Can We Travel To The PAST?

Time travel.

This mysterious phenomenon has been the subject of numerous disputes and discussions for many years. Today there are many films, TV series, and books about time travel. But despite such a wide discussion of this topic, it still remains unclear whether time travel is possible in reality. Some believe that this is possible, others on the contrary believe that it is impossible to move in time and this is just an invention of Science Fiction. So let's figure out if humanity will ever be able to rewind time and look into other time epics.

It is generally believed that the idea of time travel appeared in the 19th century. At the same time, at the end of the century, the science fiction writer Herbert Wells came up with a device for such travel; a time machine. However, long before this invention, the heroes of many myths and fairy tales made time travel without any devices although not without outside help. For example, mythical heroes could fall into the future after sleeping for some time at the will of the gods or other powerful forces. This is exactly the story that happened to the ancient Indian King Muchukunda who helped the gods to defeat the demons. As a reward, he asked the gods to immerse himself in an endless sleep but one day the sleeping King was awakened by the god Vishnu and when Muchukanda left his cave he was very surprised by how much the world had changed. Moving into the future through a long sleep is found in a variety of myths and fairy tales of many people. Although they do not correspond to Modern Ideas about time travel. Nevertheless, they are an option for irrevocably moving into the future.

What does science say about time travel? According to Einstein's Theory of Relativity, time tends to slow down with increasing speed. In other words the faster the speed of your movement in space the slower time will go for you. At the same time for everyone else, the speed of the passage of time will not change. That is if we take two people, leave one on earth and put the other in a spaceship and accelerate it to near the speed of light then time will go at different speeds for them. If the spacecraft moves at a speed equal to 90% of the speed of light then 10 minutes will pass for a person inside the ship while 20 minutes will pass for a person remaining on Earth. Thus, a person who moved at near light speed in fact will be in the future.

Time dilation was demonstrated by Joseph C. Hafele and Richard E. Keating in October 1971. On regular commercial flights, they flew around the planet Twice first to the East and then to the West. They took a high-precision atomic clock with them on board. After the flight, the time on the clock on the plane was compared with the same clock remaining on the ground. It turned out that during their flight Joseph and Richard moved in time by about 332 nanoseconds thus the very first human Journey Through Time took place in 1971. It was named the Hafele-Keating Experiment. Of course, they moved into the very near future but the main thing here is the fact that time travel is possible both in theory and in practice. This experiment demonstrated the possibility of gravitational time dilation as well as time dilation for moving objects.

We've sorted out the travel to the future but what about travelling back in time? After all many of us are much more attracted to the opportunity to jump not forward in time but backwards. What if I want to see dinosaurs? You will be surprised but it turns out that travelling to the past is theoretically quite possible and it doesn't even contradict Einstein's Theory of Relativity. Moreover, there are several ways of such movement. It is worth saying that Einstein himself did not believe in the possibility of travelling to the past. Nevertheless, the first scientific method of such a movement was demonstrated in 1949 by the Austrian mathematician Kurt Friedrich Godel. He found out that the theory of relativity allows for the existence of certain trajectories in the universe during which you can return to the starting point before your journey begins. Such trajectories are called Closed Time-like Curves, CTC. In 1949 using the theory of relativity Kurt Godel was able to obtain a model of the universe in which such curves exist. If his model is correct then it turns out that there are closed trajectories during the passage of which you will return to the original point already in the past time. In fact, Einstein could not find any flaws in the model created by Godel. After Godel's discovery, the scientific world was shocked. Physicists began to look for new models of the universe that would allow the existence of such closed trajectories and it turned out that there is not even one but several such models.

According to Godel, the space of his model of the universe rotates and the side effect of this rotation is the possibility of moving into the past. However, at the moment there is no evidence that our space-time rotates. However, for temporary displacements, it is not necessary to have a rotating space-time since the rest of the found models of universes according to calculations do not rotate. However, science has not yet confirmed a single fact corresponding to those models of universes that would be consistent with the theory of relativity and would allow the possibility of travelling into the past. Nevertheless, some facts stating that we live in such a universe have not been refuted. That is to say, at the moment we do not know whether we live in a model of the universe that allows for a trip to the past or not. But then the relevant question arises, it will be good if it turns out that we live in a model of the universe that has Closed Time-like Curves and allows for the possibility of time travel. What if it's not the case? What if there are no Closed Time-like Curves in our universe? Are we destined to live in a boring universe and never even look into the past?

