OOPS! I killed my Grandfather.

One of the most famous and intriguing concepts of the modern world is the Grandfather Paradox. It is truly one of the biggest paradoxes of time travel. The Grandfather Paradox says that if you were to go back in time and kill your grandfather before he had any children, who would give birth to your parents and thus how would you be born, and if you were not born, who would kill your grandfather in the first place?

Time is something that should be treated with utmost caution. There are no proven answers to this paradox. There are however some theories for the solution of the paradox, each more esoteric than the next. These are the two most widely accepted ones:

Quantum Superposition[1].

One interpretation is based on the concept of quantum superposition. Quantum superposition is when something exists in two states simultaneously, sub-atomic particles like electrons regularly exist in such a condition. When applied to the Grandfather Paradox, after going to the past you are in two states simultaneously, one in which you have killed your grandfather and do not exist and one in which you did not kill your grandfather and do exist. The theory of quantum superposition was proved by Young`s Double Slit experiment [2], where electrons showed a wave particle duality. The famous thought experiment on Schrodinger`s Cat [3] can also be applied here, where the cat is simultaneously dead and alive, just like you and your grandfather.

Many Worlds Interpretation [4]

Another explanation is based on the Many Worlds Interpretation according to which there are infinite timelines or realities being played out at the same time, for example as you are reading this, you could be the President of a country in another reality or you may be the opposite gender, or you may not even be a human being. When applied to the Grandfather Paradox, after going in the past, you are transported to another reality, so when you kill your grandfather, you are actually affecting some other reality not your own. A new reality is created where you do not exist but your own reality is unaffected and in your world, your grandfather is still alive. So what happened is you went to another reality to kill your grandfather, and all subsequent events in that reality reflect the fact that your grandfather is dead.

All of these solutions may be difficult to believe and wrap your head around. But for physicists it is normal, as quantum physics is one of the most intriguing concepts in modern science. Remember these are all just speculations and there is no concrete proof, after all time travel has not yet been invented and there is no way to test these theories!

References 

[1]-https://jqi.umd.edu/glossary/quantum-superposition

[2]-https://www.britannica.com/science/light/Youngs-double-slit-experiment

[3]-https://www.nationalgeographic.com/news/2013/8/130812-physics-schrodinger-erwin-google-doodle-cat-paradox-science/

[4]-https://science.howstuffworks.com/science-vs-myth/everyday-myths/parallel-universe2.htm

Image from https://aeon.co/videos/how-quantum-superposition-could-unravel-the-grandfather-paradox