Time travel is supported by films and television shows and several mythological stories like the Mahabharata, an Indian classic, and the Urashima Taro Saga, a Japanese legend. But time travel is always associated with paradoxes!
Time travel is a standard plot device in fiction, but things are far more complicated and mysterious in reality. Various scientists have extensively researched this matter, and it might be possible mathematically. Yet why is time travel still not possible in reality? The answer is the causal paradox and its temporal effects!
What is a Paradox?
Well, the word itself is pretty complicated to be defined entirely, but let me try. You see, a paradox is something that sounds highly self-contradicting and absurd in the beginning, almost something you would never believe when you hear it but would seem and feel true when investigated deeply.
I know it is still not clear, so let me explain it with an example. Think of a burning candle. How would you make a riddle out of it? “The older it gets, the shorter it grows” – that’s it! Here, you have a paradox that might sound quite conflicting initially, but it is true.
The time travel paradoxes are similar to the one I explained above. There are five central time travel paradoxes, which might be why this theory is just a theory! Let’s delve deeper into each of them.
1. The Grandfather Paradox
As the name suggests, this is the most sought-after paradox in the concept of time travel. To understand this paradox, let’s create a hypothetical situation where you are a teenager. Those raging hormones have been seeking a chance to take revenge on your grandfather, who had left your father penniless in the past.
You hate your grandfather by now! Yeah, it has started to grow on your nerves as you grew up watching how much your father had to struggle to pay your school fees every month. Somehow, you knew he used to be crazy rich but left your father in an orphanage without an inheritance because of an extramarital affair.
Now, you find out a way to travel back in time and decide to stop these events from happening. You travel to the year when your grandfather was single and hadn’t started a family yet. Being a young military enthusiast, you have some experience in long-range shooting. So, you carried your shotgun and decided to hunt down your grandfather before he could destroy lives.
Here, one situation arises. If you successfully kill your grandfather before he could have kids with your grandmother, you would prevent your father from taking birth in the first place. That makes your existence unreal! How would you travel back in time to kill your grandfather if you didn’t exist?
This contradiction gives rise to the multiverse concept, in which parallel realities coexist. If you do not vanish after killing your grandfather, you have visited a new reality where you do not exist and can yet return to the world where you do exist. The Netflix sci-fi series Dark depicts this dilemma in a very interwoven and intricate manner.
2. The Predestination Paradox
Let’s understand the situation with a story. The name sounds similar to the Final Destination movie series, and so is the concept. It would feel like you can travel back in time or be able to see events that would happen in front of you, yet you cannot do anything to change the events. A helpless situation.
Suppose you are a widower who has lost his spouse in an unfortunate car accident in the past. When you find a way to travel back in time, you decide to get to the accident spot and save your spouse from that mishap.
She had died in a car accident caused by a kid suddenly running in the middle of the road while trying to follow the traffic lights. In a few seconds, the car took a wrong turn and hit the hydrant, and the engine burst open. The water short-circuited the car and put it into flames. When the firefighters arrived, she was dead due to bleeding inside her head and partly burnt.
You arrive at the event’s date, time, and place and spot your spouse driving in your direction. You even spot the kid from the other side stepping away from his mother. You thrust yourself onto the road to push away the kid from coming near the middle, but now instead, you made your spouse take the wrong turn, and things happened as they had happened before.
Event A in the past influences Event B in the future, which then causes Event A to occur. The scientific term describing this situation is called “temporal causal loop.” The circular chain of events ensures that the time traveller does not change history and that any attempts to halt an event from occurring in the past will only serve to facilitate the cause rather than stop it.
So, this is what the predestination paradox means – nothing can change what happened in the past. If you try to change it, you become the reason behind the event you wanted to prevent. Everything is predestined to happen the same way, so whatever has happened will happen. There is no escape from this loop.
3. The Bootstrap Paradox
What comes before? The egg or the chicken? Well, I don’t have an answer to this. Because for the egg to exist, it has to come from a chicken, and a chicken has to hatch from an egg. So, it’s pretty close to what I will explain next!
The Bootstrap paradox explains how something, someone, or a piece of data, when sent back in time, creates an infinite loop in which the thing exists without an apparent genesis. Ontology – the branch of philosophy that deals with the nature and existence of objects – lies at the core of this paradox. Let’s take the reference of the book “A Journey Through Time” in the Netflix series Dark.
In the series, the clockmaker was gifted with the book by a person from the future, and he wrote the book again looped back to him from the future. Similarly, suppose Einstein met a time traveller and learnt about the “Theory of Relativity” from him, which led him to do formal research on the given subject. But then, where did the information come from in the first place?
For a person, suppose you are a man in your early twenties and travelled back in time. You meet someone and fall in love during your stay in the past. As a result, she becomes pregnant and gives birth to a baby who grows to become you a couple of decades later. So, what is your timeline of existence?
Per the laws of physics, this paradox seems the most impossible to exist practically. Even rules of genetics wouldn’t allow this! How can a person go on to give birth to himself?
The bootstrap paradox develops because the origin would have no significance if time travel is conceivable. When we talk about the origin, we certainly mean the past, and if the past cannot be precisely defined, we will undoubtedly struggle to identify the source of anything. After all, the future, present, and past would not be sure.
4. The Hitler Paradox
The Hitler Paradox eliminates your motivation to travel back in time to kill him, much like the Grandfather Paradox ironically prevents your birth. The core idea behind this theory is called the “Butterfly Effect” – seemingly minor changes can have enormous ripple responses over extended periods.
However, the butterfly effect in the case of killing Hitler would not be limited to your birth, like in the case of killing your grandfather. It can cause considerable changes in global aspects like politics, economy and even population distribution throughout the decades. You might even never come to know about Hitler if he was not allowed to create his part of history!
If you do not read about Hitler and his atrocities, you wouldn’t likely feel the urge to travel back in time and kill him. And not only that! It will also prevent central political borders and events from ever happening. It kills the entire purpose of time travel.
5. Polchinski’s Paradox
This paradox is named after an American theoretical physicist Joseph Polchinski who came up with this scenario. In this case, a billiard ball rolls towards a hole on the table and enters a wormhole that throws it back onto the table just a few seconds before the ball enters the hole.
This wormhole traversed ball then hits its previous self and changes its course so that it does not enter the wormhole now. It prevents itself from going on a time loop. Then how did it happen in the first place? This phenomenon is Polchinski’s Paradox, which is highly in accord with the rules of general relativity, i.e., it can form a closed time curve (CTC) in a space-time bound area.
Time Travel is simply impossible to achieve, thanks to the paradoxes! However, numerous other theories can bound time travel into specific rules, which might help make this a successful try. Scientists have been scratching their brains a lot on these rules, and maybe you would like to know more about them too.