here. Now, in After Doomsday (St Albans, Herts, 1975), we have a third means of faster than light travel (FTL).
The superlight drive is based on:
"The mathematical depiction of space as having a structure equivalent to a set of standing waves in an n-dimensional continuum." (p. 29)
(The "Standing Wave" is one of James Blish's means of FTL along with his Haertel overdrive, Arpe Drive and Imaginary Drive. Blish preferred to avoid the cliche of "hyperspace.")
In After Doomsday, where the waves interfere, a spaceship can move between them. In interstellar space, where gravitational distortion is minimal, interference fringes are sufficiently close that spaceships can skip most of the straight-line distance to their destination. Whereas galaxies recede as intergalactic space is generated, a superlight ship exploits zones where space is cancelled out - except that the field physicist winces at this summary by a mechanical engineer.
More interesting is the kind of interstellar society that Anderson imagines in this, and only this, novel. It is not known whether superlight ships were invented once or more than once. Explorers found primitives, civilizations not interested in space travel or those that were able to learn and apply superlight, which thus spread not like a cone of light but like dandelion seeds. Each space-traveling species deals regularly only with other species in its own civilization-cluster because there are too many for regular contact elsewhere. The galaxy is too big for any single Federation or Empire. No one can have an overview or know the full history. This makes sense and would have been a perfect premise for a series of novels.