>  Yet, this is not as simple as it sounds in any case. The slingshot effect
> is always relative to something. For example interplanetary gravitational
> assist is relative to the Sun. The Sun itself cannot be used for a 
> slingshot
> effect inside the solar system (it could be used for a slingshot relative
> to the galaxy, but not relative to the solar system).
>
>  With this taken into account, can you just go from Earth to the nearest
> black hole, get an enormous speedup and come back at 100x the speed and
> slam onto Earth at that speed? From a gravity assist only, I don't think 
> so.

I was thinking, it doesn't really matter about a gravity slingshot for time 
travel, just speeding up as you go close to the black hole will suffice to 
allow you to travel forward in time.  It doesn't matter if you slow down on 
the way back because you've already done the time-travelling bit as you went 
around the black hole close to the speed of light.

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