Most of us belief that time is the same everywhere. A minute here is the same as a minute anywhere else. Now the time has come to get over this idea. There are places in the universe where time runs slower than in other places which means if you happen to be in such places, you’ll grow old slower than others! Our understanding of time is very different from its reality. Scientists today believe that time is the 4th dimension.

The time in GPS satellites orbiting the earth runs faster than the time on earth. Everyday a third of a billionth of a second is gained by these satellites. A correction is made in the time in these satellites to match with the time on earth. The reason for this is simply the mass of the earth. As explained by Einstein, matter drags on time and slows it down. Heavier the object, the more it drags on time. Although the time gained by satellites here is very small, the same principal applies for big numbers.

Stephan Hawking says if a satellite orbits around a black hole, massive enough to fit in 4 billion suns, time travel can be observed very well. The time taken by the satellite for one orbit could be 16 minutes as observed from a space station on earth or anywhere else away from the black hole. But for the astronauts inside, the time taken would be only 8 minutes! The black hole is so massive that its drag on time is very high. But this experiment is dangerous. If a very high speed and a perfect trajectory is not maintained, the satellite can get sucked into the black hole due to its high gravity.

As strange as this, is a phenomenon of light. Imagine a high beam light source switched on behind a body moving at 2 hundred thousand kilometer per second. Simple calculations show that light rays from the source should overtake the body at 1 hundred thousand kilometer per second as the speed of light is 3 hundred thousand kilometers per second. But the calculation here isn't that simple. Light overtakes the body at its regular speed, i.e. 3 hundred thousand kilometer per second. Similarly if the body is moving towards the light source at a speed of 2 hundred thousand kmps, light ray does not overtake the body in 5 hundred thousand kmps. Light rays pass by a body at the constant speed of 3 hundred thousand kmps independent of the body's speed and direction of motion. The reason for this is the fact that time stops for anything traveling at the speed of light.Which means traveling around a black hole isn’t the only way for time travel.

Stephen Hawking explains the phenomenon with an example of a hypothetical train traveling around the earth at 99.99% the speed of light. Spend a week in that train and you will find out that the world outside is a hundred years in future when you step out. A week in this train is a 100 years outside!

While in the train, your heartbeat, pulse rate, walking, aging and even ticking of your wrist watch slows down. This cannot be observed while inside because everything including the movement of the brain signals slow down at the same rate. This idea of time slowing down at high speeds has been experimentally proven in the particle accelerator in Geneva. Short lived particles called ‘pi mesons’ which ordinarily disintegrate in 25 billionth of a second live 30 times longer when accelerated at 99.99% the speed of light.

But why does speeds close to that of light slow down time? It is to make sure the body doesn't reach the 100% of the speed of light. Laws of physics forbid a body or any particle with mass from reaching the speed of light. That explains the '99.99%'. For a body travelling close to this speed, time slows down just enough to keep the body from breaking the cosmic speed barrier!      

Now this explains time travel to future. But what about time travel to past? There are two major problems with time travel to past. First problem is that the body has to travel faster than light to go to past which is impossible according to the laws of physics. Second is paradoxes. If someone went back in the past and killed his great grandfather before he was married, then his existence doesn’t make sense. He shouldn’t be born because his great grandfather was killed before he was married which means neither his grandfather nor his father were born! This is popularly known as the "Grandfather Paradox". Considering these kind of paradoxes, many scientists including Stephan Hawking deny the possibility of time travel to past. But not all scientists believe so.

Dr. Ronald Mallet, a physics professor in the University of Connecticut has already made a blue print for a time machine that can connect with past and has started working on building it. He says, “I feel with current technology it is possible at the very least to send sub-atomic particles back into the past.” If sub-atomic particles can be sent, information can be conveniently sent in the same route. Dr.David Deutch, physicist and writer, explains how a body can be made to move faster than the speed of light near a spinning black hole. A spinning black hole drags the space around it in circles and sucks it towards the centre just like water in a whirlpool. He says, “In this kind of situation, a body can evade the rule of not exceeding the speed of light”. Dr.Mallet knows a comparatively easier and more efficient technique to twist space-time. “Space-time” might sound confusing till we understand that space and time are not independent of each other, but continuous as explained by Einstein.

“The secret lies in the Einstein’s theory even though he didn't theorize about time travel to past”, says Dr.Mallet. His idea is to use circulating high intensity laser light beam to twist space-time into a loop. High intensity laser beams today can produce temperatures higher than the center of the sun. The loop connects the past to present, present to future and since it’s a loop, it also connects future to past. “A particle fired in a cork tree shaped path into a tunnel of circulating high intensity laser beam will wind its way to the past” he says.

But in this real time machine, the particle can be sent back in past only till the date and time the time machine was first switched on unlike the one in sci-fi. There could be two possibilities after going to past. First is that even if someone goes to the past, there is no free will to change any events or the chronology of events that have already occurred. This way there are no paradoxes. Second possibility is that there is a free will! And this gives rise to another exotic theory called ‘parallel universe’. When you go to past to meet your great grandpa in his teenage, you happen to go to a parallel universe where your great grandpa is still in his teenage!! And there is not one parallel universe but billions of it. However finding out which of the possibilities is true can be done only after building a functional time machine.

Mastering time travel would be the pinnacle of scientific advancement of the human race which would not only revolutionize our technological levels by helping us understand the future technology, but also take us to a different level of understanding of the universe, time and life itself.