In 1915, Albert Einstein proposed his theory of general relativity. This theory was the basis for two people to agree upon what they see, when one of them is stationary, while, the other is moving. This theory later went on to form the ground work on developing the Global Positioning System and explain occurrences such as gravity, space-time, time dilation, black holes, the fourth dimension and the most fascinating of all, time travel.
Unlike the Newtonian theory, which states that every object that has mass exerts an attractive force (gravity), Einstein concluded that it was not the object itself that exerted the force, but it was the presence of the object in space that was warping space-time. The warping of the space-time, which can be interpreted as a very thin cloth, caused the movement of the objects within it.
Einstein discarded the concept of the past, present and future calling it “a stubbornly persistent illusion”. He believed that there was only the now. So if there were an alien, a hundred thousand light years away eating a sandwich right this instant, because of the distance to be travelled by light, which governs time, that alien would actually be eating that sandwich a long time ago on ‘earth-time’, around the time of the second world war.
In his theory, Einstein explained that the closer you get to the speed of light, the slower time gets. This is known as Time Dilation. If one, by some miracle, was to reach the speed of light, which is a constant, time for that person would stop completely. So naturally, if one went faster than that speed, one should be able to travel back in time. This concept however, was proved impossible by Einstein in his equation E=MC2. The closer one was to get to the speed of light, the more would be the mass of the object. Since kinetic energy is directly proportional to the mass of the object, the object would be of infinite mass when it reached the speed of light. It would hence, take infinite energy to move that object, which is clearly not possible given today’s technology.