We shouldn't worry here too given not everything is so clear. Dutch physicist Willem Jacob van Stockum has found out that there may be some artefacts in the universe that can create closed time-like lines in any of the possible models of universes. In other words, even if there are no trajectories in our universe that would make it possible to move into the past then we can still create such trajectories artificially or find artefacts capable of creating such lines in any of the possible universes. There is one interesting point here and it consists of this, any movement faster than the speed of light in any of the possible models of universes can send us into the past. And there is a theoretical way to travel through time using natural artefacts of the universe. One of these artefacts is wormholes. Wormholes are a topological feature of space-time. They represent something like a tunnel through space-time. Using wormholes can help us travel vast distances in as short a time as possible but they are also suitable for creating closed time-like lines and therefore time travel. This happens precisely because thanks to wormholes it is theoretically possible to travel faster than the speed of light. Of course, so far the existence of wormholes is just a theory but this theory does not contradict science. Moreover, back in 1935 Albert Einstein and Nathan Rosen fully described the mechanism of the origin and development of wormholes. Scientists willingly believe in their existence which means that they may well be somewhere in the universe and maybe we just have to find them.

In 1974 American physicist Frank J. Tipler also showed an artefact suitable for time travel. This is a long cylinder spinning around which one could make a leap into the past. However, Stephen Hawking criticized this option because it has many problems. One of these problems is that such a cylinder must be of incredible length. It should extend for tens or even hundreds of light years. Of course, humanity is not able to create a structure of such scale but scientists are also trying to find a way out of this situation. One of the possible ways out is the so-called Cosmic Strings. These are hypothetical astronomical objects representing a space-time defect. Cosmic Strings are incredibly thin but at the same time very long tubes stretching in space for tens and hundreds of light years. The scientific community readily believes in the existence of cosmic strings because too many studies confirm this with a very high probability.

In addition to the ways of moving through time using wormholes and cosmic strings, there is also a way that allows jumping through time using a completely man-made mechanism, the Alcubierre bubble. It was developed by Mexican physicist Miguel Alcubierre in 1994. Such a structure would allow us to overcome the speed of light and therefore make jumps in time. The very idea of a bubble is a theoretically possible idea of a warp engine according to which a spacecraft could reach a speed exceeding light by compressing space in front of it and unclenching behind it. As we know objects cannot accelerate to the speed of light in ordinary space-time, The Alcubierre engine shifts the space itself around the object so that the object reaches its destination faster than light in normal space without violating any physical laws. Simply put, the spacecraft will be in a bubble that compresses the space in front of it and unclenches behind it. In this way, it would be possible to achieve a speed exceeding light. The only problem is that this way of moving would require too much energy. Unimaginably a lot of energy. Imagine that about 75 million tons of hydrogen are produced on our entire planet every year. Even if we assume that Humanity was able to create a working model of the Alcubierre bubble then in order to activate it and accelerate to a speed exceeding light we would need about as much hydrogen as the whole of Humanity would produce in 25 trillion years! Yes, it would have to wait a very long time. We need not worry about this though. American engineer Harold White proposed his own version of the Alcubierre air bubble and in 2015 even provided the results of his experiments on the acceleration of micro-particles. More precisely, the particles themselves move at a constant speed and the space in front of them is curved due to this acceleration. The most important thing is that Harold White was able to reduce energy consumption from this method of movement by as much as three septillion times. To exceed the speed of light by 10 times, a generator equivalent to the power of the sun would accumulate the necessary amount of energy in just one-millionth of a second.

Summing up we can say that there are many theoretical ways to travel to the past. These are Closed Time-like Lines, Wormholes, Cosmic Strings and even man-made mechanisms such as the Alcubierre air bubble. To this day scientists are actively continuing to work on the research and improvement of each of these options. Therefore it is quite possible that in the future we will still be able to turn back and see with our own eyes what was happening in the world long before our appearance.