Science fiction movies, like Back to the future and Terminator, and books, like H G Wells’ ‘Time Machine’ and Stephen King’s ‘22/11/63’, have paved way for the idea of time travel into everybody’s minds. With ideas of the TARDIS and the DeLorean seeping into pop culture, everyone talks of travelling through time. But is it really possible, though? Can you actually just step into a machine and travel through time and space without any consequences? Let’s forget the laws of physics for a moment and delve into whether, on a philosophical scale, one can travel through time.
Time is the fourth dimension of the universe, along with the three spatial dimensions of length, breadth and height. It is often thought of to be of a linear pattern, that is, consisting of a sequential series of events, having a beginning and an end. In Newtonian terms, it is an absolute reality and regardless of human perception. With this view of time, come several paradoxes of time travel.
We are all travelling through time, every second of every day. You are not currently at the point of time you were when you started reading this sentence. You are constantly moving forward in time. However, this obviously is not the same as what we consider to be ‘travelling through time’. When you travel backwards through time, to the past, or forward, to the future, you are actually creating discrepancies between your personal time and the external time. Your personal time is what you experience, whereas external time is, well, time itself, regardless of you. So, if you choose to travel forwards in time, to say, the year 2449, the external time that will pass is 430 years, whereas your personal time that passes may only be a half an hour or so. In a normal circumstance, without any time travel involved, however, the external time and your personal time go parallelly.
Time travel, itself may occur in many ways, outside of using a time machine. For example, mental time travel or chronesthesia in psychology includes mentally reconstructing the past and imagining future scenarios (which may be how Doctor Strange saw 1400605 possibilities in Avengers: Infinity War). This is what, in mythologies, is mentioned as prophecies. However, along with this comes the Cassandra dilemma. The Cassandra dilemma gets its name from the character of Cassandra, from Greek mythology, who gets the ability to tell prophecies from the Greek god, Apollo but when she refuses Apollo’s romantic advances, she is cursed so that none of the prophecies she foretells would be believed.
Another method to get anywhere in time is by the use of a wormhole, also called the Einstein-Rosen bridge. A wormhole is a speculative structure that connects two different points in spacetime. These are, however, theoretical and haven’t yet been proven to exist.
A paradox is a situation that seems to contradict itself. A paradox of time travel or a temporal paradox is a paradox or a logical contradiction involving time and time travel. The two most commonly known paradoxes of time travel are the bootstrap paradox and the grandfather paradox.
Consider a person, say, I, travel back in time to meet Shakespeare. After a few hours of conversation, I say to him, “Some are born great, some achieve greatness, and some have greatness thrust upon them.” The bard, impressed by the quote, decides to use it in his writing. However, I, myself got my hands on the quote when I read it in ‘Twelfth Night’ by Shakespeare. So, where did this quote really come from? Shakespeare got it from me and I, from him. It has no origin. It’s always been there, stuck in a time loop. This is the bootstrap paradox. It creates a consistent causal loop.
Now, imagine if one day, you decide to go back in time and kill one of your grandparents (why would you do that?) before one of your parents was born. If the said parent was never born, neither were you. If you weren’t born, you can’t go back in time to kill your grandfather. If your grandfather didn’t die by your hands, you are alive and can kill him. This is the grandfather paradox. It creates an inconsistent causal loop. The grandfather paradox can be extended beyond just you murdering your grandpa. Like, maybe you want to go back in time and kill Hitler, because, why wouldn’t you? But if you manage to accomplish that, it would mean that the Holocaust never happened. But if the Holocaust didn’t happen, you wouldn’t decide to go back in time to kill Hitler. But if you didn’t kill Hitler… This is also referred to as the Hitler paradox.
Another variation of the grandfather paradox is the Polchinski paradox, named after its proposer, the American theoretical physicist and string theorist, Joseph Gerard Polchinski Jr. He raised a paradoxical situation, wherein a billiard ball is fired into a wormhole at just the right angle, such that it will travel back in time to hit the billiard ball being fired into the wormhole, knocking it off course.
Yet another variant of the grandfather paradox is the retrosuicide paradox, wherein someone might go back in time to kill their younger self, consequently making themself cease to exist.
Imagine yourself sitting in a classroom on a Monday morning. Then, on a Wednesday night, you decide to go back to the Monday morning, to a coffee shop. But you, or a version of you, is in the classroom at the same time. How can you be at a coffee shop and a classroom at the same point in time? This is the double occupancy paradox. The double occupancy paradox also brings us to the issue of the paradox of freedom, because, on the Wednesday night you will always ‘decide’ to go back to the Monday morning. So, you may think you choose to do so, but you are, in fact, predestined to, which brings the question to, ‘do you have any free will?’
Then there’s the predestination paradox, according to which, the time traveller who goes back in time to change the past, themself becomes responsible for creating the past as they know it. Going back to the example of killing Hitler, consider if you decide to go back in time to kill Hitler by shooting him with a gun. You get there, take your aim but as you shoot, someone pushes Hitler from off your bullet’s trail and just then a train passes by where Hitler was standing. So, your shooting, in fact, caused Hitler to not be hit by the train, which could have killed him. In altering the past, you created your future.
Most of these paradoxes, if not all of them, are removed if we consider the non-linear model of time, which is most accepted today. As per this model, if you travel back in time and change the events of the past, you will have created a new timeline, thus removing the possibilities of paradoxes.
Stephen Hawking was once quoted as saying, “If time travel is possible, where are the tourists from the future?” This, in fact, is the reasoning of the Fermi paradox too. However, it is very much possible that you may only be able to travel back in time such that the furthest you can go is to the point when the time travelling device was invented. This is especially true if the mode of time travel is by the use of a wormhole. Or it may even be possible that future time travellers do actually walk among us, just without us realising it.