In this blog post, we will learn about the possibility of humans traveling back in time and the multiverse theory that explains this possibility.
Have you ever seen the movie “The Terminator”? This movie is based on time travel and deals with the war between machines and humans. It depicts a battle between a machine that tries to prevent a person from being born in the future by traveling back in time in a time machine to change the future and a human who travels back in time in a time machine to stop it. Looking at the plot, it may seem unrealistic because it causes various paradoxes due to time travel. However, physicists say that time travel is possible and that the paradoxes it causes can be resolved. One of the two solutions to the paradox they have proposed is the intervention of an “invisible hand,” and the other is the theory of multiple universes. I believe that the invisible hand is not an appropriate solution to the paradox and that the theory of multiple universes is a valid solution.
There are several ways to travel through time, including using a Kerr black hole and orbiting a cosmic string with a huge mass. First, let’s take a look at the method using a black hole. A black hole is an astronomical object with an enormous density, and its gravity is so strong that even light cannot escape. If there is enough negative matter (matter that only exerts gravitational force) or negative energy (gravity) inside a black hole, a wormhole can exist, which can act as a passage connecting two different points in space or time in our universe. However, in order to use a wormhole, one must enter the center of a black hole, which would expose them to enormous gravity and cause all the atoms to disintegrate. The solution to this problem is possible through a rotating black hole, which is called a Kerr black hole. Objects passing through a black hole can pass through a wormhole without being torn to pieces like they would be in the case of other black holes. This is because the centrifugal force and gravity balance out and exert enough force to prevent the time machine passenger from being killed.
Let’s take a look at the second method, which uses cosmic strings. Here, cosmic strings are objects that are thinner than an atom but millions of light-years long. Time travel is possible when these two strings approach each other and are about to collide. If you travel back and forth to the point where the two strings collide, the space between them contracts, making the angle of rotation around the cosmic string 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 the special theory of relativity because the speed of the spacecraft is less than the speed of light when observed in the coordinate system directly next to the spacecraft. Of course, this cosmic string has very strict conditions. This cosmic string must have an incredible density of 1 million × 1 billion tons per centimeter and move at 99.999999996% of the speed of light. Although this condition is very restrictive, the universe is infinitely large, so the probability of the existence of the cosmic string is not zero. If a monkey types for an infinite amount of time, it is possible that it will write out Einstein’s paper identically. If the probability of an event occurring in an infinite universe is not zero, it is not impossible for this event to occur when the universe is searched thoroughly.
Based on the above two reasons, time travel is considered to be possible. However, there are paradoxical situations in time travel. The first is the grandfather paradox. If we go back in time and kill our ancestors, we cannot logically exist. The second is the information paradox. If a scientist invented a time machine and traveled back in time to tell his past self how to build it, the information about the time machine would lose its source. The third is the Birkhoff’s paradox. Suppose a person who has seen the future changes his mind about it. Then we have no idea what to think about the future he saw.
To solve these three paradoxes, physicists have proposed two solutions. The first solution is that the “invisible hand” intervenes to prevent a person who has traveled through time from changing the future, no matter what choice they make. The second solution is the multiverse. If you change the future through time travel, it means that the universe splits into multiple branches at that moment and you will be transferred to another universe.
First, the first solution is that it is impossible to explain the existence of that “invisible hand.” For example, let’s consider the following. 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 to make the target miss or the gun malfunction, which cannot be explained causally by any force that exists in the natural world. And the future can change rapidly just by the fact that I myself went back to the past and stood in that place due to the butterfly effect, so the “invisible hand” is an undesirable explanation. On the other hand, the second solution is also well-suited to the perspective of quantum mechanics, and I believe it is an appropriate solution. According to quantum mechanics, the position of a particle is determined probabilistically, which can be interpreted differently as follows: when the position of an atom is determined, the world splits into as many states as there are possible states in which the particle could exist, and one of those states is the universe in which we live. If we go back in time and kill our ancestors, we will be transported to a different universe from the one we lived in at that moment.
Time travel is theoretically possible, and the various paradoxes that this entails can be resolved by the theory of multiple universes. I hope that the day will come when humanity, which has gradually developed its technology to dominate three-dimensional space, will be able to travel between various worlds and dominate four-dimensional space.
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