AFAIK the Epstein drive from The Expanse is pretty much at the edge of known physics in terms of being plausible, with a few additional caveats. The biggest of

AFAIK the Epstein drive from The Expanse is pretty much at the edge of known physics in terms of being plausible, with a few additional caveats. The biggest of these is that IRL those ships would have to have large heatsinks. Even if the drive were phenomenally efficient you're going to have a minimum of hundreds of megawatts of waste heat to radiate into space to avoid melting the ship. The second one is that the drive itself would be a deadly weapon. Those scenes in The Expanse where they fire up the drive near other ships or near space stations? Nope, not unless you want to fry everyone on board with X-rays. You wouldn't fire up a drive anywhere near anything you didn't want to cook with hard radiation. This last point is known as Jon's Law in space engineering and sci-fi: any sufficiently powerful space drive is also by definition a weapon of mass destruction.