We can conclude that time travelling is possible if we(humans) are able to travel in the speed of light.
BUT, what about going back to the past? Is that possible? Explain.

Yes, what you guys are saying may be true. But according to Michio Kaku, there are THEORIES about time travelling to the past(Check out on YouTube).Therefore, I personally think that time travelling to the past is possible, it is only a problem of engineering,...
Check out this article that i've found:


"If we could journey back into the past, history would be impossible to write. As soon as a historian recorded the history of the past, someone could go back into the past and rewrite it. Not only would time machines put historians out of business, but they would enable us to alter the course of time at will. If, for example, we were to go back to the era of the dinosaurs and accidentally step on a mammal that happened to be our ancestor, perhaps we would accidentally wipe out the entire human race. History would become an unending, madcap Monty Python episode, as tourists from the future trampled over historic events while trying to get the best camera angle.

But perhaps the thorniest problems are the logical paradoxes raised by time travel. For example, what happens if we kill our parents before we are born? This is a logical impossibility. It is sometimes called the 'grandfather paradox'.

There are three ways to resolve these paradoxes. First, perhaps you simply repeat past history when you go back in time, therefore fulfilling the past. In this case, you have no free will. You are forced to complete the past as it was written. Thus, if you go back into the past to give the secret of time travel to your younger self, then it was meant to happen that way. The secret of time travel came from the future. It was destiny. (But this does not tell us where the original idea came from.)

Second, you have free will, so you can change the past, but within limits. Your free will is not allowed to create a time paradox. Whenever you try to kill your parents before you are born, a mysterious force prevents you from pulling the trigger. This position has been advocated by the Russian physicist Igor Novikov. He argues that there is a law preventing us from walking on the ceiling, although we might want to. Hence, there might be a law preventing us from killing our parents before we are born.

Third, the universe splits into two. On one timeline the people whom you killed look just like your parents, but they are different, because you are now in a parallel universe. This latter possibility seems to be the one consistent with the quantum theory.

The film Back to the Future explored the third possibility. Doc Emmett Brown (Christopher Lloyd) invents a plutonium-fired DeLorean car, which is actually a time-machine for travelling to the past. Marty McFly (Michael J. Fox) enters the machine and goes back and meets his teenage mother, who then falls in love with him. This poses a sticky problem. If Marty's teenage mother spurns his future father, then they never would have married, and he would never have been born.

The problem is clarified a bit by Doc Brown. He goes to the blackboard and draws a horizontal line, representing the timeline of our universe. Then he draws a second line, which branches off the first line, representing a parallel universe that opens up when you change the past. Thus, whenever we go back into the river of time, the river forks into two, and one timeline becomes two timelines, or what is called the 'many worlds' approach.

This means that all time-travel paradoxes can be solved. If you have killed your parents before you were born, it simply means you have killed some people who are genetically identical to your parents, with the same memories and personalities, but they are not your true parents."

Extracted from 'Physics of the Impossible' by Michio Kaku (Allen Lane); available from Telegraph Books for £18 plus £1.25 p&p. Call 0870 428 4115 or order online at books.telegraph.co.uk
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