The closer you get to the speed of light, the slower time progresses relative to everyone on earth.. We are travelling at a fixed rate, so time moves at a fixed rate.. If we take a spacecraft up to a speed faster than earth travels, time slows ever so slightly.. If you get really close to the speed of light, its alot more pronounced.. If you could spend 50 years moving at that speed alot more time will have passed on earth during the same timeframe..
Travelling back will only be possible if you can bend light into a circle.. Theres a lab called CERN where they are attempting this with mirrors.. Essentially if you can make a laser go in a circle, you could pass particles through the void created in the center.. The thing is, you can only send them back to a time after the "portal is open".. I read a bit about this, and that was the theory
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