E=mc2;
E=energy
m=mass
c2=speed of light squared
can be rewritten as m=c2/E
since energy cannot be made, as you speed up, mass increases exponentially
before you reach the speed of light you will have an infinite mass, which is impossible
I remember this from ages ago so its probably not 100% right, but basically, we cant travel at the speed of light.
Also, it was only thought that black holes could be used as "portals" if light cannot escape them. Unfortunately for us , especially sam, Big steve hawkins found out that light can escape from them so time travels not possible that way either.
We gotta keep dreaming though...