Slipstream/Slipspace can essentially be compared with the fictional ''subspace''. That is something parallel to everything in our universe, but the travel times in ''subspace'' are way less. For example if you enter this ''slipstream/subspace whatever'' on point A, then go to point B IN SUBSPACE, and exit subspace at point B, you will be at point B in normal space. You will have travelled that certain distance in subspace, but that distance is way greater in normal space, and thus you have travelled, probably FTL, across space. Similar principe to the warp drive, really, if only there is something like subspace.
Another explanation of it, is that subspace is some place with alternate physics laws, which do allow you to physically travel faster than light without gaining infinite mass. So if you have an FTL engine but it can't go FTL because of physics.. slip into slipspace and use it there. Yeah, strangelove is pretty right, Sci Fi BS.
With slipstream it's more complicated than that, read it upon wikipedia, I can't be bothered to read all that up.
Some scientists have probably considered it but there is not even remote evidence that even such a thing exists, so it's purely fictional.
Edited by Aftershock, 10 July 2008 - 12:49.