Grandfather Paradox and Other Problems Related to Time Travel.

Time Travel - Part 2

Nov 19, 2022

“Time travel is against reason”, said Filby.

What reason?” said the Time Traveler.

- H. G. Wells

Greetings, Fellow Bohron

In the last issue, we saw how difficult it is to achieve time travel. Even if we have adequate technology and an understanding of physical laws, a number of questions still trouble us. So let’s discuss them now.

Time travel poses a number of problems. *Source*

Paradoxes and Time Conundrums

The possibility of time travel is overshadowed by a number of technical and social problems. Suppose you commit a crime in the present world and then flee into the past to take refuge. Can you be punished for the crime you committed in future?

The most popular problem associated with time travel is this:

Suppose you build a time machine, go into the past and kill your own grandfather before he had kids. This means your parents and eventually, you would never be born. But if you were never born, how could you build a time machine and go into the past in the first place? This logical impossibility is known as the "grandfather paradox."

A diagram illustrating of Grandfather Paradox. *Source*

Resolving the Paradox

Logicians have proposed three solutions to resolve the grandfather paradox:

Repeating the Past

Some believe that when you revisit the past, you only repeat history; you are performing the same actions as had been already performed. You possess no free will and are forced to complete the past as it was written. It was destiny

The first possibility is analogous to a read-only file. Just like you can’t edit the contents of a read-only file, you are not allowed to change the past.

Novikov's Self-Consistency Principle.

Russian physicist Igor Novikov proposed a second solution in which you have free will but within limits. He explains it with this simple example: you possess the free will of walking on a ceiling, but a physical law (of gravitation) prevents you from doing so. Igor argues that the same applies to his solution. You are only allowed to perform those actions in the past that do not create a time paradox. An unknown natural law prevents you from killing your parents before you are born. This is knowns as Novikov's self-consistency principle.

The Self Consistency Principle helps Donnie Darko to sacrifice himself in order to save his universe from collapsing onto itself. *Source*

The second possibility is explored in the movies Terminator 3 and Donnie Darko.

Many-Worlds Interpretation

The third solution explores the possible existence of multiverses. It says that whenever we make a decision, the universe is split into a number of parallel universes. So when you go to the past and kill your parents, you have actually entered a different universe than you started from. On one timeline, the people you killed look just like your parents, but they are not your “true” parents because you are now in a parallel universe. This is known as the “many-worlds interpretation” of quantum mechanics".

An example of the many-worlds situation. *Source*

1985 movie Back to the Future explored this third possibility. It is also the central theme of the sci-fi novel Dark Matter.

Can radiation inside a wormhole time machine kill us?

We know that a traversable wormhole can serve as a time machine. However, any radiation that enters a time machine is sent back to the past which eventually returns to the present and then enters the portal again. So an infinite amount of radiation builds, so strong that it can kill anyone who enters.

The "many worlds" idea solves this problem. Any radiation entering the time machine once is sent to the past in a different parallel universe, so it cannot reenter the same universe again.

The wormhole distorts the space-time sheet so much that a tunnel is formed that connects two distant events in the universe. *Source*

In 1997, three physicists Bernard Ray, Marek Radzikowski, and Robert Wald showed that time travel was consistent with all the known laws of physics, except in one place, the event horizon of a black hole. The event horizon is one of the most mysterious places in the universe. So much unknown that even Einstein's theory of gravitation breaks down at that point. At such a place, quantum effects have to take over.

To calculate radiation effects inside a wormhole, we need a theory that combines general relativity and quantum mechanics. However, all the attempts to merge these two theories have been fruitless, the answers that such a theory provides make no sense. A Theory of Everything awaits to be discovered.

The theory of everything may or may not answer all our fundamental questions about the universe. *Source*

A theory of everything would unite the four fundamental forces of the universe and enable us to calculate the radiation effects inside a wormhole and clear the doubts regarding the stability of a wormhole to be used as a time machine.

Since time travel is closely linked to the wormholes, Michio Kaku argues that time travel would be possible only in a million years.