I believe that time travel is theoretically possible, and that many of the paradoxes it entails can be resolved by multiverse theory.
Have you ever seen the movie “The Terminator”? It’s a movie about a war between machines and humans based on time travel, in which machines travel back in time to change the future and prevent people from being born in the future, and humans travel back in time to stop them. On the surface, time travel may seem unrealistic because it creates a number of paradoxes. However, physicists have shown that time travel is possible and that the paradoxes it creates can be solved. One of the two solutions to the paradoxes they propose is the intervention of an “invisible hand” and the other is multiverse theory. I believe that the invisible hand is not an adequate solution and that multiverse theory is a valid solution.
There are two ways to travel through time: through a Kerr black hole, or by traveling around a cosmic string of massive mass. Let’s start with the Kerr black hole method. A black hole is a celestial object with such a huge density that its gravitational pull is so great that not even light can escape. Inside a black hole, if there is a sufficient amount of negative matter (matter that exerts only the force of gravity) or negative energy (the force of attraction), a wormhole can exist, which can act as a pathway between two different points in space or time within our universe. However, in order to use a wormhole, you would have to enter the center of a black hole, which would expose you to its enormous gravitational pull and cause all of your atoms to shatter. The solution to this problem is a rotating black hole, which is called a Kerr black hole. Objects passing through a Kerr black hole can pass through a wormhole without being disintegrated as catastrophically as in other black holes. This is because the centrifugal and gravitational forces are balanced to create a force that would not kill the occupants of the time machine.
The second way is to use cosmic strings. Cosmic strings are objects that are thinner than an atom in thickness but millions of light-years long. Time travel is possible when two such strings approach each other and are about to collide. As the two strings travel back and forth to the point of collision, the space between them contracts so that the angle of rotation for a single trip around the cosmic string is less than 360 degrees. So, from the perspective of another observer, the spacecraft can travel faster than light and go back in time. This does not violate special relativity because the speed of the spacecraft is less than the speed of light when observed in the coordinate system immediately adjacent to the spacecraft. Of course, this cosmic string has very demanding conditions. It must have an enormous density of 1 million by 1 billion tons per centimeter and move at 99.999999996% of the speed of light. Although these conditions are very restrictive, the universe is infinitely large, so the probability of a cosmic string existing is not zero. Just as a monkey typing for an infinite amount of time could produce an exact copy of Einstein’s thesis, if the probability of an event occurring in an infinite universe is non-zero, it is not impossible to search the universe for it.
Based on these two rationales, time travel is certainly possible. However, there are two paradoxes in time travel. The first is the grandfather paradox. If we traveled back in time and killed our ancestors, we would logically cease to exist. The second is the information paradox. If a scientist invents a time machine and travels back in time to tell his past self how to build it, the information about the time machine loses its source. The third is Bilker’s paradox. If a person sees the future and doesn’t like what he sees and changes his mind, then he doesn’t know what to think about the future.
To resolve these three paradoxes, physicists have proposed two solutions. The first is that an “invisible hand” intervenes and prevents the time-traveler from changing the future, no matter what choices they make, and the second is the multiverse. This means that if you change the future through time travel, the universe will split into multiple universes at that moment.
The first solution has no explanation for the existence of the invisible hand. Consider an example. Suppose I go back in time, point a gun at myself, and decide to pull the trigger. In this situation, the invisible hand would act as a force that causes the aim to miss or the gun to malfunction, which cannot be causally explained by any force in nature. And since the butterfly effect allows you to travel back in time and change the future radically just by being there, the invisible hand is an undesirable explanation. On the other hand, the second solution fits well with the view of quantum mechanics. According to quantum mechanics, the position of a particle is determined probabilistically, which means that at the moment an atom is positioned, the world is split into the number of states it can be in, and one of those states is the universe we live in. If we traveled back in time and killed our ancestors, we would proceed to a different universe than the one we were living in at that moment.
Time travel is theoretically possible, and many of the paradoxes it entails can be solved by multiverse theory. I look forward to the day when humanity, which dominates three-dimensional space with increasingly advanced technology, will dominate four-dimensional space by traveling between different worlds.
