Controls
Diagram is a spacetime diagram: position across, time up, light at 45°. The other observer starts one light-unit away, so one time unit is the time light takes to cross that gap. Everything above the dashed line happened after you sent. If the return leg lands below it, it arrived before you sent.
Things to notice
An instant jump needs almost no motion to break
Pick any shortcut, hyperspace or ansible drive and nudge the other observer's speed to 0.05c. The return leg already lands before you sent. The faster the drive, the lower the speed that breaks it, all the way down to zero.
The thresholdWarp and hyperfast ships get a speed dial
Pick a warp drive or a faster-in-flat-space ship and the speed slider wakes up. At 1.1c the other observer needs nearly 0.995c to cause trouble. At 20c, a tenth of light speed does it.
Why the dial mattersThe sublight ships cannot build the diagram
Pick a generation ship, a ramjet or a hibernation liner. Slide the other observer's speed anywhere you like. The return leg always lands after you sent, and if they outrun your message it never reaches them at all.
Why nothing goes wrongSome stories own the bill
The register marks a drive C-owns when the story admits the reply can beat the question and does something with it. Among them: . Pick one and read its note under the diagram.
All the ownersThe stories marked C-dodges take a preferred frame
A few stories keep their drive and avoid the paradox by giving "instant" one fixed meaning for everyone, a rule the register files under CAU-4. Pick Le Guin's ansible or the Foundation jump and read the readout's mark.
Register CAU-4Switching frames changes nothing
Draw it in their frame instead. The return leg still lands on your worldline below the send event. Whether you got the reply before you asked is a fact about your worldline, not about a viewpoint.
What is invariantWhat is going on
The register's first causality entry, CAU-1, says that any signal faster than light in one frame runs backwards in time in some other frame, and that two such signals between people in relative motion deliver the answer before the question. That is true of any mechanism at all. A warp bubble that moves the space, a wormhole that shortens the path, a jump through hyperspace and an ansible that sends only the message all count as the same kind of signal once they beat light between two points. This page treats each drive as exactly that signal and draws the two-leg exchange from the antitelephone demo.
The only way out that keeps the drive is CAU-4, the preferred-frame escape: if "instant" means instant in one special frame for everyone, the two legs no longer add up. The cost is that special relativity is no longer the whole story for the FTL sector, and the story has to say what picks the frame. The catalogue marks each fictional drive on what it does about all this. Most say nothing. A few own it and build the plot on it. A few dodge it with a stated rule. The sublight ships, kept as a control group, never owed the bill in the first place.