Time travel, as it will be used in this article, refers to a physical movement to a different point in time while remaining in the same space. Before delving into the possibility of time travel, it is important to understand the concept of time. In the 17th century, Newton proposed a concept of time called absolute time, which signifies time that flows at a constant speed regardless of any change in space. Even if all substance should disappear from the universe, leaving only empty space, time would still continue to flow at a steady rate. On the other hand, Einstein completely redefined the concept of time by coming up with his special theory of relativity. This theory states that the time of moving objects is slower than that of stationary objects. In addition, he proposed in his theory of general relativity that gravity can reduce the speed of time, as well. Both the concepts of absolute time and relative time achieved public recognition. However, the two concepts failed to distinguish the past from the future, because they did not specify thedirection inwhichtime flows.

   Time only flows in one direction – forward. It is impossible for time to defy this movement by going backward. The reason that time can only flow toward the future can be found in the second law of thermodynamics. The law states that in an isolated system, entropy can only increase. Here, an isolated system is a term used in physics to denote a physical system that cannot be affected by outer influences, because the system blocks any external energy from passing through. For example, the universe is an isolated system because it exists as itself. In other words, there is nothing that exists outside the universe. In addition, entropy is a measure, like temperature, which numerically signifies the flow of energy. Entropy measurement is a way of distinguishing the past from the future. The distribution of energy in space-time changes as entropy increases. Entropy increases as energy spreads throughout space-time. The space-time with increasing entropy becomes the future. Because entropy does not decrease in an isolated system, it is impossible to go back to the past, where there is less entropy than there is in the present.

   For instance, let’s suppose that there is a batch of separated salt and sand and another batch of the two mixed together. It is clear that the batch of separated salt and sand is the previous state of the mixed batch; the process of blending salt and sand is irreversible because they cannot be separated once mixed. The difference between the two batches is how the molecules of salt and sand are distributed. While the salt molecules in the first batch are clustered together, the salt molecules in the second batch are distributed equally among the sand molecules. In this example, the distribution of salt and sand molecules is a metaphorical representation of the flow of energy, spreading outward from an originally clustered state. Thus, the first batch of separated salt and sand – representing the past - has low entropy, while the second batch of mixed sand and salt - representing the future - has high entropy.

 

   Einstein’s theory of relativity not only established a new concept of time, but also proved that time travel to the future can be possible. The overarching theory can be divided into thespecial theory of relativity and thetheory of general relativity. There are three main points in thespecial theory of relativity. The first one is relativity of simultaneity, which states that the concept of simultaneity is relative to the object or person’s position. For instance, a person standing on coordinate A could observe two events occurring at the same time, but another person standing on coordinate B could say that the two events did not happen concurrently. The second point is time dilation, which says that the time of moving objects passes more slowly than that of motionless objects. The last point is length contraction, which refers to the fact that the length of a moving object becomes shorter than that of a still object. All told, thespecial theory of relativity overturns the prior conventional wisdom that time flows at equal speed for everyone and everything.

   According to the principle of length contraction, astronauts can experience time travel to the future. As a spaceship rockets towards outer space, its length will seem to have shrunk from the perspective of people on Earth. On the other hand, astronauts in the spaceship will feel as if the distance between Earth and the planet of their destination has shortened. Thus, the astronauts are able to travel the distance of a million light years in just a year. Though it seems paradoxical that it takes a ray of light a million years to reach the planet but only a year for the spaceship, this is possible because time does not flow at equal speed for everyone. According to the principle of time dilation, the speed of time differs depending on how quicklyobjects move. From the perspective of people on Earth, it takes the spaceship onemillion years to reach the planet, but from the perspective of the astronauts, it takes a year to reach their destination. Therefore, when the spaceship returns to Earth, the astronauts will be able to see the future. In fact, according to the special theory of relativity, if a spaceship moves almost at the speed of light, three years in the spaceship will equal 300 years on Earth.

   Even so, the idea of time travel to the future, corroborated by thespecial theory of relativity, has a flaw. While people on Earth feel that time inside the spaceship passes more slowly, astronauts feel that time outside the spaceship passes more slowly. In short, a paradoxical situation occurs in which both people on Earth and the astronauts inside the spaceship assert that the other’s time is passing more slowly. A solution for this predicament is to physically check whose time passed more slowly compared to the other. While a spaceship is on its journey, both an astronaut and a person on Earth can simultaneously observe each other’s clocks and make records. When the astronaut comes back to Earth, the two can check each other’s clocks and conclude whose time passed more slowly.

   TheTheory of general relativity is the second part ofEinstein’s theory of relativity. This theory deals with the relationship between gravity and time. According to the theory, the time of an object that is under stronger gravity flows more slowly. If an astronaut should get close to a black hole, his or her time would flow more slowly than for others because of extremely strong gravity, thus enabling time travel to the future after coming back to Earth. This case is similar to the case mentioned above, in whichthe astronauts are able to see the future when they come back to Earth by traveling on the spaceship moving at almost the speed of light. Thus, Einstein’s theory of relativity explains how one can experience time travel to the future, either by moving at a very fast speed or going near a place that has strong gravity.

