Before I begin to talk on time travel, I would like to talk about time itself. Time is a property of the universe, in fact it is the fourth dimension of space-time. That is space has three dimensions while time is the fourth dimension. Albert Einstein has taught us in his relativity that there is nothing like absolute time. This means that different people observing a particular event might measure different time for the event.
In this post, I want to talk about what is called the arrow of time. Why do we feel that time flows in the forward and not in the backward. According to Stephen Hawking in his book A Brief History Of Time, there are about three arrows of time and they are- the thermodynamic arrow, the psychological arrow and the cosmological arrow of time. The important thing to note is that they all point to the same direction. Forward.
Entropy defines the degree of disorderliness of a body and according to the second law of thermodynamics, if things are left to themselves, entropy would increase. This means that the disorderliness of a body would increase. Let’s take an egg for an example. An egg has a more likelihood of cracking, that is its entropy tends to increase. And when it cracks, it is quite not possible for it to form back as a whole egg. At least, it doesn’t happen this way. Time follows a similar pattern, it can only move forward.
The psychological arrow deals with the fact that we remember things from the past and not from the future. If this arrow of time doesn’t follow the same direction as the thermodynamic arrow, we would remember things from the future and not from the past. This is not what happens and so the arrow points in the forward direction.
The cosmological arrow deals with the fact that the universe is expanding rather than contracting.
In 1935, Einstein and Nathan Rosen wrote a paper in which they showed that general relativity allowed what they called “bridges,” but which are now known as wormholes. The Einstein-Rosen bridges didn’t last long enough for a spaceship to get through: the ship would run into a singularity as the wormhole pinched off. However, it has been suggested that it might be possible for an advanced civilization to keep a wormhole open.
But a question we might ask is that why haven’t we seen anybody from the future if time travel is indeed possible.
A possible way to explain the absence of visitors from the future would be to say that the past is fixed because we have observed it and seen that it does not have the kind of warping needed to allow travel back from the future. On the other hand, the future is unknown and open, so it might well have the curvature required. This would mean that any time travel would be confined to the future. There would be no chance of Captain Kirk and the Starship Enterprise turning up at the present time. This might explain why we have not yet been overrun by tourists from the future, but it would not avoid the problems that would arise if one were able to go back and change history. Suppose, for example, you went back and killed your great-great-grandfather while he was still a child. There are many versions of this paradox but they are essentially equivalent: one would get contradictions if one were free to change the past. There seem to be two possible resolutions to the paradoxes posed by time travel. One I shall call the consistent histories approach. It says that even if space-time is warped so that it would be possible to travel into the past, what happens in space-time must be a consistent solution of the laws of physics. According to this viewpoint, you could not go back in time unless history showed that you had already arrived in the past and, while there, had not killed your great-great-grandfather or committed any other acts that would conflict with your current situation in the present. Moreover, when you did go back, you wouldn’t be able to change recorded history. That means you wouldn’t have free will to do what you wanted. Of course, one could say that free will is an illusion anyway. If there really is a complete unified theory that governs everything, it presumably also determines your actions. But it does so in a way that is impossible to calculate for an organism that is as complicated as a human being. The reason we say that humans have free will is because we can’t predict what they will do. However, if the human then goes off in a rocket ship and comes back before he or she set off, we will be able to predict what he or she will do because it will be part of recorded history. Thus, in that situation, the time traveler would have no free will. The other possible way to resolve the paradoxes of time travel might be called the alternative histories hypothesis. The idea here is that when time travelers go back to the past, they enter alternative histories which differ from recorded history. Thus they can act freely, without the constraint of consistency with their previous history. Steven Spiel-berg had fun with this notion in the Back to the Future films: Marty McFly was able to go back and change his parents’ courtship to a more satisfactory history.
Source: A Brief History Of Time