The concept of time machines is paradoxical. If we would be able to go into the past, we might have changed things which affects our past and our present, therefore creating a paradoxical endless loop. If we could predict the future, we would be able to gamble and win the lottery, or in any casino game. Therefore, it’s not possible to change the past nor to predict the future. It is possible to travel into remote future, using relativity for example – but it’s not possible to come back. Time, as we perceive it, is a one way direction – past, present, future.
I read two good books related to the issues of time:
1. A Brief History of Time (Hawking)
2. The Arrow of Time
(You can also read about it in Wikipedia)
I believe that the arrow of time as we perceive it, or what is called the thermodynamic arrow of time (the second law of thermodynamics), is not inherent in the physical world itself, but only in the way we perceive it. Therefore, it is possible to “go to the past”. But although it’s physically possible to go to the past, we will never be able to perceive it. This is because the way we perceive time. Our common sense just can’t handle such things, due to our biological limitations. But it doesn’t mean that’s impossible.