FTL Is a Causality Problem, Not an Engine Problem

What we actually know

Chemical, nuclear and even antimatter propulsion all live comfortably below c and all take decades to centuries between stars. No amount of additional thrust crosses the line, because the line is not a speed the engine must reach; it is a structural feature of spacetime.

Where the extrapolation begins

Wormholes and warped spacetime appear in legitimate solutions to Einstein's equations. The Alcubierre metric is a real solution. But solutions are not permissions: the known versions require exotic matter with negative energy density, quantities comparable to planetary or stellar mass-energy, and no one knows whether nature permits such geometries to form, remain stable, or carry anything through them intact. Genuine faster-than-light motion could also rearrange the order of events and permit closed loops through time — the causality problem returns wearing an engineering costume.

What the novel invents

The Servator arrived. That is the only datum. The novel deliberately refuses to specify the mechanism, because specifying it would make the builders' physics a variant of ours — and the point is that it may not be.

Where to read next

• — Alcubierre, 'The warp drive: hyper-fast travel within general relativity'
• — Morris–Thorne wormholes; energy conditions in general relativity