Here it is:
(I used both the metric system and American system as I posted this on a different site first)
Why time travel to the future is possible but time travel to the past is much more difficult and may even be impossible.
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 available 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 pushes 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:
The nearest black hole is light years away
There is no such machine that can do such things
In a scientific study it showed that Astronauts aboard the ISS are in fact moving so fast that they are 0.07 milliseconds ahead of there relatives on earth. The reason why is because the ISS orbits Earth at a speed of 4.76 miles (7.660477 Kilometers) a second so in an hour it moves at 17,000 miles/ 28,000 Kilometers an hour.
This is only a fraction of the speed of light but they are moving fast enough to experience time dilation
And technically, we are moving into the future at a rate of 1hr/hr.
Even if you did manage to travel back in time three things would happen to you:
1. The Earth is constantly moving in the universe, not only around the sun but through space. So the position of something on Earth, say... an hour ago, isn't the same spot in space. You'd end up appearing in the vacuum of space.
2. If you could anchor yourself so #1 doesn't matter, you'll materialize in a space with matter. And two atoms can't coexist in the same space so at best, you'd have air bubbles form inside your body and you'd die. At worst, you'd fuse your atoms with the surrounding material and undergo nuclear fusion.
3. You can't change anything in the past so why would you bother?