Since I only learned special relativity but not yet general relativity, please bear with me that our discussion will be limited to the scope of special relativity, that is, suppose we would not utilize special spacetime topology, such as worm hole and black hole. I ran into some video about time machine on Discovery Channel. You can watch it on youtube at , which talks about the time travel to the past and the actual time machine is being built. However, I doubt this idea very much, with my knowledge of special relativity. In special relativity, time travel to the future is believed possible and exemplified by the twin paradox. The twin paradox is well resolved. When the traveling brother returns to the earth, his brother is older and more time has passed on the earth. This looks as if the traveling brother manages to time travel to the future. But this so called time travel has the following nature. Please confirm: 1. Suppose the traveling brother has a powerful telescope on the spaceship, he would see the time on the earth passes slower, during his trip away from the earth. 2. He could not interact with his brother or anything else on the earth, except sending radio signal back. 3. He would only see the time on the earth passes much faster, during his trip back to earth. 4. Once he is back to the earth, he can interact with his brother again. 5. Both of them are older than when they separated and commerced the trip. Because of this, some people argue that time travel to the past is also possible. Now let us think of this scenario: the traveling brother wants to age faster than the brother on earth. There are probably two ways to achieve this: send his family for high speed travel, or he stays up high up in the space where gravity is lower than on the earth. In this case, when the brothers unite, the aging faster brother claims he has traveled to the past because his family is younger. I think the above scenario is possible. But it is still not time travel to the past. It is merely that the time passes in different rates. Both of the brothers are still older than when they separated. Either of the brother can ever go back to any time before they separated. This can also be very easily seen from a spacetime diagram. No matter how much the speed is, the time axis is always warped in a way that, it is still pointing to the positive direction in time, and positive or negative direction in space. This indicates that time can only move forward but pass in different rates. As for the grandfather paradox, found at http://en.wikipedia.org/wiki/Grandfather_paradox, I think it is also easy to explain. The man simply is not able to travel back to kill his grandfather. If the grandfather is still alive, say 60 years old and the man is 10 years old, it is possible that when the man meets his grandfather, the grandfather is 70 years old and the man is 80 years old, but, even now, if the man kills his grandfather, he will still be alive, and his grandfather still did give birth to his father before and his father to him.