How General Theory of Relativity proved the possibility of Time Travel.

Time travel has always been a fascinating topic to think about. Will we ever be able to see the future or travel back in time? Would there ever be a time machine or are there any other means to travel through time? What would be the consequences of time travel? What is the Grandfather Paradox? These are just some of the questions that arise when we talk about time travel. But let's start from the beginning. 

Sir Issac Newton is considered the greatest scientist of all time. His understanding of Gravity is one of the major leaps in the field of physics. He came up with the Inverse Square Law based on the works of Kepler who told that the celestial bodies "somehow magnetically" moved in an elliptical path around the Sun. Newton wasn't satisfied with the explanation and studied the revolution of the Moon around the Earth. His theory states that the gravitational force of an object depends on its size and the distance between 2 objects. But he always believed that gravity is an innate force that can act on 2 bodies at a distance and also time is linear and it can move only in one direction - forward.
Then came Albert Einstein who said that space and time are not two different entities but are interwoven into one, called Space-time. He came up with a thought experiment in which he is in a rocket traveling at 9.8m/s/s directly upward and weighed himself on a scale, he would weigh exactly what he would if he were on Earth. This means that if he didn't know he was on a rocket, he would definitely tell that he was standing on Earth. This sums up the fact that gravity was somehow related to acceleration. He then came up with another thought experiment where he thought, what would happen if we're to switch on a flashlight pointing it at one wall of the rocket while standing against the opposing wall? He realized that if he had a sensitive measuring instrument he would be able to measure the slight curvature in the path taken by the light to reach the opposite wall because the rocket is moving at a constant speed perpendicular to the beam of light.

He then thought about what would happen if he were to measure the same effect here on Earth. We may think that light would travel straight and hit the wall at the same height as the source (the flashlight) but that would violate the Principle of Equivalence. Since the effect of Earth's gravity and the acceleration of the rocket has the same effect, the light should bend and reach the opposite end of the room slightly lower than the source.

Now a problem arises where we know that light always takes the shortest path, so how can it bend in the presence of gravitational field?. Einstein then realized that the light is taking the shortest distance, but that shortest distance is not a straight line but a curved one. He later hypothesized that in the presence of mass and energy, space becomes curved. This means that the shortest distance taken by a beam of light in the presence of a massive body is a curved one. 

But why did he call it Space-time? How was time affected by this? To understand that, he used one of his old theories, Special Relativity. This states that the speed of light is always the same regardless of the perspective of the observer. It means that the speed of light will be the same in an accelerating reference frame as well as in a resting reference frame. Now, since we know that acceleration is relative to gravity, he said that the speed of light in gravity will be the same as it's the speed in empty space. We know that,

Speed = Distance / Time.

But if the distance traveled by the beam of light in the presence of gravity, due to the curvature of space, is greater than the distance traveled by the beam in empty space, how can the speed be the same? 

He understood that for this to happen, time itself must slowdown in the presence of a gravitational field. This implies that an observer who is experiencing no gravity at all will see a clock running in a gravitational field run slowly. So the clocks at the International Space Station run ahead of the clocks here on Earth. 

The General Theory of Relativity thus explains how the celestial bodies move around the sun. Imagine our Solar System to be the surface of a trampoline. Since the Sun has a huge mass and energy, it bends the spacetime the most in our solar system. This causes the sun to be at the center at a certain depth on the trampoline. Now if we let a marble to roll around the edge of the trampoline it would revolve around the edge, gradually making its way to the center. The marble represents the planets that move around the Sun. The question arises that why doesn't the Earth collide with the Sun? This doesn't happen because the Sun itself is moving in a particular direction which prevents the planets from falling into the curved spacetime. 
 
Now that we have established that time runs slower in the presence of the gravitational field, what would happen if you stand next to a Blackhole? Yes, you would die for sure, but let's not get into the details. The gravitational effects of a black hole are so enormous that it doesn't even let light pass through it, hence the name. This makes time travel into the future every convenient. All you have to do is travel to the nearest black hole, spend some time around it, come back to Earth to find yourself in the future. Since the gravity of the black hole slows downtime to such an extent that if you go to the very edge of the event horizon of a black hole, being careful not to fall into the black hole, one week spent there for you could be a million years for someone staying at Earth. This implies that by spending time near a black hole we would effectively come back to Earth in the future! This is one of the methods of time travel into the future. 
Traveling at the speed of light is another way we can speed up time. This is called Time Dilation. It can be explained by the following example. Five years on a ship traveling at 99 percent the speed of light (2.5 years out and 2.5 years back) corresponds to roughly 36 years on Earth. 

Traveling back in time is rather tricky. Many paradoxes arise when we think about the consequences of time travel to the past. One of those is called the Grandfather Paradox. Imagine you travel back in time and kill your grandfather. This means that your parents won't be born, which implies that you won't be born which then means that you won't be able to travel back in time to kill your grandfather. Stephen Hawking has come up with a solution in which he tells that we simply cannot travel back in time because there is a presence of a Chronology Protection Conjecture which prevents the appearance of closed timelike curves. He simply denies the possibility of backward time travel by telling that somehow the physical world will stop us from changing the time-line of the past. But some scientists believe the existence of parallel universes which would make time travel possible. This means that if you travel back and kill your grandfather, a branched universe will form in which you would never be born but you would be able to return to the present in which you are alive. But to this day traveling back in time has no substantial evidence to support it. 

Newton believed that space is constant and time is linear. He said that no matter where we are in the universe, time would pass at the same rate. But Einstein's theory proved him wrong. So, if you put them face to face in an intellectual battle, who do you think will win? I believe that it would be Newton because, without any Godfathers to guide him or any previous study of a topic, he was able to come up with something that changed the course of our understanding of the universe. Einstein had the works of Newton himself to start his research. In one of the interviews, Stephen Hawking was asked "what would you ask Newton if you ever get a chance to meet him?". Stephen Hawking said that he would ask Newton "what would happen to a star that cannot support itself against its own gravity?". This question is so complicated because it deals with the topic that came right after where Newton ended his research. So, would he be able to come up with the solution to the question he never even thought of? It would be one heck of a conversation that no one would want to miss!