According to the laws of nature we cannot travel back in time even if we had the capability to time travel. Every event in the universe has a cause before an effect. The event must have a cause before you can see the effect of the event.
For example the Sun produces heat (the effect) because of the thermonuclear reactions of the hydrogen atoms occurring in its interior (the cause).
This what the "Grandfather paradox" of time travel is all about. You cannot go back in time and kill your grandfather to stop yourself from being born. The paradox here is if you were never born you will not be able to go back in time to do this act because you will not exist to do the act. You being born is the cause of the event to go back in time to kill your grandfather. That is the effect of the event. It cannot be done in reverse.
It doesn't make sense , the the effect of you killing your grandfather cannot happen before you were born to cause this effect. That is the universal law of the universe, every event has a cause before an effect of the event can be detected.