jammed pistol and an awkward expression on your face while your granddad runs pell-mell from a freakish encounter that he never speaks of to anyone.
But if you do this, are you really changing the past? Or have you just travelled into different universe that always existed, in which your grandfather was shot by some passing maniac when he was young and you were never born, while the universe in which you were born, stepped into a time machine and were never heard from again carries on existing?
In the many worlds view of time travel, it is the latter. Going back to the simpler experiment with the billiard ball, the you in universe A sees the ball vanish into the time machine, never to be seen again, while the you in universe B tries to shoot the ball into the time machine but is frustrated when an identical ball appears and knocks into it in mid-flight, and is left with two billiard balls rolling around on the floor.
So we’ve achieved a self-consistent physical picture in which time travel exists without paradoxes, but at the expense of ditching – or at least heavily modifying – our intuitive ideas about what it means to change the past.
To see the implications of this, imagine that you are poised, gun aimed, ready to shoot your poor innocent grandfather, when some grumpy old bugger comes storming out of a police box and disarms you with a whack of his cane. “You can’t be allowed to change history,” he declares, “and it’s up to me to stop you!” But this aggressive senior citizen hasn’t stopped you changing history at all. All he has done is prevent you from moving from one universe to another, forcing you to remain in one where your grandfather lived through a perplexing and frightening event in his youth. There are still universes out there where your grandfather died in his teens, in which you do not exist at all.
So is there any point in trying to stop someone from changing the past? Well, no more than there is any point in doing anything. All possible futures might happen, but they do not all happen with equal probability, and from each choice branches off a cascade of universes, branches upon branches, and the higher the probability of an outcome, the more versions of the universe will spring from it. Perhaps the best and worst of all possible worlds exist simultaneously, but the number of different versions of their inhabitants, and the number of times their joy or suffering is duplicated across the universes, depends on how likely each outcome is.
As a time traveller, you have no more or less responsibility than anyone else to try to make a better future more probable. There is nothing intrinsically special about the history that you are familiar with – just because William won the Battle of Hastings in your history book doesn’t mean you shouldn’t give Harold a helping hand should you turn up in 1066. But the time traveller may have some insight into the consequences of their actions that is denied to the rest of us, some reason to think a particular outcome will be better or worse, some moral drive to make as many universes as possible as good as they can be.
A frequent time traveller, with vast experience of traversing a multitude of histories, might even take on the task of visiting key decision points, influencing the outcome as far as possible, saving at least some timelines from future horrors. Such a person would be special indeed, using knowledge of the future to bend whole universes towards their vision of a better life. One might almost call them a Lord of Time.