Why would we wonder how it works if it's possible?

In order to time travel:

1.)Flux Capacitor
2.)1.21 gigawatts
3.)A Ferrari

Its the butterfly affect, if you havnt watched that movie i recomend you do, its a great movie!

who cares about time travel. Lets get to a new planet first, so we can actually get enough resources. We have way to many people on this planet. We are killing it.

Time travel has already been done (:
'Time Dilation' is the key word here! No body notices though because it is such a small amount and not like doctor who or something.
The closer you are to the speed of light time passes slower. Humans can't notice it but if a clock was on the train the hands would be off by a 2 billionth of a second to a clock just at the station
if the train went to 99.999% the speed of light, then a year would pass for every 223 years at the station. ;)

And it's not just speed, the closer you are to the centre of mass the slower time goes which is similar with if you had a clock on a satellite further from the centre of mass, i.e earth and the high speed of it's orbit (:
So gravity and speed are needed.

P.S none of this is copy and pasted (: