Backward Time Travel Is Impossible

Everyone knows that time travel is real – it happens every day. Every time an astronaut launches into orbit, he or she experiences very slight time dilation, which means that they experience time at a slower rate than people on Earth. There are also creatures that freeze themselves during the winter, experiencing a state of suspended animation. Each of these techniques could allow a being to theoretically transport themselves far into the future. Unfortunately, without the ability to return to the present, this isn’t very useful.

Have you ever wondered what it would be like to go back in time? Well if you have any imagination at all, chances are you have. After all, who wouldn’t want to go back and walk with the dinosaurs, hang out with Abraham Lincoln or invest in Microsoft? The possibilities are endless – at least they would be if backward time travel was possible.

Before we can discuss its impossibility, we must first discuss the different theories about how backward time travel could potentially work. Of course, the technological requirements remain unknown, but the theory regarding how backward time travel would affect our world can be divided into the following general categories, summarized in the following table:

Characteristic Time Travel Theory
Fixed History Flexible History Alternate History
Description There is only a single, unchangeable timeline There’s only a single timeline, but we can change it Each action in the past produces a new timeline
In Film 12 Monkeys, The Terminator Back to the Future, Looper Terminator 2: Judgment Day, Star Trek (2009)
Problems No free will Paradoxes Can’t return to original timeline

The difference between the three interpretations can be illustrated using the story of a man who travels back in time to assassinate an infant Adolf Hitler in order to prevent World War II.

If history is fixed, then the man will not be able to prevent Hitler from coming to power. In fact, his actions will actually prove necessary for this to happen. Perhaps the man fails in his attempt to murder the child, and by doing so, scars the child and plants the seed necessary for Hitler’s evil actions. Or maybe he successfully dispatches the infant, but the family later adopts another child, naming it after their lost son, and the adopted child becomes the Hitler from history. Whatever the case, the man’s actions can’t affect the future because, from the vantage of the future, they’re already a part of the past.

With a flexible history, the man may successfully kill Hitler and prevent World War II. However, by preventing such a monumental historical event, the entire future is changed. Perhaps his grandparents never immigrate to the United States, so his parents never meet and, consequently, he’s never born. And if he’s never born, then how can he go back in time and kill Hitler? Of course, if the man doesn’t exist, then World War II happens, and the man now exists. This is known as a grandfather paradox, and it is inescapable, which is one reason why the third theory seems so attractive.

Perhaps the man does travel back in time and successfully assassinates Hitler, but instead of changing the future, an entirely new future is created. This understanding of backward time travel allows the man to change the past while preventing paradoxes from occurring. However, because the man has created a new timeline – a new universe – he is now unable to return to his own. After all, if he really has changed the past, then the future he knew no longer exists, or it exists in an alternate reality.

Another reason why this view seems to make sense is the growing belief that there are an infinite number of parallel universes. In a feeble attempt to understand the implications of quantum physics, popular culture has sifted a few concepts, including the many-worlds interpretation, which implies that all possible timelines exist in alternate realities (or universes). Although this idea would seem to support the possibility of backward time travel, we’ll see that no matter which theory we subscribe to, backward time travel is impossible.

Let’s begin by addressing the first option: fixed history theory. Imagine that you have possession of a working time travel machine, so you decide to travel back in time ten minutes to give yourself a high five. Well if history is immutable, then you won’t be able to high five yourself, or even see yourself, because you don’t remember seeing yourself ten minutes ago. See, this theory only works if we assume that the term history implies grand, complex events that no living person remembers (or of which they deny memory). If we try to change something simple and knowable, then it becomes obvious that we really should be able to change the past. So we’re left with two options: either backward time travel never ends up occurring, or history is not fixed.

Before we move on, let’s stop to discuss what we mean when we say that backward time travel is impossible. This doesn’t necessarily require that the technology is never invented; it could be that it’s just never implemented. After all, in order for us to do something, we need the both the opportunity and desire to do it. Perhaps backward time travel never happens because we decide that it shouldn’t.

Anyway, what if our history is flexible and allows us to go back and change things? Well, aside from the previously mentioned grandfather paradox (and others), there’s also no record in our history of anyone back in time and messing around. This could be due to the skill and secrecy of the travelers, but it’s difficult to imagine that no one in all of time was accidentally discovered or decided to reveal their secret. Of course, if a hidden organization tightly controlled the technology, then it might be safe. However, this would require that the secret would never be revealed throughout all of history. It would also mean that no one else ever invents time travel, otherwise they would both be editing each other’s pasts, producing competing time travelers. Basically, backward time travel can’t happen with a flexible history because it produces paradoxes, and it won’t happen because anyone who invents it will be assassinated by an opposing group. This is because backward time travel is power – the power to make the world the way you want it to be – and it’s pretty likely that no one would be comfortable with anyone else holding such power.

And so we come to alternate history theory (also known as parallel universe theory). This concept seems plausible. After all, it allows us to change history, it doesn’t produce any paradoxes and it also seems to be supported by science. However, a problem occurs when we imagine the ramifications of an infinite number of alternate realities combined with backward time travel. If there are an infinite number of universes, then all possibilities have occurred an infinite number of times. So if backward time travel is possible, then there are an infinite number of realities in which it exists. That means there’s a universe where a person from the future decided to visit you right now. But no one is visiting you from the future, so this can’t be the case.

Some would argue that the number of universes only increases (via branching) when a backward time traveler changes something, but this implies that there’s a specific number of universes, which violates the zero, one or infinity rule. It also would mean that there’s only one original universe, which means that there’s only one chance for time travel to be invented and implemented. In addition, if traveling back in time produces a new and separate future, then there really isn’t any reason to go back in time. Think about it. If you go back time to assassinate Hitler and succeed, you didn’t really kill him, you only created a new universe in which he’s dead. In this way, backward time travel is actually completely useless since we’re unable to affect anything in the present, only create a new present where things are different. It’s like trying to save your dog from cancer by getting a new dog.

And just in case you still think time travel might happen, there’s a secret society whose members have sworn an oath, passed down through generations, that should backward time travel be invented, they will go back and stop it before it starts.