Everyone has, at least, once in a lifetime thought of the idea of time travel. “What if I could go back in time and change the course of my life.” Time travel was made extremely popular with the release of the blockbuster movie, “back to the future.” Since then many science fiction movies have emerged with time travel as the center of their plot. Time travel has always been linked to science fiction and unachievable but Einstein's theory of relativity has opened new doors. It has shown us that time is relative and does not pass at the same speed for everyone. But still many scientists believe that time can be squashed or stretched, but it cannot run backward. Even though if we achieve time travel, another problem arises and that is, the right position. Nothing in the universe is stationary, as the moon revolves around the earth, the earth around the sun and the sun around the milky way, time travel isn't merely about moving through time but also distance. If you were to travel back in time 100 years to the exact same position you are in right now, the earth would not have arrived yet, it would still be millions of kilometres away. So not only would we need to figure out how to travel in time, but also where everything would be relative to that time period. However, if we resolve this vicious cycle of time travel then we have another problem knocking our doors and that is time paradoxes.
A paradox is a flaw in logic. It is a statement which contradicts itself, like the statement “nobody goes to x restaurant anymore as it is too crowded.” Time travel is filled with many paradoxes. Some of the most famous paradoxes are, “the grandfather paradox”, “the predestination paradox” and “the bootstrap paradox.” This post covers various types of paradoxes and their possible solutions. Therefore bear with me to dive into the mind melting theories and paradoxes of time.
The paradoxes of time:
Grandfather paradox: Grandfather paradox is by far the most famous time paradox. It says that if a person travels to a time before their grandfather had children and kills him making their own birth impossible. So how was he able to move back in time in the first place?
Bootstrap paradox: Bootstrap paradox says that if you send an object back in time, it would be stuck in an infinite time loop with no origin. For example, you are a child studying the works of Shakespeare at your school. You travel back in the time of Shakespeare with a copy of “Hamlet” and you hand over the book to Shakespeare who later publishes the book by his name. Then who actually authored the book? You read Hamlet approximately four centuries later the book was published and you hand over the book to Shakespeare back in time who copies the book and publishes it by his name, then where does the book actually come from? The book has no origin. It is stuck in an infinite time loop.
Predestination paradox: A predestination paradox arises when a person traveling back in time becomes part of past events and may even have caused the initial event that caused that person to travel back in time in the first place. For example a fire set up in a forest and you decide to investigate the source of the fire. You go back in time, but you accidentally knock over a box of kerosene which ignites the fire, the same fire that would inspire you, years later, to travel back in time.
Time paradoxes can be pretty mind bending, but we have many hypotheses which solve these paradoxes. The most straightforward solution is that you can’t go back in time, thus these paradoxes would never arise. But this theory easily dodges the paradox ignoring the fact that everything is possible (given time constraints to achieve it). Thus, what if time travel is possible as we never know what the future looks forward to. If it is possible, then what could be the solution of the paradoxes?
The multiverse theory: The multiverse theory states that when a choice is made (no matter how insignificant), a plane of existence is created for each outcome. For example, there's a universe where that person didn't eat the bat. There's a universe where you partook of breakfast two seconds later than you did today. There's a universe where the wind blew at 5.00000001 MPH instead of just five MPH. Every outcome exists in parallel universes.
Now back to the paradox. You can visualize the theory as a tree graph, where every node represents a choice and every branch is an outcome. Let's make a simple chart that represents the aliveness of your grandfather.
/ (Yes, you are born)
(Grandfather Alive?) ⟶
\ (No, you aren't born)
Hence, you are in the top branch. Let's say you travel back to the first node and kill your grandfather. Do you break time and cease to exist? No!
You simply make a change in another universe where the change already existed. When you go back in time to kill your grandfather, you're not going back to your own history, but a copy of your history, and everything you do in this version of your history will affect the alternate future of that universe, not your own. It is the universe where your grandfather was dead, and it would not affect the timeline you are living in.
Information takes time to travel: Changes you make can never catch up to the present. Those changes can't move through time instantly. It's subject to a speed limit. We all are time travelers. We all are moving in the future every second. The speed of time is.. one second per second so if you go back five years in time and make a change, then for it to reflect, it should take another five years.
Imagine that a timeline is like a giant history book where a time traveler is erasing the past chapters, but the author who is writing the book is not aware of the changes. With this another possibility arises: the changes you make will just remain there and never ever affect the future. It's because the information is lost between dimensions and time. Maybe that's what is deja vu. Your future self, visiting the past and doing a particular task which is stuck in the dimension, and you also do the same task which leads you to think that you have already done that!
Time loop: If you go back in time and kill your grandfather, you will never be born thus you can't move back in time, so your grandfather isn't killed, thus you are born, so you go back in time and kill your grandfather, and so on… It's two entangled events happening together. This is possible at quantum states, and subatomic particles do this almost always which is called superposition and this is responsible for fusion in the sun's core, double slit experiment, etc. So if the world was to exist in the superposition state, then your grandfather is both dead and alive forming a closed time loop. But could a closed time loop actually exist, according to the laws of physics? We don't know.
Even though all these solutions are completely hypothetical as no individual has visited the past (as far as we know) but they provide satisfactory answers nonetheless this could also be a possibility that we are knocking the wrong doors. Whatever the true answer may be, we are miles away to achieve it. We have to attain tons of knowledge of time itself, leave alone time travel. Even though time travel could appear impossible to us, this field is growing and maybe one day quantum physics could answer the time paradoxes and maybe our dreams of time travel could be true, one fine day.