have you herd of mini black holes? they theroticly the size of a proton and have a gravity well not much bigger
now I have thought, if you could atrifually make those mini black holes, possibly by having 2 protons decay from an energy state (a thery of mine) if would form a black hole that has very little mass, but is infonatly dence
now if you could form those blackholes into a series of rings that have trillions of blackholes a square micrometer, than their combined gravity wells would make the ring have a donut shaped gravity field
if you would be able to have a ship genarate those rings in frount of itself, and it would fly through the center, it would experience a linier slingshot effect simmiler to going around a star, after a series of these it could theroticly approach the speed of light (it wouldnt be able to reach it however, because the energy beam genarating the rings would be traveling at the speed of light
even if the thery that mass becomes infonate as speed increases is true, it would have minimal effect on this system, because the heavyr it is, the more the gravitional field effects it, and the better the system works
it would gather the protons needed by colecting them from intersteller plasma via a mag scoop (simmiler to a mag sail that you guys have been talking about, but instead this is an intake for intersteller hydrogen)
of corse the best thing about this system, would be that it would be able to be stationary, the ship I described had the system internal because it would need it to slow down as well (I doubt that a mag sail would be an effective intyersteller brake at those speeds)
but once the ship is there, it would be able to build a stationary system consisting of physical rings supporting the blackholes, wich would be much larger because of their permenent nature (the rings of the ship would only last a few millionths of a second due to hawking radiation, although some small blackholes may combine to make bigger ones that would leave a trail)
those rings would allow a ship to acheave almost infonate speeds (therareticly) depending on the size of the black holes, and the amount of stages of rings
a ship would simply fly into the ring system, and it would be accelarated, it would cruse untill it reached a simmiler ring system at the destanation that would slow it back down
of corse, lower power rings would be perfact for inter soler system commerse, it would take the round trip time from Earth to Saturn from 1.5 years with fusion, to mabe hours or days
of corse, the rings would require more than just power, they would require physical matter to constantly feed the blackholes to keep the gravity field stable, but the profit potentual that the fast traveling system would make, would be much greater than the upkeep costs
I came up with that idea in about 5 min while taking a shower about 6 months ago lol