It may be that there is no other concept that has attracted the imagination as the idea of time travel – the ability to travel to any point in the past or future. What could be cooler? You could jump into the time machine to go back and see major events in history and talk with people who were there. Who could you find? Julius Caesar? Leonardo da Vinci? Elvis Presley? You could go back and find yourself at an earlier age, go forward and see how you look at the future … It is these possibilities that make time travel the subject of many books and science fiction movies. 

Turns out, somehow, we are all time travelers. While we’re at the computer, with mouse clicks, the clock is ticking. The future is constantly being transformed into the past with this lasting only a fleeting moment. Everything is being done now quickly moving into the past, which means that we continue to move through time. 

Ideas about time travel have existed for centuries, but when Albert Einstein released his special theory of relativity, he laid the foundations of the theoretical possibility of time travel. As we all know, nobody could prove a trip back in time, but no one could rule it out too 
Understanding time 
The astronomer Carl Sagan was right when he said that time is “resistant to simple definition.”Many of us think we know what is the time, but it is difficult to define it. Literally, you can not see or touch time, but can see its effects. The proof that we are moving in time is everywhere: our bodies age, buildings are exposed to weather and crumble, trees grow. Most of us feel the pressure of time as if we were compelled to meet deadlines and schedule appointments.Generally, our lives are dictated by the time they need to be somewhere. 
Ask anyone to define the time and probably most will look at your wristwatch or clock to another. We see time as the ticking of the hands of these devices. We know that there are 60 seconds in one minute, 60 minutes in an hour, 24 hours a day, 365 days a year. These are the basic numbers of time that we all learn. 

Time is also defined as the fourth dimension of our universe. The other three spatial dimensions are: right-left, forward-backward and up-down. Time can not exist without space and, similarly, the space can not exist without time. This intimate relationship between time and space is called space-time continuum, which means that every event that happens in the universe involves both space and time. 

According to the theory of special relativity of Einstein, time will pass more slowly as an object approaches the speed of light. This leads many researchers to believe that travel faster than light speed could open the possibility of time travel, for both the past and the future. The problem is believed that the speed of light is the highest speed at which something can travel, then it is unlikely that we can travel to the past. As an object approaches the speed of light, its relativistic mass increases and becomes infinite when reaching that speed. Accelerating an infinite mass faster than that is impossible, at least until now. 

But time travel in the other direction does not seem so difficult, and one day, the future may be a possible destination. 

Space phenomena 
While the writers have big ideas for time machines over the years, yet none was built in the real world. Most theories of time travel does not rely on any machine. Instead, time travel will be made, probably by natural phenomena that will transport us instantly to a point in time to another. These space phenomena, which we are not sure that there include: 
rotating black holes 
wormholes 
cosmic strings 

We will see each one in the following sections. 

Black Holes 

Image courtesy NASA 
When the stars who have more than four times the mass of our sun reach the end of life and have burned up all their fuel, they collapse under the pressure of its own weight. This implosion creates “black holes” which have gravitational fields so strong that even light can escape them.Anything that comes in contact with the event horizon of the black hole is swallowed. The event horizon is the admission of a black hole from which nothing can escape. 
You can imagine the shape of a black hole into something like an ice cream cone. It is wide at the top and ends at a point called a singularity. At the singularity, the laws of physics cease to exist and all matter is crushed beyond recognition. This type of non-rotating black hole is called the Schwarzschild black hole, named after the German astronomer Karl Schwarzschild. 

Another type of black hole, called a Kerr hole, is also theoretically possible. Kerr holes are rotating black holes that could be used as portals for time travel or travel to parallel universes. In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory for a rotating black hole. In his theory, dying stars would collapse into a rotating ring of neutrons that would produce sufficient centrifugal force to prevent the formation of a singularity. Since the black hole would not have a singularity, Kerr believed it would be safe to enter it without being crushed by the infinite gravitational force at its center. 

If Kerr holes do exist, may be able to pass through them and leave a “white” hole. A white hole would have the reverse of the black hole. So instead of attracting all that is within range of its gravitational force within himself, he would use some kind of exotic matter with negative energy to push everything out and away from you. These white holes would be our way into other times or on other worlds. 

Given the little we know about black holes, perhaps Kerr holes may exist. However, physicist Kip Thorne of the California Institute of Technology, believes that the laws of physics prevent their formation. He says there is no way to enter and exit a black hole and that anything that attempts to enter will be sucked in and destroyed before it even reaches the singularity. 

Wormholes 
Thorne believes that the universe might exist another type of structure shaped tunnel, which could be used as a portal to time travel. It is considered that the wormholes, also called Einstein-Rosen bridges, have the greatest potential for time travel, if they indeed exist. They could not only allow travel in time and also travel to many light-years from Earth in only a fraction of the amount of time that would be required with conventional methods of space travel. 
The Wormholes are considered possible based on Einstein’s theory of relativity, which says that any mass curves spacetime. To understand this curvature, think of two people holding a blanket and stretching well. If a person put a baseball on the sheet, the weight would roll the ball into the middle of the sheet, causing it to bow at that point. Now, if a marble is placed at the edge of the same sheet it would travel toward the baseball because of the curve. 

Imagining that space is a curved two-dimensional plane, wormholes like this could be formed by two masses applying enough force on spacetime to create a tunnel connecting distant points of the universe 

In this example, space is viewed as a two-dimensional plane instead of the four dimensions that actually make up spacetime. Imagine that this sheet is folded, leaving a space between the tops and bottom. Putting baseball on the top side will cause a curvature to form. If an equal mass were placed at the bottom of the sheet at a point corresponding to that occupied by baseball at the top, the second mass would eventually meet with the baseball. It is more or less how the wormholes can be formed. 

In space, masses that place pressure on different parts of the universe could eventually form a tunnel, that is, a wormhole. We could travel from Earth to another galaxy and back relatively quickly. For example, let’s imagine a scenario in which we wanted to travel to Sirius (or Sirius), a star seen in the Canis Major constellation just below Orion. Sirius is about 9 light-years from Earth, which is equivalent to about 90 trillion kilometers. Obviously, this distance would be too large for the space travelers to traverse and return in time to tell us what they saw there. Until now, the farther people have traveled into space is to the moon, which is about 400 thousand kilometers from Earth. If we could find a wormhole that connects the space around Sirius, we could save considerable time by avoiding the trillions of miles we had to go with traditional space travel. 

How all this relates to time travel? As discussed previously, the theory of relativity says that as the speed of an object approaches the speed of light, time slows down. Scientists have discovered that even at the speed of a spacecraft, astronauts can travel a few nanoseconds for the future. To understand this, imagine two people, one person A and person B. The person A stays on Earth, while person B takes off in a space rocket. At takeoff, their watches are in perfect sync. The closer the speed of light traveling the rocket person B, the slower time will pass for it (relative to person A). If the individual B for a few hours to travel 50% of the speed of light and return to Earth, will be obvious to both that person A has aged much faster than individual B. This difference in aging is because time passed much faster for person A than to the individual B, who was traveling closer to the speed of light. Many years may have passed for person A, while person B experienced a time lapse of few hours. Learn more about this twin paradox in How Special Relativity Works. 

If wormholes can be discovered, this may allow us to travel both to the past and the future.Work like this: say that the entrance of the wormhole is portable. Thus, the individual B in the previous example, who traveled in space for a few hours to 50% speed of light, it could take an incoming wormhole into space, while the opposite end would remain on Earth with individual A.The two people continue to see themselves as the person B traveled into space. When person B returned to Earth a few hours later, for the individual A few years could have passed. Now, when person A looks through the wormhole that traveled into space, he will realize at a younger age, the age he was when person B launched into space. The cool thing is that, on entering the wormhole, the older person could go into the past, while the younger person B could step into the future. 

Cosmic strings 
Another theory on how we could travel back and forth in time uses the idea of cosmic strings, proposed by Princeton University physicist J. Richard Gott in 1991. They are, as their name suggests, string-shaped objects that some scientists believe to have been formed in the early stages of the universe. These strings can cover the entire expanse of the universe and are under immense pressure: millions and millions of tons. 
These cosmic strings, which are thinner than an atom, would generate an enormous amount of gravitational pull on any objects that pass near them. Objects trapped in a cosmic string could travel at incredible speeds, and since the gravitational force them distorts the space-time, they could be used for time travel. Approaching two cosmic strings, or a rope and a black hole, it might be possible to warp spacetime enough to create closed timelike curves. 

A spacecraft could become a time machine using the gravity produced by the two cosmic strings (or by a rope and a black hole) to throw into the past. For this, she would make a circular path in a loop around the cosmic strings. However, there is still much speculation about whether these strings exist and, if so, what form would have. Gott himself said that for just one year back in time, it would take a loop of string that contained half the mass-energy of an entire galaxy. And as with any time machine, you could not go back further than the point at which the time machine was created. 

Problems with time travel 
If someday we can develop a theory for time travel, would open a Pandora’s box of very complicated problems called paradoxes. A paradox is defined as something that contradicts itself. Here are two common examples: 
Suppose you could go back to a time before his birth. The mere fact that you can exist in a time before you were born creates a paradox. If you were born in 1960, how could there be in 1955? 
  
Perhaps the most famous paradox is the grandfather paradox. What would happen if a time traveler went back and killed one of her ancestors before her own birth? If the person killed his grandfather, as she could be alive to come back and kill him? If we could change the past, this would create an infinite number of paradoxes. 
Another theory regarding time travel brings the idea of parallel universes, or alternative histories.Say you actually go back to find his grandfather when he was a child. In the theory of parallel universes, you may have traveled to another universe, one similar to ours, but it has a succession of different events. For example, if you go back in time and kill one of your ancestors killed that person in a universe that is no longer the universe that you exist. And if you then try to get back to your own time, can end up in another parallel universe and never get back to the universe you started. 
The idea here is that every action causes the creation of a new universe and that there is an infinite number of universes. When you killed your ancestor, you created a new universe, a universe similar to yours until the time you changed the original succession of events. 

Still confused? Welcome to the world of time travel. Just imagine how complicated are the ticket prices.