 

   While thetheory of relativity increases the possibility oftime travel to the future, there are a number of paradoxes that lower the likelihood for time travel to the past. First of all, there is the Polchinski paradox, which signifies a situation in which a particular series of events continues to occur in space-time over and over again without a specific beginning. Event A causes event B to occur, while event B is also the cause of event A. For instance, suppose that Ball 1 is dropped into the entrance of a time tunnel, whileBall 2 comes out through the other side of the tunnel, arriving in the past. Upon coming out from the exit, Ball 2 touches Ball 1, yet to be dropped into the time tunnel, and gives Ball 1 a slight push, which makes it fall into the entrance of the tunnel. Ball 1, which has fallen into the time tunnel again, exits the tunnel and arrives inthe past as Ball 2, which again gives Ball 1 a push. This process will continue on and on without a definite beginning.

   Furthermore, the grandfather paradox concerns a situation in which a person goes back to the past and kills his grandfather as a boy. If he should kill his young grandfather, his mother/father would not be born, which leads to the conclusion that the killer himself would not be born. The man becomes an uncanny being that should exist yet also should not exist. Cause (grandfather) and effect (grandson) cannot be inversely related to each other. The inverse causality shown by the grandfather paradox attests that time travel to the past cannot be possible.

   TheHitler paradox is similar to the grandfather paradox, but in this case, it is supposed that a mangoes back to the past and kills Hitler before he instigates the Holocaust. The reason that the man travelled to the past is because he wanted to stop Hitler before he massacred so many people. However, by killing Hitler, the man himself is eliminating the very reason he travelled to the past. If he should succeed in killing Hitler before the Holocaust, thenthe Holocaust would never havehappened, and thus the man neverwould have wanted to kill Hitler. Moreover, the butterfly effect that follows the assassination of Hitler would be much greater than killing one’s own grandfather.

   There are two mainstream counter-arguments to these claims that challenge time travel to the past. First, there is Everett’s Many Worlds Interpretation, which assumes that there are multiple twin worlds similar to our own in different space-times. Assuming that time travel takes a person to the past to one of these twin worlds, contradictory situations such as the case of the grandfather paradox would not occur. For instance, a man killing his own grandfather in a doppelganger universe only means that the man will not be born in that twin universe in the future. A second counter-argument is called Novikov’s self-consistency principle. This argument states that nothing can stop a past event from happening because it is fated to occur. In the end, the person who travelled back in time to stop an event fails to do, and eventually gets caught in an infinitely repeating loop of time.

   As an illustration of Novikov’s self-consistency principle, a man goes back in time to stop his wife from getting hit by a car, but fails to save her. The scenario starts when the man enters the tunnel of time and arrivesinthe past. The man plans on finding his wife and preventing the accident from happening. However, an unexpected event prevents him from saving his wife. For instance, someone could stop the man from throwing himself in the road to save her, or he could get hit by a car himself before he can reach her. His wife eventually gets hit by a car no matter how hard he tries. The man, who has just witnessed his wife’s death, goes into the tunnel of time again to travel to the past. In the end, it is impossible to alter history, because the act of traveling to the past cannot affect the present situation. Despite such counterarguments to time travel paradoxes, time travel to the past still seems difficult when analyzed logically.

 

   In 2014, a research team of the University of Queensland in Australia published its research in which a simulation of time travel using wormholes and photons** brought about positive results. The team discovered a closed time-like curve (CTC) during the simulation, which indicates that one could arrive at the same spot at an earlier point in time by travelling to the past. Thetheory of general relativity states that strong enough gravity can modify space-time. Time only flows in one direction, but it is theoretically possible to bend this one-way passage using gravity, and make a hypothetical passageway (wormhole) which ultimately allows for time travel to the past.

   In the simulation, the research team first created a time machine with a wormhole. They named the two entrances to the wormhole A and B. The two entrances’ time were synchronized at noon at the beginning of the simulation. They first fixed A, and moved B away from its original position at almost the speed of light. B’s time slowed down as it moved, according to the principle of time dilation in thespecial theory of relativity. At the moment when A’s time became 3 p.m., B’s time hadonly reached 2 p.m. Then the researchers moved B back to its original position at almost the speed of light. A’s time became 6 p.m. while B’s time reached 4 p.m. At this point, a photon was sent at 6 p.m. to travel atnearly the speed of light from A towards B and arrived there at 5 p.m. Then the photon went into the wormhole through B and came out from A at 5 p.m., because the time of the two entrances had been synchronized. In the end, the photon time-travelled to the past by starting from A at 6 p.m. and arriving at the same spot at 5 p.m.

   These findings of the Queensland University research team are free from the major time travel paradox – the grandfather paradox. The irony in the grandfather paradox is that the man who attempted time travel got caught in an enigmatic situation in which he was on the line of existing and not existing at the same time. However, photons, unlike most objects, can exist in any form and are thus unaffected by the grandfather paradox. It is still difficult to prove the possibility of time travel to the past with matters other than photons, including humans. Nonetheless, the research result is still significant in that it indeed verified a form of time travel to the past thatpreviously was thought to be theoretically impossible.

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   Queensland University’s research team broke new ground on the study of time travel to the past, which was deemed almost impossible until recently. In addition, thetheory of relativity proves that time travel to the future - though the effects are marginal - is already a reality. There is still a plethora of mysteries to be uncovered and studied, but the latest research on time travel gives us hope that all types of time travel mightbecome possible in the future.

 

*Wormhole: A hypothetical tube-like passage in space-time thatlinks the black hole and the white hole; A white hole, the contrary concept of a black hole, spits out objects that the black hole absorbs.

**Photons: Particles of light