Although that may seem facetious, the truth is that the debate remained unsettled until Robinson and the crew of the Daedalus made the first FTL flights. It took empirical experimentation to answer the question of time travel for certain, and it was only after the fact that the supporting math and physics were fully developed.

What was found was this: no matter how fast a superluminal voyage—no matter how many multiples of c your spaceship travels—you will never be able to return to your origin point before you left. Nor for that matter can you use FTL signals to send information into the past. Some amount of time will always elapse between departure and return.

How is this possible? If one is at all familiar with light cones and Lorentz transformations, it should be blindingly obvious that exceeding the speed of light results in being able to visit the past and kill your own grandfather (or something equally absurd).

Yet we cannot.

The key to understanding this lies in the fact that all three luminal realms belong to the same universe. Despite their seeming separation (as it appears from our normal, subluminal point of view), the three are part of a larger, cohesive whole. And while local violations of physical laws may appear to occur in certain circumstances, on a global scale, those laws are upheld. Conservation of energy and momentum, for example, are always maintained across the three luminal realms.

Adding to that, there is a certain amount of crossover. Gravitational distortions on one side of the luminal barrier will have a mirrored effect on the other. Thus, an object moving in subluminal space will leave an STL gravitational distortion in the equivalent FTL space. Waves from the distortion will propagate outward at c no matter what, but the movement of the gravitational center will be less than c. And the reverse is true for a superluminal gravitational mass, which would leave an FTL track of spacetime ripples through normal, subluminal space. (Of course, no such FTL tracks were detected prior to the invention of the Markov Drive, but that was a result of—in most cases—their extreme weakness and the distance of most superluminal matter from the main body of the Milky Way.)

Note: it’s important to remember that just as anything moving faster than c in subluminal space could theoretically be used to arrange a causality violation, so too could anything moving slower than c in superluminal space. In FTL, c is the minimum speed of information. Above that, relativity and non-simultaneity are maintained, no matter how fast you might be going.

Even without the existence of a Markov Drive, we now have a situation where natural phenomena seem to be violating the light-speed barrier on both sides of the spacetime membrane, but again, without inducing any causality violations.

The question returns: Why is that?

The answer is twofold.

One: no particle of real mass ever breaks the light-speed barrier in either the sub- or superluminal realm. If one did, we would see all of the paradoxes and causality violations predicted by traditional physics.