I have read many books and this question appear again and again. As we know, we learned to not asking, which Feynman has mentioned many times. Even this simple question we cannot ask people when we were young, since they normally would call you insane.

People do not know the reason, however, people would ask a lot about other things which have no reason as well. Recently, I have asked myself and I have come up with an answer. "Why can't we go back in time?" A bit similar to those I read from Hawking, it is something related to thermodynamics.

It takes energy for us to remember, to draw an ordered pattern in our brain. Our brain do have lots of disordered pictures (which may appear in different forms in our imaginations or our dreams), however, only the ordered types, which somehow we think hard to remember were burnt into our brain, just like DVD.

As we all know, in order to go back in time, we need sufficient engery, swap particles to anti-particles, mirror image and do everything in reverse order. You can run backwards and use your left hand as your right hand, anti-particles maybe just like we go back in time to see everything in anti-particles. I believe go back in time is possible, and the reason why we cannot see people from future can be concluded into 2 reasons:

1, people from future are in form of our anti-particles, which after hitting our 'real' world particles, then annihilate to flash....

2, people from future would require the use of energy, which will change his ordered pattern in brain to a more disordered status. By the time he come back to our time, it is not possible for him to remember what to do here.

A bit like science fiction, but it is true in a sense that it is analogue to our anthropic principle. Give yourself a space to think questions you dare not to ask.