In her seminal study of the geometry drive, Rani Spengler posited that three elements had to be considered when assessing the capabilities of the drive:

• Relativity.
  
• The ability to go faster than light.
  
• Causality.

These three elements cannot exist together. Specifically, relativity and causality existing at once mean that faster than light travel cannot exist, she noted. So far, that was the accepted paradigm: relativity works and effects are always preceded by causes. The problem is that the geometry drive does allow for faster than light travel. Point to point instantaneous travel by way of space-time folding, to be accurate.

And there's a concern with that. A massive one. Under relativity, the faster you travel, the slower time passes, as far as your frame of reference is concerned. Once you go past lightspeed, you start going back in time. Because there is no absolute frame of reference, you can witness an event before it ever happened. FTL is time travel. And time travel breaks causality — one may for instance create effects that have no cause.

And it's the crux of the issue. Relativity holds very well. We've been testing it for several centuries, and we have yet to find a hole in Einstein's theory. But faster than light travel also works! So the only possible solution — and Rani quickly understood it — is that causality has to go. This is precisely why she held the geometry drive in such regard. It wasn't just a tool for interstellar travel. It was an artifact that could, potentially, break the very foundation of the universe. It was a tool that could violate the very foundation of the world: causality.

Now the question becomes: why didn't the geometry drive turn the world upside down? Why didn't we start seeing ships returning from battles before they even happened? Courier ships sending letters to people who aren't even born? Admirals and politicians planning cross-time schemes?

Because the geometry drive was created by geniuses. Much like it cannot translate a ship into solid matter, the geometry drive has a built-in safety mechanism to prevent the creation of paracausal loops. Many people who tried to leverage a geometry drive to create such paradoxes have encountered what Rani called the “causal barrier”: it seemed that the entire universe acted so that their attempt at time travel would not have any measurable effect. For instance, when during the Pleiades wars admiral Azches tried to ambush the Sequence by performing a time-loop manoeuvre, none of her ships managed to jump that day, due to a highly improbable combination of mechanical failures, human errors and well-timed Sequence counter-attacks. The most likely explanation is that the failsafe mechanisms of his ships' geometry drives had engaged, manipulating the structure of the universe and thus preventing Azches from sending help to her past self.

The causality failsafe doesn't always work. This is where things get interesting. We know that causality breaches can happen when using the geometry drive. They are rare, but they exist. The Eletharna, a courier ship on the Elora-Earth lane, is known for carrying letters addressed to people who aren't born yet. On Mundis, the descendants of the first settlers are seemingly capable of seeing in the future when near a working geometry drive. During its famous rim-bound journey, exploration ship Laniakea often reported FTL probes identifying now-barren planets as habitable, bringing information back from the past.

Cracks in reality follow in the wake of faster than light ships.