First, understand that time does not exist apart from the other three dimensions.
Einstein’s acceptance of time as a dimension of space-time along with height, length, and depth negates his own assertion that time is linear and that travel along it is possible in only one direction. One can go up or down in the dimension of height. Once can go forward or backward in the dimension of length. One can go left or right in the dimension of width.
Therefore, one should be able to go to the future and to the past when the technology to do so becomes available. However, I hypothesize that in either direction doing so will be like viewing a “Read Only” file in Word or similar programs. In other words, one will be able to see both but to affect neither.
Time isn’t really linear; it’s circular. With thosee other three dimensions of space-time, it’s actually spherical. Therefore, from any single point in time, the future has already happened, and the past is still yet to be.
The only point at which we can truly exist is the moment we are at right now; all others are past or future, gone away or yet to be. But that one moment where we are now is the beginning, and the end, and every moment in between. In other words, each and every moment we are alive, we are living at the moment of the Big Bang, the moment of Creation, in a manner of speaking.