(I posted this on the other site and decided to copy and paste it here for the same reasons but I edited it and added more detail)
According to Einstein's theory of General relativity. As something speeds up, time slows down.
This, in fact, can lead to time travel into the future if you travel at almost the speed of light but you can't travel exactly at the speed of light or time would stop completely, plus, it would take more energy than avalibile in the universe, as Einstein observed.
Time travel into the past, however, is more difficult as in order for time travel into the past, the time machine must exist.
You can only travel back in time to where the time machine existed as without the time machine you may simply disappear.
If you travel back into time the time machine would disappear and you would be stuck even if you did manage to travel to whatever time it is.
So therefore, time travel into the future is as simple as going near the speed of light.
A theory says that black holes might allow you to time travel into the past if you could use a machine to open the blackhole for long enough.
The end you would come out of would be what's known as a "whitehole" (it pushs things out unlike blackholes which suck things in)
It says that if you did make it out then you might be able to go back in time to a random time.
However, testing this theory would be impossible for many reasons, but not limited to:
Death
The nearest black hole is light years away
There is no such machine that can do such things
THIS IS A SERIOUS POST ABOUT MY RESEARCH ON TIME TRAVEL