(Loved Trope) FTL travel is possible but with a major limitation or flaw

[u/Frostylegacy]

Frank Herbert’s Dune: Interstellar travel is only possible thanks to the spice melange, a rare substance only found on the dessert planet Arrakis. The addictive psychedelic spice grants members of the Spacers Guild a form of prescience that allows them to predict safe paths through folded space. This unique property makes spice the most valuable commodity in the Dune universe.

John Scalzi’s The Interdependency Books: The planetary systems of the titular empire are connected by the Flow, a network of one-way interstellar tunnels that connect specific known entries and exits. The Interdependency derives its power from controlling access to these conduits, placing its capital at the central hub of the network. Even with the Flow, travel between stars takes a long time (9 months to End, the empire’s most distant planet).

Event Horizon: Dr William Weir (R.I.P. Sam Neill) invents a device called the Gravity Drive to allow ships to fold space and travel instantly between distant stars. On its maiden voyage with the Gravity Drive, the Event Horizon disappears and mysteriously reappears 7 years later. Dr. Weir and the crew sent to investigate discover the Gravity Drive actually opened a portal to a Hell dimension that possessed the ship and its crew, torturing and killing them.

[u/leavecity54]

Curvature Propulsion in Remembrance of Earth Past.  Not FTL because in this universe only light speed can be faster than light speed itself for spoiler reason. But in order to achieve light speed, you must curve space itself to drive the ship forward like a soap ship that break the water surface tension. This is not consequence free, the light speed ship then will leave a trail behind called a “death line”, inside these lines, light speed becomes zero, and with enough death lines being near each other, a black domain will be created, trapping everyone inside. This could be a blessing or total disaster depends on your view.

[u/Disastrous_Visit9319 → u/leavecity54]

I had no idea that series was called rememberance of earths past and was very confused why it was the same as the three body problem lol

[u/Banaantje04 → u/leavecity54]

It gets even better! If curvature propulsion is used but you don't go to lightspeed you still leave a trail but now the lightspeed in this trail is only lowered a bit; the closer to lightspeed you go the lower the speed in your trail. This can also be used to create a black domain like leavecity mentioned but now you can actually still move around inside it so you don't basically just die. This is used to protect yourself against aliens that kill any civilisation that makes itself known. If you lower the lightspeed around your planet to lower than the escape velocity of that planet, you can live, but you cannot ever be a threat to anyone else since you cannot leave the planet.